# Gibson RS. Principles of Nutritional Assessment: Food consumption of individuals

3rd Edition
Octber 2020

Abstract

Quantitative methods for measuring food consumption of indi­viduals consist of 24h recalls or estimated or weighed food records. These measure actual intakes for indi­viduals over one day, or by increasing the number of measurement days, usual intakes of foods and nutrients. For retrospective inform­ation on usual intakes of foods or nutrients over a longer time period, a dietary history or food frequency question­naire (FFQ) can be used. Each method has advantages and limitations.

Technical improvements for measuring dietary intakes of indi­viduals include telephone-assisted inter­views, graduated portion-size photographs printed or digitally displayed. Web-based inter­viewer or self-admin­istered 24h recalls that stan­dardize the inter­view and data entry are being used in national food consumption surveys. For low-income countries, 24h recall data can be collected off-line using a mobile application and linked to a web databased application allowing access to food composition data. Several image-based methods utilizing camera-enabled cellular phones or smartphones, digital, wearable or digital video cameras are being tested, some relying on active image capture by users and others on passive image capture whereby pictures are taken automatically. Under development are wearable devices which objectively measure diet without relying on user-reported food intake.

Selection of the appropriate dietary method depends primarily on the study objectives. For average usual intake of a group, a single 24h recall or record is appropriate, whereas for the prevalence of inadequate intakes, replicate observations on each indi­vidual or at least a representative subsample are required. For assessing usual nutrient intakes for ranking within a group, multiple replicates of 24h recalls or 1d food records are essential, with an even larger number per indi­vidual for diet counselling, correlation or regression analysis. The number, selection, and spacing of the days depends on the within-in person day-to-day variation of the nutrient of inter­est and level of precision required. Alternatively, a semi-quantitative food frequency question­naire or dietary history can provide data on usual nutrient intakes for ranking or diet counselling etc.

CITE AS: Gibson RS. Principles of Nutritional Assessment: Food consumption of indi­viduals
https://nutritionalassessment.org/indi­vidual/
Email: Rosalind.Gibson@Otago.AC.NZ

## 3.1 Methods for measuring food consumption of indi­viduals

This chapter describes methods commonly used for measuring the food consumption of indi­viduals. Subsequent chapters discuss the factors associated with the reproducibility and validity of each of these methods (Chapters 5-7) and the calculation and, subsequently, evaluation of nutrient intakes (Chapters 4 and 8).

Two groups of methods are used to measure the food consumption of indi­viduals. The first group, known as quantitative daily consumption methods, consists of recalls or records designed to measure the quantity of the indi­vidual foods consumed over a one day period. By increasing the number of measurement days, quantitative estimates of the usual intakes of indi­viduals can be obtained, using the same methods. The number, selection, and spacing of the days depend on the food intake, the nutrients of inter­est, the day-to-day variation in nutrient intake, and the level of precision required. Determination of the usual intake of indivduals is partic­ularly critical when relationships between diet and biological parameters or health or chronic disease are assessed. Estimates of usual intakes of individuals in a group or population are also needed to estimate the prevalence of inadequate intakes.

The second group of methods includes the dietary history and the food frequency question­naire. Both obtain retrospective inform­ation on the patterns of food use during a longer, less precisely defined time period. Such methods can be used to assess the usual intake of foods or specific classes of foods. With modification, they can also provide data on usual nutrient intakes of individuals.

Quantitative daily consumption methods such as 24h recalls and records, when adjusted appropriately statistically to assess usual intakes, provide less-biased estimates of dietary intake than those such as food frequency question­naires and dietary histories that are designed to generate data on usual intakes directly

The measurement of food consumption at the indi­vidual level is costly and time consuming. Hence, such studies should be planned with care Even with the advent of new technologies to measure dietary intakes (Section 3.2), challenges still remain. Consid­eration should be given to the cost-effective collection of additional data from the same indi­viduals at the same time; such additional inform­ation may significantly enhance the inter­pretation of the dietary data. At a minimum, socioeconomic and health-related inform­ation, simple anthropometric measures, possibly a physical activity question­naire, and biological samples for the determination of important biomarkers (Chapter 15) should be collected when time and resources permit

The accurate assessment of the food intake of infants is partic­ularly difficult, espe­cially when infants are receiving both breast milk and complementary foods WHO (1998) has published guidelines that can be used to evaluate nutrient intakes of breastfed infants receiving complementary foods.

### 3.1.1 Twenty-four-hour recall method

In the 24h recall method, the respondent and, where necessary, their parents or caretakers are asked by the nutritionist, who has been trained in inter­viewing techniques, to recall the respondent's exact food intake during the previous 24h period or preceding day. Thus the method assesses the actual intake of indi­viduals. However, a single 24h recall is not sufficient to describe an indi­vidual's usual intake of food and nutrients; multiple 24h recalls on the same indi­vidual over several days are required to achieve this objective (Section 3.1.2). Nevertheless, multiple single-day recalls on different indi­viduals can give a valid measure of the usual intake of a group or population (Section 3.3.1).

A multiple-pass inter­viewing technique is recommended for the 24h recall methods. The USDA has developed an automated multiple-pass method (AMPM)consisting of five steps to collect dietary data using the 24h recall method; these five steps are shown in Table 3.1.
Table 3.1 USDA's Automated Multiple Pass Method. From
USDA's Automated Multiple Pass Method
Design AMPM Step Memory Cues
Unstructured
Respondents use
their own strategy

### Quick List ⇓

Collects recalled foods
Yesterday, midnight to midnight,
day of week, activities, snacks
and beverages, location
Structured
Questions for specific
types of foods

### Forgotten Foods⇓

Probes for categories
of forgotten foods
Seven question: Beverages,
alcoholic beverages;
sweets; savory snacks;
fruits, vegetables or
and one for anything else.
Structured
Times and names
of eating occasions

### Time & Occasion⇓

Collects eating times
and meal names
Breakfast, lunch,
dinner, snack
Structured
Standardized questions to
collect details of each food

Review foods for each
eating occasion & intervals
between each occasion

### Detail Cycle⇓

Collect food details
and amounts.
a bagel & coffee
Anything else?

Did you have anything
and 10am snack?
Unstructured
Respondents use
their own strategy

### Final Probe⇓

Anything else recalled
Situations where foods
may have been easily
forgotten: in the car, at
meetings, when shopping,
cooking, or cleaning up?
The AMPM is used by the US  in their National Health and Nutrition Examination Surveys (NHANES). Branded food products have now been added to the NHANES generic database to facilitate easier selection of foods consumed by the participants.

The first step in the AMPM is the Quick List where respondents list all food consumed in the previous 24h period. The second step, called Forgotten Foods, includes a series of questions that probes for categories of foods that are commonly forgotten. The third step, Time & Occasion, collects the time each food was eaten and the name of the eating occasion. The fourth step is the Detail Cycle which elicits descriptions of foods and amounts eaten, aided by the inter­active use of the USDA Food Model Booklet and measuring guides ; see Chapter 5 for more details. The Detail Cycle also includes questions that review each eating occasion and each inter­val between eating occasions. The fifth step is a final review question, the Final Probe, which provides the respondent a final opportunity to recall any foods that had not been reported previously during the inter­view. The AMPM has been validated by comparing reported energy intake in young children with total energy expenditure assessed using the doubly labeled water method and in an observational study in which actual intakes of energy, protein, carbohydrate, and fat were compared with recalled intakes in men

A modification of the multiple-pass 24h recall consisting of four passes — termed an interactive 24h recall — has been developed to collect information on rural populations in low- and middle-income countries; details are given in the technical monograph by Gibson and Ferguson (2008).
In the first pass a complete list of all the foods and beverages consumed during the preceding day is obtained.

In the second pass, a detailed description of each food and beverage consumed, including cooking methods and brand names (if possible) is collected. Standardized probe questions, specific for the population subgroup and setting, should be used to elicit specific details for each food item. For example, for milk products, probe questions should include the kind of dairy product, brand name (if appropriate), and percentage fat (as butterfat or milk fat). Further examples of probes that can be used to obtain detailed descriptions of specified foods are also provided in the monograph.

In the third pass, estimates of the amount of each food and beverage item consumed are obtained, generally in household measures, and entered either on the data sheet Appendix 3.1 or a computer-based data-entry form. Care must be taken to record whether any fortified foods or beverages have been consumed. Graduated photographs, a set of measuring cups, spoons, and rulers, local household utensils (calibrated for use), play dough, or food models of various types (Chapter 5) can be used to assist the respondent in assessing portion sizes of food items consumed See guidelines on portion-size estimation methods by: www. intake.org. Information on the amount of any mixed dishes consumed by the respondents, the amount of each ingredient in the mixed dishes, and the total amount of each cooked mixed dish prepared must also be collected at this time. These details are usually recorded on a separate data sheet or computer-based data-entry “recipe form".

In the fourth pass, the recall is reviewed to ensure that all items, including use of vitamin and mineral supplements, have been recorded correctly. Methods for coding the completed 24h recalls and potential sources of coding errors are discussed in Chapter 5.

Additional modifications that can be used in the interactive 24h recall are listed in Table 3.2
Table 3.2: Interactive 24-h recall modifications suggested for rural populations in developing countries to improve the recall of food items. From Gibson and Ferguson
Interactive 24-h recall modifications
Provide group training on portion size estimation
before the actual recall.
Supply picture charts on the day before the recall
for use as a checklist on the day the food is
actually consumed, and for comparison with the
recall to reduce memory lapses.
Provide bowls and plates for use on the recall
days to help the respondents visualize the amount
of food consumed.
Weigh the portion sizes of salted replicas of the
actual foods consumed by the respondent.
and are discussed in more detail in the technical monograph Whenever possible, recall inter­views in rural settings in low and middle-income countries should be conducted in the respondent's home, because the familiar environment encourages partic­ipation, improves the recall of foods consumed, and facilitates calibration of local household utensils by the inter­viewer. However, major challenges arise when assessing food consumption at the indi­vidual level in countries or cultures where the majority of the food consumed is as shared plate eating. Strategies used to overcome some of these challenges are reviewed by

A major advantage of a 24h recall is that the respondent burden is small so that compliance is generally high. In addition, the method is quick and relatively inexpensive, and can be used equally well with both literate and illiterate respondents. Nevertheless, any 24h inter­view protocol must be stan­dardized, pretested,and then piloted prior to use. Standardization is partic­ularly important in large-scale national surveys and for comparisons across countries Adherence to the inter­view protocol and accuracy of food coding by the inter­viewers should be checked periodically during the survey, and the inter­viewers must be retrained if required to minimize inter­viewer bias (Chapter 5). Detailed sugges­tions on how to conduct the inter­view can be found in Hughes (1986) who stressed that leading questions and judgmental comments should be avoided. An indi­rect approach employing open-ended questions is recommended. This enables respondents to freely express their feelings so that answers are not biased. Piloting should be undertaken in an area near the study site, using respondents similar to those who will partic­ipate in the actual study.

When 24h recalls are used to characterize the average usual intake of a population group, the respondents should be representative of the population under study. In addition, the survey should be conducted in such a way that all days of the week are equally represented. In this way, any day-of-the-week effects on food or nutrient intakes will be taken into account (Chapter 6). Seasonality must also be consid­ered as it affects food availability.

Guidelines on how to plan, design, and conduct large-scale 24h recall dietary surveys in low-and middle-income countries have been developed by the Intake — Center for Dietary Assessment "Deitchler M, Arimond M, Carriquiry A, Hotz C, Tooze JA (2020a). Planning and design consid­erations for quantitative 24-hour recall dietary surveys in low and middle-income countries. Intake - Center for Dietary Assessment/FHI Solutions: Washingon, DC." Available from www.intake.org An outline of the main tasks that must be completed prior to the conduct of a large-scale 24h recall survey is presented in Figure 3.1

A 24h recall has been used in several national nutrition surveys in low-, middle-, and high-income countries. Examples include the Cameroon Nigeria New Zealand (MOH,1997), and the US NHANES III survey The AMPR method, used in the US NHANES III, has been adapted for use by both the Canadian and Australian national health surveys.

The US National Cancer Institute have developed an Automated Self-Administered 24h recall (ASA24) based on the US AMPR method which is suitable for large-scale epidemiological studies The ASA24 web-based system was tested for use with children and adults in Canada Overall partic­ipants were receptive to completing ASA24, although the ASA24 inter­face and the steps in completing the recalls were not necessarily intuitive, espe­cially for younger children and elderly adults. The investigators emphasized the importance of piloting protocols using online tools and the potential need for tailored resources to support certain age groups.

In general, recall inter­views can be conducted on children aged > 12y and on most adults, except for persons with poor memories (e.g., some elderly). Parents are typically relied upon as proxy reporters for capturing dietary intake for children under five years of age. Children aged from 5–12y should be inter­viewed along with their primary caretaker, usually the mother, an approach termed assisted reporting. It may be necessary to inter­view several people if the children are at school, day-care, or play in the homes of friends, to ensure that all foods eaten away from home are reported. Recipe data for school or day-care meals may be required to supplement the food intake data collected in the home. In a study of Canadian preschool children 2–5y in which parents used the online self-admin­istered 24h recall (ASA24), they were able to report the food and beverages that their child had consumed with reasonable accuracy but the accuracy of portion size estimates was low

In the end, the success of the 24h recall, irrespe­ctive of the method applied, depends on the respondent's memory, their ability to convey accurate estimates of portion sizes consumed, the degree of motivation of the respondent, and the persistence of the inter­viewer

### 3.1.2 Repeated 24h recalls

Twenty-four-hour recalls can be repeated to estimate the average food intake of indi­viduals over a longer time period (i.e., usual food intake). The number of 24h recalls required to estimate the usual nutrient intake of indi­viduals depends on the day-to-day variation in food intake within one indi­vidual (i.e., within-person variation). In turn, this variation is affected by the nutrient under study, the study population, and seasonal variations in intake.

Repeated 24h recalls were recommended as part of a system for measuring food consumption patterns in the United States to account for within-person variability as early as 1981 (NRC, 1981). In the US NHANES 2011–2014, food intakes on two non-consecutive days were recorded, on the first day through using computer-assisted personal recall inter­views (CAPI), and on the second day via a computer-assisted telephone inter­view (CATI), using the AMPR on both occasions The second recall inter­views were not conducted on all respondents, but instead were repeated on a randomly selected subsample of the population. Tooze (2020) recommends that the repeated 24h recalls should be performed on non-consecutive days 3–10d apart, and when only a random subsample of the population is used, repeated on at least 50 indi­viduals per stratum. Moreover, when episodically consumed foods, food groups or nutrients are of partic­ular inter­est, then more replicates per person should be collected rather than increasing the sample size from which two replicates are obtained (Tooze, 2020).

Repeated 24h recalls assisted by inter­viewers were also used for the Dutch National Food Consumption Survey (DNFCS) and the French Nutrition and Health Survey Several research groups in the UK have incorporated new technologies to collect 24h recalls, including the two web-based methods, myfood24 and INTAKE24 see Section 3.2.3 for more details.

#### Estimated food records

For the estimated food record, also referred to as a food diary by some researchers, the respondent is asked to record in household measures, at the time of consumption, all foods and beverages eaten (including snacks), for a specified time period. Detailed descriptions of all foods and beverages (including brand names) and their method of preparation and cooking should also be recorded. For mixed dishes such as spaghetti Bolognese, the amount of each raw ingredient used in the recipe, the final weight of the mixed dish, and the amount consumed by the respondent should be recorded, wherever possible. The inform­ation is recorded on a form similar to that shown in Appendix 3-1 , except that household measures are used for food amounts. Usually, the respondent, parent, or caretaker completes the food record, although in low or middle-income countries a local field investigator may perform this task Reactivity, defined as a change in behavior due to awareness that behavior is being or will be measured, may be an issue with this method. The respondent has the opportunity to modify his or her “usual” diet, potentially in a more socially desirable manner or to simplify the recording task (Chapter 5).

Food portion sizes can be estimated by the respondent in a variety of ways. Standard household measuring cups and spoons should be used if possible, supplemented by measurements with a ruler (for meat and cake) and counts (for eggs and bread slices). Unfortunately, errors may arise because the respondent may fail to quantify portion sizes correctly. Additional errors may also arise during the conversion of volumes to weights (Chapter 5), although this latter step is usually completed by the investigator. Details on how to convert portion sizes to weight equivalents are given in

The number of days included in an estimated record varies, depending on the study objective. When the objective is to obtain an average intake for a group, then only one day per person is required, provided all days of the week are equally represented in the final sample. However, when estimates of usual intakes of each person are required, then the number, selection, and spacing of the days required per person depends on the factors described for the repeated 24h recall (Section 3.1.2). Weekend days should always be proportionately included in the dietary survey period for each person, to account for potential day-of-the-week effects on food and nutrient intakes. This problem is discussed in more detail in Chapter 6.

The European Prospective Investigation of Cancer (EPIC) study in Norfolk, U.K. collected food and nutrient intakes from 2117 men and women using a 7d estimated food diary. EPIC was a large multicenter prospective study aimed at investigating the relationship between nutrition and various life-style factors and the etiology of cancer and other chronic diseases. The study involved 23 regional centers located in ten countries, and involved a total cohort of about 480,000 subjects. The respondents of the EPIC study in Norfolk were provided with a “diet diary” — a 45-page colored booklet in which they were asked to record the description, preparation, and amounts of foods eaten over seven consecutive days. Food portion sizes were estimated by the respondents in terms of household measures, with the help of 17 sets of color photographs of small, medium, and large portions of the different foods. Details are given in The Danish National Survey of Diet and Physical Activity (4–75y) also used a seven-day estimated food diary with a paper format

A four-day estimated food diary, recorded using a paper form, is used by the U.K. National Diet and Nutrition Survey rolling program. This is a continuous cross-sectional survey designed to collect detailed inform­ation on food and nutrient intakes and nutritional status for a range of ages and across the social strata in the UK. The field work for this rolling program began in 2008 and is on-going with about 1000 people surveyed per year

#### Weighed food record

Weighed food records are more frequently used in the United Kingdom and Europe because weighing scales are often used for food preparation in these regions. In the earlier British National Diet and Nutrition Surveys of adults and children seven-day weighed food records were used. However, when the rolling program was introduced in 2008, the seven-day weighed food records were replaced by a four-day estimated food diary due to concerns about respondent burden.

A weighed food record is the most precise method available for estimating usual food and nutrient intakes of indi­viduals. It is the preferred method when diet counseling or correlation of intakes with biological parameters are involved. In a weighed record, the subject, parent, or caretaker is instructed to weigh all foods and beverages consumed by the subject during a specified time period. Details of methods of food preparation, description of foods, and brand names (if known) should also be recorded. For mixed dishes such as spaghetti Bolognese, the weight of the portion consumed should be recorded, along with the weights and description of all the raw ingredients, including flavors and spices used in the recipe, as well as the final total weight of the mixed dish. The method of recording is similar to that shown for a 24h recall (Table 3.1), with the weight of the food items being recorded under "Amount."

If occasional meals are eaten away from home, respondents are generally requested to record descriptions of the amounts of food eaten. The nutritionist can then buy and weigh a duplicate portion of each recorded food item, where possible, to assess the prob­able weight consumed. Alternatively, if appropriate, the nutritionist can telephone a restaurant to obtain details of the portion sizes consumed.

As with the estimated record, the number, spacing, and selection of days necessary to characterize the usual nutrient intakes of an indi­vidual using the weighed record depend on the within-person variation in food intake, which, in turn, depends on the nutrient of inter­est, the study population, and any seasonal variation of intake. Again, week-end days should be proportionately included to account for any weekend effect on the nutrient intake. If a weighed food record method is to be used, respondents must be motivated, numerate, and literate. Reactivity may be an issue with this method, and inter­fere with the assessment of usual dietary intake. Respondents may change their usual eating pattern to simplify the weighing process or, alternatively, to impress the investigator (Chapter 7). In addition, respondent burden for a weighed food record is higher than for an estimated record or for a 24h recall, so indi­viduals may be less willing to cooperate. Reproducibility, however, is greater in the weighed record than in the estimated record method because the portion sizes are weighed, although significant underreporting (Chapter 5) may still occur.

### 3.1.3 Dietary history

The dietary history method attempts to estimate the usual food intake and meal pattern of indi­viduals over a relatively long period of time — often a month. This inter­view method was originally designed to be carried out by a nutritionist trained in inter­viewing techniques. More recently, computerized versions have been developed which provide stan­dardized methods for data collection and probing, and minimize potential inter­viewer bias in responses

Initially, the dietary history had three components. The first component was an inter­view about the usual overall eating pattern of the subject, both at mealtimes and between meals. Such inform­ation included detailed descriptions of foods, their frequency of consumption, and usual portion sizes in common household measures. “What do you usually eat for breakfast?” is a typical question that might have been included in the inter­view.

The second component served as a crosscheck and consisted of a question­naire on the frequency of consumption of specific food items. This part was used to verify and clarify the inform­ation on the kinds and amounts of foods given as the usual intake in the first component. Questions asked related to specific foods, such as: "Do you like or dislike milk." A 24h recall of actual intake may also have been included at this stage.

In the third component, subjects recorded their food intake at home for three days. Portion sizes at this stage were estimated using a variety of techniques, including stan­dard measuring cups and spoons, common utensils, commercial plastic food models, photographs, or real foods. Today, the original dietary history method is seldom used in this three-part format, the third component being commonly omitted.

The time periods covered by the dietary history method vary. The maximum time period that can be used has not been definitely established. When shorter time frames (i.e., one month) are used, reproducibility and validity are apparently higher than for longer periods (see Chapter 7). Measurements of food intake over one-year periods are prob­ably unrealistic unless seasonal variations in food intakes are taken into account.

Dutch investigators used a three-part dietary history method covering one month to record usual food consumption on weekdays, Saturdays, and Sundays separately This approach takes into account the potential effect of weekends on nutrient intake. The portion size of foods most frequently consumed in this study were weighed by a dietitian in the home. A weighted daily average intake was then calculated from the data, using the following formula:

$\small ((5 × weekday) + Saturday + Sunday) / 7$

A modified version of this dietary history method was adopted in the Survey in Europe on Nutrition in the Elderly: A Concerted Action (SENECA). This multicenter survey was designed to examine cross-cultural variations in the nutrition, lifestyle, health, and perform­ance of elderly Europeans The method involved the completion of a three-day estimated record, followed by an inter­view during which respondents were questioned about their usual dietary intake over the past month. Portions of the most commonly eaten foods were weighed by the inter­viewer

The recording of a dietary history can be very labor intensive, with inter­views taking up to 2h per respondent Several investigators have reported that the dietary history tends to overestimate nutrient intakes, when compared with results from weighed records. Nes et al. (1991), for example, used the dietary history developed for the SENECA study and showed that the method generated consistently higher intakes of energy and nutrients than three-day weighed records. Livingstone and Robson (2000) reported similar findi­ngs in a study of children and adolescents, but claimed that the results obtained from the dietary history were more representative of habitual intake than those obtained from seven-day weighed records. In general, because dietary histories, unlike food frequency question­naires, do not limit the variability in the responses, they overcome many of the limitations of a food frequency question­naire. However, the absence of a stan­dardized format for the dietary history method limits comparison of its usefulness across studies

### 3.1.4 Food frequency question­naire

The food frequency question­naire, sometimes referred to as a diet history question­naire, aims to assess the usual frequency with which food items or food groups are consumed during a specified time period. It was originally designed to provide descriptive qualitative inform­ation about usual food-consumption patterns. With the addition of portion-size estimates and the introduction of improved computerized self-admin­istered question­naires, the method has become semi-quantitative, allowing the derivation of energy and selected nutrient intakes

In its simplest form, the question­naire consists of a list of foods and an associated set of frequency-of-use response categories ( Appendix 3-2 ). The list of foods may focus on specific groups of foods, partic­ular foods, or foods consumed periodically in association with special events or seasons. Alternatively, the food list may be extensive to enable estimates of total food intake and dietary diversity to be made. The frequency-of-use response categories may be daily, weekly, monthly, or yearly, depending on the study objective.

Specific combinations of foods can be used as predictors for intakes of certain nutrients or non-nutrients, provided that the dietary components are concen­trated in a relatively small number of foods or specific food groups. Examples include the frequency of consumption of fresh fruits and fruit juices as predictors of vitamin C intake green leafy vegetables and carrots or fruit and vegetable intake as predictors of carotenoid intakes whole grain cereals, legumes, nuts, fruits, and vegetables as predictors of dietary fiber intakes and dairy products as predictors of calcium intakes The method can also be used to assess the intake of fats and cholesterol artificial sweeteners certain contaminants present in specific foods alcohol and condiments

The food frequency question­naires should feature simple, well-defined foods and food categories, and open-ended questions should be avoided as preformatted lists of food categories act as a memory prompt. The method may use a stan­dardized inter­view, a self-admin­istered machine-readable printed question­naire, or a computer-admin­istered question­naire. Most question­naires usually list 80 to 120 indi­vidual items and take from 15–30 minutes to complete (see abbreviated example given in Appendix 3.2). Hence, the food frequency question­naire imposes less burden on respondents than most of the other dietary assessment methods. The results are easy to collect and process and are generally taken to represent usual intakes over an extended period of time, and hence are not affected by day-to-day variability. As food frequency question­naires are designed to provide retrospective inform­ation about diet, they are often used in retrospective case-control studies However, their validity and feasibility for estimating food intakes in the remote past has not always been clearly established

Food scores can be calculated from qualitative food frequency data and the frequency of consumption of certain food groups The 2015–2020 US Dietary Guidelines for Americans or an equivalent stan­dard such as the WHO Healthy Diet, list the optimum number of servings of the major food groups per person per day, and serve as a basis for the scores. The scores can then be examined in relation to psycho­social influences (e.g., level of education, income), as well as vital statistics, season, geographic distribution, and biomarkers of dietary patterns

WHO has issued guidelines on a Healthy Diet and a (HEI‑2015) has been developed by the USDA, that measures alignment of US diets with the 2015‑2020 Dietary Guidelines for Americans. The HEI‑2015 has 13 dietary components in total, of which nine are “adequacy” components (those recommended for inclusion in a healthy diet) and four “moderation” components that should be consumed sparingly. These dietary components are listed in Table 3.3.
Table 3.3 HEI–2015 Components & Scoring Standards. From HEI (2020).
1Intakes between the minimum and maximum standards are scored proportionately.
2Includes 100% fruit juice.
3Includes all forms except juice.
4Includes legumes (beans and peas). 5Includes all milk products, such as fluid milk, yogurt, and cheese, and fortified soy beverages.
6Includes legumes (beans and peas).
6,7Includes seafood, nuts, seeds, soy products (other than beverages), and legumes (beans and peas).
8Ratio of poly- and monounsaturated fatty acids (PUFAs and MUFAs) to saturated fatty acids (SFAs).
HEI–20151 Components & Scoring Standards
ComponentMax.
points
Standard for
max. score
(per 1,000 kcal)
Standard for
min. score (0)
Total Fruits2 5 ≥ 0.8 cup equiv. No Fruit
Whole Fruits3 5 ≥ 0.4 cup equiv. No Whole Fruit
Total Vegetables4 5 ≥ 1.1 cup equiv. No Vegetables
Greens and Beans4 5 ≥ 0.2 cup equiv. No Dark Green
Veg. or Legumes
Whole Grains 10 ≥ 1.5 oz equiv. No Whole Grains
Dairy5 10 ≥ 1.3 cup equiv. No Dairy
Total Protein Foods6 5 ≥ 2.5 oz equiv. No Protein Foods
Seafood and
Plant Proteins6,7
5 ≥ 0.8 oz equiv. No Seafood or
Plant Proteins
Fatty Acids8 10 (PUFAs + MUFAs)
/SFAs ≥ 2.5
(PUFAs + MUFAs)
/SFAs ≤ 1.2
Moderation:
Refined Grains 10 ≤ 1.8 oz equiv. ≥ 4.3 oz equiv.
Sodium 10 ≤ 1.1 gram ≥ 2.0 grams
Added Sugars 10≤ 6.5% of energy ≥ 26% of energy
Saturated Fats 10 ≤ 8% of energy ≥ 16% of energy
This table also shows the scoring stan­dard for each of the dietary components. The scores for each of the thirteen components derived from the indi­vidual's intake, are added to give an HEI Index score (maximum = 100) for the indi­vidual.

Kant et al. ( 2000) used food frequency inform­ation to calculate a “Recommended Food Score” (RFS) to evaluate the consumption of foods consid­ered to be consistent with the U.S dietary guidelines existing at that time. In a prospective study of diet quality and mortality in women from the United States, they showed that the RFS was inversely associated with all‑cause mortality. Since that time, the RFS has been used in Australia as a diet quality index for preschoolers and in Korea to investigate links with physical perform­ance among the elderly and depression, anxiety, and quality of life

Many recent users of food frequency question­naires have quantified portion sizes of food items of inter­est, often using photographs Portion sizes can be ranked as small, medium, and large, preferably based on age and sex-specific portion size data generated from country-specific national nutrition surveys Note that inclusion of inform­ation on portion sizes produces semi-quantitative food frequency data (Appendix 3-3 ). This can be converted to data on energy and nutrient intakes by multiplying the fractional portion size of each food consumed per day by its energy and nutrient content, obtained from appropriate food composition data. The results are then summed to obtain an estimate of an indi­vidual's total daily energy and nutrient intake.

Block et al. (1986) derived a food list for a food frequency question­naire with portion sizes from the NHANES II results. Food items selected were based on the frequency of consumption of certain specific food items and which contributed significantly to the total population intake of energy and each of 17 nutrients. Serving sizes were estimated from observed portion size distributions in the NHANES II data. Medium serving sizes for each food were specified in the food frequency question­naire, and the respondent indi­cated whether his or her usual serving size was small, medium, or large, as shown in Appendix 3.3. A specialized food composition database was developed for use with this food frequency question­naire. A very similar approach has been used to design semiquantitative multi-ethnic food frequency question­naires

An improvement to the Block food frequency question­naire termed the Diet History Question­naire (DHQ) has been developed. The DHQ consists of 124 food items, portion sizes, and dietary supplement questions and takes one hour to complete. Results of a comparative validity study showed that the perform­ance of the DHQ is better than that of the food frequency question­naires of both Block and Willett

The semi-quantitative food frequency question­naire has become a widely used tool in dietary assessment. Country-specific semiquantitative food frequency question­naires containing between 130 and 300 food items were used in the EPIC study to estimate indi­vidual usual food intakes In the EPIC study in Norfolk, U.K. respondents (n = 23,003) estimated how frequently foods were eaten over the past year, from nine possible frequency-of-use response categories from a list of 130 foods. Reduced versions containing only 60 food items that require only 17 min to admin­ister by an inter­viewer are available; even the full 98‑item Block question­naire requires only 30‑35 min of inter­viewer time In some countries, a semi-quantitative food frequency question­naire has been used in national nutrition surveys (e.g., 1995 Australia National Dietary Survey)

More recently in Australia, an on-line suite of validated semi-quantitative food frequency question­naires, the Australian Eating Surveys (AES) have been developed. The AES is a 120‑item food frequency question­naire with fifteen supplementary questions, including food and sedentary behaviors and supplements, with the aim of capturing the usual dietary intakes of children, adolescents, and adults over the previous six months. The AES takes only 15‑20 min to complete on-line and can generate a personalized dietary feedback in real time as an incentive to encourage partic­ipation and enhance the response rate

In many low-income countries no national food consumption surveys have been conducted so the key inform­ation needed to develop a food frequency question­naire is often not available. Such inform­ation includes a listing of foods commonly consumed in the study population, details on the way the foods are typically prepared or consumed, and the usual range of portion sizes consumed. In response to this need, have developed some simple methods to obtain food listing and portion size distribution estimates for use in semi-quantitative food frequency question­naires.

Table 3.4 Ref = reference quintile. Multivariate model: age at baseline, total calorie intake, smoking history, cancer (yes/no) hypertension diagnosis (yes/no), use of antidepressants, elevated cholesterol, physical activity level, body mass index, cardiovascular disease, multivitamin use, intake of alcohol, total calorie intake, profession, missing indicator for SCF, and number of dietary assessments during 1986–2002. From Yuan et al. (2019)
Odds Ratios (95% CIs) for poor Subjective Cognitive Function (SCF), compared
with good function, associated with total vegetable, fruit, and fruit juice intakes
Quintile of intake
(n = 27,842 men)
Q1Q2Q3 Q4 Q5
Total vegetable intake
Median, servings/d
1.7 2.5 3.2 4.1 5.7
Odds Ratio
95% Confidence Interval
Ref 0.92
(0.79, 1.08)
0.85
(0.73, 0.99)
0.71
(0.60, 0.83)
0.62
(0.52, 0.74)
Total fruit intake
Median, servings/d
0.5 1.1 1.5 2.0 3.1
Odds Ratio
95% Confidence Interval
Ref 1.00
(0.85, 1.18)
0.96
(0.81, 1.13)
0.81
(0.68, 0.96)
0.79
(0.66, 0.94)
Total fruit juice intake
Median, servings/d
0.1 0.4 0.7 1.0 1.5
Odds Ratio
95% Confidence Interval
Ref 0.78
(0.67, 0.91)
0.81
(0.70, 0.94)
0.66
(0.57, 0.77)
0.65
(0.55, 0.76)
Food frequency question­naires are often used by epidemiologists studying associations between dietary habits and disease. For example Yuan et al. (2019) in their study of the relationship between long-term intake of vegetables and fruits and subjective cognitive function in US men, used this approach (Table 3.4). See also: In such studies, the food frequency question­naires must be semi-quantitative, with the ability to rank subjects on the basis of their intakes, so that subjects with low intakes can be separated from those with high intakes. This permits the calculation of the odds ratio or relative risk of disease in relation to intake of certain foods, food groups, or nutrients

Box 3.1 summarizes the five methods discussed above that can be used to assess the food consumption of indi­viduals and their uses and limitations. For a more detailed review of the strengths and weaknesses of each of these methods for use in low- and middle-income countries, see (FAO, 2018).

Box 3.1 Uses and limitations of methods used to assess the food consumption of indi­viduals.

24h recall. Subject or caretaker recalls food intake of previous 24h in an inter­view. Quantities estimated in household measures using food models as memory aids or to assist in quantifying portion sizes. Nutrient intakes calculated using food composition data.

• Uses and Limitations Useful for assessing average usual intakes of a large population, provided that the sample is truly representative and that the days of the week are adequately represented. Used for inter­national comparisons of relationship of nutrient intakes to health and susceptibility to chronic disease. Inexpensive, easy, quick, with low respondent burden so that compliance is high. Large coverage possible; can be used with illiterate indi­viduals. Element of surprise so less likely to modify eating pattern. Single 24h recalls likely to omit foods consumed infrequently. Relies on memory and hence unsatisfactory for the elderly and young children. Multiple replicate 24h recalls used to estimate usual intakes of indi­viduals.
Estimated food record. Record of all food and beverages “as eaten” (including snacks), over periods from one to seven days. Quantities estimated in household measures. Nutrient intakes calculated using food composition data.
• Uses and Limitations Used to assess actual or usual intakes of indi­viduals, depending on number of measurement days. Data on usual intakes used for diet counseling and statistical analysis involving correlation and regression. Accuracy depends on the conscientiousness of subject and ability to estimate quantities. Reactivity may be an issue. Longer time frames result in a higher respondent burden and lower cooperation. Subjects must be literate.
Weighed food record. All food consumed over a defined period is weighed by the subject, caretaker, or assistant. Food samples may be saved indi­vidually, or as a composite, for nutrient analysis. Alternatively, nutrient intakes calculated using food composition data.
• Uses and Limitations Used to assess actual or usual intakes of indi­viduals, depending on the number of measurement days. Accurate but time consuming. Setting must permit weighing. Subjects may change their usual eating pattern to simplify weighing or to impress investigator — termed — reactivity. Requires literate, motivated, and willing partic­ipants. Expensive.
Dietary history. Interview method consisting of a 24h recall of actual intake, plus inform­ation on overall usual eating pattern, followed by a food frequency question­naire to verify and clarify initial data. Usual portion sizes recorded in household measures. Nutrient intakes calculated using food composition data.
• Uses and Limitations Used to describe usual food or nutrient intakes over a relatively long time period, which can be used to estimate prevalence of inadequate intakes. Such inform­ation is used for national food policy development, for food fortification planning, and to identify food patterns associated with inadequate intakes. Labor-intensive, time-consuming, and results depend on skill of inter­viewer. The absence of a stan­dardized format limits comparison of its usefulness across studies.
Food frequency question­naire. Uses comprehensive or specific food item list to record intakes over a given period (day, week, month, year). Record is obtained by inter­view or self-admin­istered question­naire. Questionnaire can be semi-quantitative when subjects asked to quantify usual portion sizes of food items, with or without the use of food models.
• Uses and Limitations Designed to obtain qualitative, descriptive data on usual intakes of foods or classes of foods over a long time period. Useful in epidemiological studies for ranking subjects into broad categories of low, medium, and high intakes of specific foods, food components, or nutrients, for comparison with the prevalence or mortality statistics of a specific disease. Can also identify food patterns associated with inadequate intakes of specific nutrients. Method is rapid, with low respondent burden and high response rate, but accuracy is lower than for other methods.

## 3.2 Technical improvements in food consumption measurements

The increasing evidence of the relationship between diet and chronic disease has led to a number of technical advances in measurements of food consumption for indi­viduals. They include the use of the telephones in surveys, digital photographs of food portions displayed on a computer/tablet or captured with a cell-phone camera, as well as web-based tools, mobile apps, and image-based assessment. These innovative technologies aim to improve the speed, accuracy, and partic­ipation, and reduce reporting burden, bias, and the cost of collecting and analyzing dietary intake data during large scale epidemiological studies as well as national surveys. A summary of the strengths and limitations of these innovative technologies designed to improve dietary assessment methods is shown in Table 3.5.
Table 3.5 Strengths and limitations of innovative technologies to improve dietary assessment methods. From: Dietary Assessment: A resource guide
All innovative technologies used in dietary assessment
StrengthsLimitations
Costs for data collection can be
lower (less need for person-to-person
interaction)
Larger up-front investments (i.e.
purchase of mobile phones, cameras,
computers, software development, etc.)
Convenient for users, good acceptability,
may improve compliance
Risk of losing devices
Do not rely on respondent's memory Risk of technical problems (i.e. low
battery, loss of Internet connection)
could impede data collection
Record of qualitative information (e.g.
date and time of recording)
A backup method is required to collect
information, if technical problems occur
Significantly cuts down data processing
time
Personal Digital Assistant (PDA)
StrengthsLimitations
Is portable and can be easily carried by
study participants
Face-to-face training of the participants
is required
Facilitates real-time data collection,
entry and coding
Low level of dietary data details
because of pre-coded food listings
It is possible to set an alarm within the
PDA to alert participants to record their
food intake
Increases the respondent burden
compared with pen and paper records,
due to the extensive list of foods
(depending on each PDA)
Can be programmed to allow partic-
software only
Reports of difficulty using the search
function and inability to find certain
foods
Image-assisted methods i.e. digital photographs
StrengthsLimitations
Easy to use Participants may forget to take some
images
Suitable for low literacy populations (if
the technologies are easy-to-use, e.g.
digital cameras)
Not all information can be captured
with a single photograph/image
Quality of digital cameras keeps
improving and pictures with higher
resolutions can help improve the
accuracy of analysis
Difficulties in estimating portion size of
food consumed from common mixed
dishes
Suitable for subjects with memory
impairments and for children
Lack of details about cooking methods
Lower under-reporting compared with
Probably more limited accuracy for
countries with a wide range of mixed
dishes (e.g. Asia)
Still needs a written record of foods
obscured in photos, and when details
of ingredients are required
Image-assisted methods i.e. mobile-based technologies
StrengthsLimitations
Possible higher quality control of data
because of shorter delays and real-time
responses
It is costly and time-consuming to
develop the application as an interface,
and the software for automated portion
size estimation
Possibility of sending reminders Certain types of foods, such as mixed
dishes (soups, stews, casseroles,
etc.) can be difficult to analyse with
automated image analysis
Internet access would allow
respondents to send instant photos,
thus minimizing systematic mistakes
Requires certain level of literacy
Requires network/Internet access (for
real-time data collection)
Interactive computer and web-based technologies
StrengthsLimitations
Efficiency in terms of recording
information and data processing
(i.e. reduced costs and interviewer
Some imaging algorithms presently
fail to identify foods correctly and
to accurately estimate the quantity of
food in the computerized images
Increased levels of quality control Need adaptation of the software to
local settings
Include interactive visual and
audible aids
Require high levels of literacy and
computer skills from the participants
Suitable for large, geographically
dispersed samples; can include different
countries/languages (web-based)
Possibility of collecting less food
details (i.e. ingredients, methods of
preparation, etc.)
Data processing can be finalized at any
time and location (web-based)
Can provide personalized dietary
feedback (web-based)
Possibility of sending interactive
reminders (web-based)
Require Internet access (web-based)
A robust assessment of the perform­ance, cost, and response rates should be undertaken, however, prior to the adoption of any new-technology-based method.

### 3.2.1 Telephone

A telephone survey that is well-designed and carefully-admin­istered appears to be a promising method of obtaining dietary inform­ation. In partic­ular, telephoned 24h recalls are being increasingly used. The USDA has conducted several studies to examine the feasibility of using telephone follow-up surveys instead of mail follow-up for 24h recalls. Results have been promising: response rates for telephone follow-up were much greater than those for mail follow-up. Casey et al. (1999) carried out a validation study in which the results of 24h recalls conducted over the telephone were compared with in-person recalls collected in the 1994-1996 USDA Continuing Survey of Food Intakes by Individuals; strong correlations were reported. As a result, since 2002 NHANES have used the telephone to admin­ister the second recall day 3–10d later using the AMPR method

Several other large-scale telephone dietary surveys have been used in the United States, some of which have used a food frequency instrument rather than a 24h recall. Lyu et al., (1998) successfully showed that agreement between telephone and face-to-face inter­views of a semi-quantitative food frequency question­naire in Hawaii made up of 115 food items was good and unaffected by age, gender, ethnicity, or education of the respondents. These investigators did recommend mailing photographs of foods in three portion sizes in advance, to help respondents estimate amounts eaten more accurately.

Many of the smaller telephone dietary studies have been conducted on adult women Very few have been carried out on adolescents and adult men In college students, food intakes by telephoned recalls have been compared against actual intakes determined surreptitiously in the college cafeteria More studies are needed, however, among certain life-stage groups, to establish the validity of 24h recalls or food frequency question­naires admin­istered over the telephone. Under-reporting of self-reported food intakes may still occur in telephone recalls, as they do with in-person recalls; this was observed, for example, when total energy intakes derived from telephone-admin­istered 24h recalls were compared with total energy expenditure measured by doubly labeled water (Chapter 7).

Telephone surveys do have several advantages. These include their ability to reach a large number of persons at perhaps less than half the cost of face-to-face surveys. As well, with the advent of computer-assisted telephone inter­viewing, the inter­views can be readily stan­dardized, queries can be clarified, and responses can be coded immediately, during an inter­viewing time that is much shorter than a face-to-face inter­view. As a result, the response rate is enhanced.

Potential inter­viewer biases can be eliminated by using computer-assisted telephone inter­viewing, an approach used with the AMPR in the NHANES surveys. However, other sources of bias may occur with telephone surveys; these may arise from noncoverage and nonresponse. In the United States, although over 87% of the population owns a telephone, subgroups such as the poor, certain minorities, and the elderly still have fewer telephones than the general population. Such differential coverage can introduce bias in national surveys unless alternative compensating strategies are employed. Strategies may include using supplementary face-to-face or mail inter­views for persons without telephones; statistical adjustments employing weighting for sex, age, race, and income; selective over-sampling of the non-telephone users; and random-digit dialing to contact those with unlisted numbers. Nonresponse is also a source of potential bias that influences telephone surveys. This is espe­cially a prob­lem among certain subgroups such as the ethnic minorities for whom language barriers may be consid­erable, the elderly who may have hearing difficulties, and persons with less education.

Despite these limitations, telephone dietary surveys are a practical, economical, and valid alternative to the conventional face-to-face methods for large-scale epidemiological studies and nutrition surveys and for developing and evaluating community-based nutrition inter­ventions in higher income countries.

### 3.2.2 Portion size estimation aids

Errors in quantifying the portions of food consumed are often the largest error in most food consumption surveys, unless weighed food records are used. As a result, several types of portion-size measurement aids (PSMAs) have been developed in an effort to enhance the accuracy of portion size estimates (see Chapter 5 for more details). PSMAs can be divided into two categories: (1) non-photographic aids and (2) photographic aids. Canada was one of the first countries to use non-photographic aids in its National Nutrition Survey in 1973. The aids consisted of a collection of three-dimensional graduated food models of various volumes and surface areas prepared from paper-mache, wood, or hardboard. A range of graduated food models was used to prevent the tendency to generate a “direct” response. The latter phenomenon is observed when plastic food models representing only “average” portion sizes are used The Canadian surface-area models were accompanied by stan­dard thickness indi­cators made of hardboard squares. These were used during the 24h recalls to assist in assessing the overall size and thickness of foods such as cheese, cold meats, cakes and cookies. Use of thickness indi­cators is critical for assessing portion sizes of intact cuts of meat, espe­cially when irregular in shape.

Since 1973, graduated food models (Figure 3.2) have been used with 24h recalls in many national food consumption surveys, including those in the United States and New Zealand (MOH, 1997). In the dietary component of the NHANES nationwide surveys, the tool developed to measure portion sizes in the 24h recalls consists of a food model booklet of 32 life-size two-dimensional drawings of household vessels (glasses, mugs, bowls), abstract shapes (mounds and spreads), and geometrical models (circles, a grid, wedges, and thickness bars), together with a set of measuring cups, spoons, and rulers More details are given in Chapter 5. In low-income countries, PSMAs for 24h recalls may be salted replicas of staple foods, real foods, and local household utensils (e.g., bowls, plates, spoons). These are often used in combination with items to simulate liquid and solid foods (e.g., water or raw rice), manipulative models (modelling clay or play dough), tape measures or rulers and graduated food models Guidelines on PSMAs for use with 24h recall methods in low-income countries are available from www.intake.org. The use of less than five different PSMAs are recommended for surveys in low-income countries.

More recently, photographs are being increasingly used as PSMAs. They may include pictures of indi­vidual foods or meals which have been drawn, printed, or displayed digitally, often in color and sometimes shown next to a stan­dard sized object (e.g., a coin) or gridlines to aid the respondent's perception of scale. For surveys using 24h recalls, a series of graduated portion-size photographs (i.e., amount consumed on any one occasion) for each food item are commonly displayed during the inter­view, whereas for surveys employing food frequency question­naires, either an average serving size for each food (i.e., average amount served in one helping) or a series of serving sizes (small, average, large) are often used. The graduated portion-size photographs are often bound together in a photographic atlas.

Practical guidelines on how to develop a photographic atlas are given in and at www.intake.org Factors that must be consid­ered in relation to the format of the photographs include size of the image, number and range of portions sizes depicted, and the inter­val between portion sizes. In the EPIC study, for example, there was a 25% difference between portion sizes to allow a real visual perception of differences in size Nelson et al. (1994) used portion weights from the British Adult Dietary Survey ranging from the 5th to the 95th percentile for a series of eight photographs for each food. Other important factors that should be stan­dardized include the order of presentation of the photographs, labels used, angle at which the photograph was taken, background and use of reference objects for scale, color versus black and white, and use of one versus several foods on a plate.

Increasingly digital photos of food portions with multiple images displayed on a computer or a tablet are replacing printed photos to reduce the cost of printing atlases and their transportation to different inter­view sites. Nichelle et al. (2019) compared the accuracy of three portion sizes (small, medium, large) from printed photographs in the Brazilian GloboDiet atlas with digital photos displayed on a computer screen for 20 foods selected from the Brazilian GloboDiet atlas. The mean error (difference between the estimated and true portions) was not significantly different between the printed and digital photos, with agreement on using the printed and digital photos being 91% and 90%, respe­ctively.

During the development of the Internet-based, Automated, Self-Admin­istered 24h recall (ASA24) Subar et al. (2012) investigated the best way to provide digital images as portion-size estimation aides. They compared the photographs said to represent portion sizes of foods consumed by the respondents on the previous day with the actual weights obtained by surreptitious weighing. The images were also varied by the angle in which the photographs were taken, the type of image, and the number and size of digital images presented in an effort to determine how best to present the digital images to facilitate accuracy. None of the photographs were labeled with portion-size inform­ation. Results indi­cated that accuracy was not affected by the type of image, whether the images were presented simultaneously vs. sequentially, or the size and number of images presented, although there was a tendency for eight rather than four images to be more accurate. Based on these results, aerial photographs are used for portion size estimation in the ASA24.

Alternatively, real-time images of foods and beverages consumed by the respondent, and captured passively, or actively with a cell-phone camera digital camera or wearable camera can be used as PSMAs during a 24h recall the following day. The active methods require the respondents to take pictures in the appropriate way (e.g., 45°angle, arms distance from the plate, with a size reference included for scale) before and after eating all meals, and to add accompanying notes. However, all of these tasks necessitate training the respondents. As a result, for low-literacy populations, the passive techniques are more suitable, where an indi­vidual wears a camera to capture photographs, thus requiring less involvement by the respondent.

In many food consumption surveys in both lower and higher income countries, a mixture of non-photographic and photographic aids have been used as PSMAs. A systematic review that included both non-photographic and photographic PSMAs concluded that culturally appropriate digital or hard-copy photographic PSMAs were more accurate than non-photographic food models and household utensils The investigators also emphasized when selecting an appropriate tool for estimating portion sizes, both the setting and population under study must be consid­ered. Most of the studies included in this review were from high income countries. In a study in rural Bolivia, however, the mean energy and nutrient intakes of 45 women (20–52y) obtained using an inter­viewer-admin­istered atlas of graduated photos of local foods combined with digital food photographs taken by the respondents on the day prior to the 24h recall, were compared with those derived from a weighed record. No significant mean differences were reported in estimated and actual food amounts between the 24h recall using the food photo album and the digital photographs and weighed food records, except for a few foods (rice, potatoes, eggs, vegetables) with differences ranging from -5.4% for vegetables to -6.8% for rice). However, there was a slight underestimation in values been the two methods for some nutrients

### 3.2.3 Web-based systems for dietary assessment

Table 3.6 Some of the web-based 24-h recall tools for dietary assessment
Web-based 24-h dietary recall tools for dietary assessment
AuthorTool
(Country)
Notes (All tools use photographs
to aid portion size estimation)
(UK)
Large database with generic and branded food items;
meal-based approach; simplified AMPM approach,
(INTAKE24),
(UK)
Meal-based approach; based on AMPM approach;
Multipass recall for subjects 11–24y.

(France)
Web-based 24h diet record;
Food portion pictures for 250 foods
( ASA24),
(USA)
Meal-based approach; uses the five steps of the
AMPM; questions about food preparation; available in
English and Spanish; optional supplement module
DietDay
(USA)
Multiple steps similar to the AMPM approach;
recording of supplements; provides dietary feedback
WebDASC
(Denmark)
animated; guides response to six eating occasions;
1300 foods selected via category browse or search.
CANAA-W
(Belgium)
A feasibility study on 131 children and 53 parents;
to refine the lay out and structure of the tool;
and the list of food items included.
On-line dietary assessment tools based on 24h recalls, food frequency question­naires, and dietary records are becoming increasingly available. They can be inter­viewer- or self-admin­istered so data can be collected at a time and location that is convenient to the respondent. Examples of web-based 24h dietary recall tools are shown in Table 3.6. These include the US National Cancer Institute ASA24 and DietDay the UK INTAKE24 and UK myfood24 (Cade, 2017), the French NutriNet Sante WebDAS in Denmark and the CANAA-W in Belgium Some of these have been modified for use in other high-income countries such as Canada, Australia and Germany.

The use of automated 24h recall inter­views in web-based systems stan­dardizes the inter­viewing process so there is consistency in the format in relation to the questioning and sequence, food descriptors, and methods used to estimate food portion sizes. Some (e.g., ASA24; INTAKE24; myfood24) use a meal-based approach. Figure 3.3 shows an example of a screenshot from ASA24 for the recall of breakfast. Most are based on a version of the AMPM inter­view format developed by USDA

Groupings of foods and recipes, as well as the level of detail used to describe foods varies across systems. In most of the applications, ingredients of mixed dishes are entered separately in either the raw or cooked form, with an option to select a generic mixed dish if the ingredients of the recipe are unknown. Some of these on-line systems use only food portion photographs to help respondents report the amount of each food consumed (e.g., ASA24; DietDay), whereas others (e.g., INTAKE24) use multiple options for portion size selection. These may include graduated photographs or images of actual foods, guide photographs for selected prepackaged items (e.g., snacks), with or without additional options such as photographs/pictures of household measures (glasses, cups, bowls, teaspoons etc.) Generally, additional automated prompts for potentially forgotten items are programmed and quality controls are implemented at each step of the inter­view procedure, with checks made on outlier values. Standardized algorithms are often used to calculate automatically grams of the food consumed, and the amounts of ingredients (either raw or cooked) consumed from a mixed dish. Some of the applications, espe­cially for children, record the amounts served and leftovers rather than the amounts consumed. Most applications have a separate screen for describing and quantifying dietary supplements. Usually, at the end of the inter­view, a summary is displayed with the opportunity for the respondent to correct or edit inform­ation at each stage of the inter­view. Each web-based system is linked to an in-country food composition database, some of which include branded food items (e.g., ASA24, myfood24), with automated coding for those reported foods and beverages that are in the food composition database. Items not in the database are coded manually.

When developing web-based systems, the potential technology readiness of the user must be consid­ered. Certain population groups, including older indi­viduals with lower computer skills and technical experience, may have difficulty using a computerized assessment tool. Currently, only a few of these web-based 24h recall systems have undergone rigorous evaluation using either independent biomarkers (i.e., Intake24) or measures of true intakes (i.e., ASA24) although for most when compared with the stan­dard 24h recall method, agreement is generally close (Cade, 2017). More well-designed validation studies of web-based systems employing biomarker measures to investigate misreporting of dietary intakes, by age, educational level, BMI, and gender are needed before they can be applied in epidemiological studies involving multi-center sites

In low-income countries where reliable inter­net access may be lacking, espe­cially in rural areas, web-based methods are not feasible for assessing dietary intakes, espe­cially when self-admin­istered, due to the low literacy rates that often exist. To address these concerns in low-income countries, the International Dietary Data Expansion (INDDEX) Project has developed a dietary assessment platform called INDDEX24 The platform consists of a mobile application for the collection of off-line indi­vidual-level dietary data using the 24h recall method with the AMPM, linked to a web database application so that the users can download the required country-specific food composition data. The dietary recall mobile application uses an open source mobile data collection platform (Commcare) so that it is accessible to researchers at no or low cost. The application is designed so that it is straightforward to use and contains a number of quality control features such as controls to prevent entry of implausible values, pre-defined portion-size estimation methods depending on the type of food reported, and real time calculation of the respondent's reported energy intake in order to identify possible under- and over-reported intakes The mobile app is planned to convert food portions reported in non-stan­dard units into gram-weight equivalents by linking to context-specific portion size conversion factors that will be housed within the web database

### 3.2.4 Image-based methods of dietary assessment

These are defined as any method in which images are used for identifying foods, or for portion size estimation. Personal digital assistants (i.e., handheld computers) were used initially for these image-based methods, but have now been replaced by camera-enabled cellular phones or smartphones, digital, disposable, or 35mm cameras, wearable cameras, or digital video cameras. Many of these image-enabled devices rely on users manually taking photographs of foods (i.e., active image capture). Alternatively, for those methods termed 'passive image capture', pictures are automatically taken, usually at timed inter­vals (e.g., every 10 seconds) by the devices (predominantly wearable digital cameras), so that no inter­vention is required on the part of the user. The majority of tools that use active image capture have been designed for research purposes to enhance the accuracy of self-reported intakes. Fewer passive image capture methods exist and none is fully automated, most relying on specialists to manually estimate portion size

A few image-based methods are based on a combination of tools. Examples include: SmartPlate (SRI International, 2015), which consists of a camera and sensor-enabled dinner plate, the Diet Data Recorder System (DDSR), a three-part system comprised of a camera-enabled smartphone, laser module, and circuit module and the Dietary Intake Monitoring System (DIMS). The latter is a five-part system with a digital camera, weighing scale, infrared thermometer, radio-frequency identification reader (RFID), and a user RFID transponder card Most of the image-based methods are portable. Exceptions are the DIMS which is mounted on a small cart and the SmartPlate which is dinner-plate size and hence less convenient to carry compared to the other smaller devices.

Image-based methods can be further subdivided into three groups: image-based food records, image-assisted food records, and image-assisted 24h recalls. Image-based food records are defined as methods in which images of foods captured during eating episodes serve as the entire basis for the dietary assessment, whereas in image-assisted food records, the images captured are used to enhance or supplement a traditional text-based food record (written or electronic). In image-assisted 24h recalls, the food images captured during eating episodes are used to aid self-reporting during the 24h recall. For more details of image-based methods, the reader is referred to the following reviews

Several studies have confirmed that image-based methods of dietary assessment are preferred over traditional dietary records, and can enhance self-reported dietary intake by revealing unreported foods and misreporting errors. However, study costs will be increased when respondents have to be provided with a suitable device With the additional dietary inform­ation from the images, there is likely to be an increase in reported energy intake and a decrease in reporting errors, except when the methods are followed incorrectly so the images are of poor quality, or the user forgets to capture images before eating episodes. In addition, details of mixed dishes are difficult to extract from the images, some ingredients may be hidden, and cooking methods omitted. To avoid these prob­lems, respondents should be trained to review the images after taking them, to take a second image if necessary, and to record descriptions of the mixed dishes, including lists of ingredients and quantities. Coding errors associated with image analysis, however, are likely to be less random and less prob­lematic compared to the systematic bias observed when food type and portion size are self-reported.

Figure 3.4 depicts the entire system-termed the Technology Assisted Dietary Assessment (TADA) system that commences with the respondent capturing an image with the mobile food record. In this method, respondents use the camera of a mobile devise to capture images of their intake of foods and beverages which they send to a secure server, where trained analysts estimate the reported intakes of energy and nutrients. Figure 3.5 compares the mean total energy expenditure (TEE based on doubly labeled water (DLW), with the reported energy intake (rEI) using images from a mobile food record (mFR) recorded over 7.5d by 45 community dwelling men and women, 21–65y In this study, mean energy intake for TEE and rEI were 2932 vs. 2353kcal/day with a significant mean difference of 580kcal/d, p < 0.0001). A moderate statistically significant correlation was found between rEI and TEE (Spearman correlation coefficients 0.58; p < 0.0001), with reporting accuracy that was consistent across all energy intake levels. The mFR was well received by the respondent and usability was rated as easy by Boushey et al., (2017. Figure 3.5). For details of the development of the mobile food record (mFR), see

Unfortunately, there are few validation studies of image-based methods among the elderly, children, and adolescents, and the feasibility of their use in large studies (n > 100) has not yet been demonstrated. Burrows et al. (2019) provide a review of validation studies based on image-based methods using doubly labeled water.

### 3.2.5 Wearable devices

Under development are wearable devices worn around the neck, wrist, ear, or attached to clothing and designed to objectively measure diet without relying on user-reported food intake Some (e.g., WearSens, worn around the neck) lack the ability to detect specific types of foods, and thus quantify nutrient intakes, while others (e.g., GoBe, worn as a wristband) only measure calories, fat, protein, and carbohydrate. Several chewing and bite-related wearable sensors are also being developed which aim to differentiate between food intake and non-food intake, with some aiming to estimate energy intake (e.g., BitBite) Discomfort has been reported by some users of these wearable devices which suggests that their format needs to be modified Privacy issues also need to be addressed when using wearable devices

## 3.3 Selecting an appropriate method

The method of choice for assessing food or nutrient intakes depends primarily on the objectives of the study. No method is devoid of random or systematic errors (Chapter 5), or prevents alterations in the food habits of the respondents. The US National Cancer Institute has developed a  Dietary Assessment Primer (2020), a web resource to aid researchers choose the best available dietary assessment approach to achieve their research objective. Readers are advised to consult the primer before selecting a self-report dietary instrument.

Box 3.2 provides guidance on the most appropriate methods for assessing food or nutrient intakes in relation to four possible levels of objectives.

Box 3.2: Selection of methodology to measure nutrient intakes to meet four possible levels of objectives.

Level One: Mean nutrient intake of a group

• Preferred approach A single 24h recall, or single weighed or estimated food record, with large number of subjects and adequate representation of all days of the week
Level Two: Proportion of population “at risk”
• Preferred approach Replicate observations on each indi­vidual or a subsample using 24h recalls or weighed or estimated one day food records with an adequate representation of all days of the week
Level Three: Usual intakes of nutrients in indi­viduals for ranking within a group
• Preferred approach Multiple replicates of 24h recalls or food records or a semiquantitative food frequency question­naire
Level Four: Usual intakes of foods or nutrients in indi­viduals for counseling or for correlation or regression analysis
• Preferred approach Even larger number of recalls or records for each indi­vidual. Alternatively, a semiquantitative food frequency question­naire or a dietary history can be used.

Note that the number and selection of replicate 24h recalls, or weighed or estimated one day food records required to obtain level two, level three, or level four data, depends on the day-to-day variation within one indi­vidual (i.e., within-person variation) of the nutrient of inter­est (Chapter 6). This variation depends on the nutrient, the study population, and the seasonal variations of intake. Generally, for nutrients found in high concen­trations in only a few foods, such as vitamins A and D and cholesterol, the number of replicates needed is greater than for those found in a wide range of foods (e.g., protein). Noncon­secutive days should be selected for the replicates when possible, to enhance the statistical power of the inform­ation: day-to-day correlations between intakes often occur when food intake data are collected over consecutive days. The length of time needed between the observation days also depends on the nutrient a 3–10d inter­val after the previous recall is recommended (Tooze, 2020).

Additional factors that should be consid­ered when choosing a method for assessing the food consumption of indi­viduals are the characteristics of the indi­viduals within the study population, the respondent burden of the method, and the available resources. For instance, certain methods are unsuitable for elderly subjects with poor memories, for busy mothers with young children, or for illiterate indi­viduals. Other methods require highly trained personnel and specialized laboratory and computing facilities, which may not be available. Generally, the more accurate methods are associated with higher costs, greater respondent burden, and lower response rates. Unfortunately, compromises often have to be made between the collection of precise data on usual nutrient intakes of indi­viduals and a high response rate.

### 3.3.1 Determining the mean nutrient intake of a group: level one

Level one is the easiest objective to achieve and can be met by measuring the food intake of each subject in the group using a single 24h recall or a one day food record, provided the indi­viduals are representative of the study population and all the days of the week are proportionately represented in the final sample. Data on mean usual nutrient intakes of a group can be used for inter­national comparisons across countries of the relationship of nutrient intakes to health and disease. However, this method based on a single intake day per subject should never be used for reporting the distribution of intakes (i.e., as percentiles of intake) (Tooze, 2020).

To calculate n, the number of subjects required in the group, an estimate of the between-person variation for the nutrient of inter­est is needed. This is usually obtained from the literature but may be determined during a pilot study.

As an example, assume the expected mean iron intake obtained from the literature is 10mg/d, with an anticipated stan­dard deviation (s) of 3mg/d. Also assume that we want to be 95% confident that the true mean lies between 9.2 to 10.8mg/d (i.e., the confidence inter­val has limits which are 0.8mg/d on either side of the mean). A 95% confidence inter­val is calculated approximately as the mean ±2 × e, where e is the stan­dard error of the mean — a measure of the precision of the estimated mean. Hence , the required e = 0.8/2 = 0.4. We can use the following formula to calculate n, the desired group size. $\small n = s^{2}_{b} /e^{2 }$ where s2
b
is the between-person variance of the nutrient of inter­est, and e is the desired stan­dard error — a measure of precision required for the estimate of the mean intake of the nutrient of inter­est. Hence $\small n = 3^{2 } / 0.4^{2 } = 56.25$ and so 56 subjects are required. Alternatively, if we wanted to be 99% confident that the true mean lies between 9.2 and 10.8mg/d, then more subjects must be studied. A 99% confidence inter­val is calculated approximately as the mean ±3 × e. Hence the required e = 0.8/3 = 0.27 Therefore

$\small n = 3^{2 } / 0.27^{2 } = 123$ and 123 subjects are required.

Clearly, the size of the group (n) necessary to characterize the group mean usual nutrient intake depends on the degree of precision required: more indi­viduals must be studied to achieve higher precision. This calculation of the required group size should be repeated for each of the nutrients of inter­est, and the largest n (i.e., the worst case) should be used if possible.

If the study objective is to demonstrate a significant difference in the mean intakes of two groups, or a significant change in the mean intakes, based on unpaired or paired data, then alternative formulae must be applied; details are given in

### 3.3.2 Calculating the population percentage “at risk": level two

To determine the percentage of the population “at risk” of inadequate nutrient intakes, an estimate of the distribution of usual intakes of the indi­viduals is required. This, in turn, requires that the food consumption of indi­viduals be measured over more than one day. Hence, repeated 24h recalls, or replicate weighed or estimated one day food records are the methods of choice, again ensuring that all days of the week are proportionately represented in the final sample. Often, it is not feasible to carry out repeated observations on all the indi­viduals, as in the case of a national dietary survey, and the recalls or records are repeated on a subsample of the indi­viduals only.

To achieve a level two objective, at least two independent measurements of food intake should be obtained on at least a representative random subsample of indi­viduals in the survey. The U.S. Food and Nutrition Board recommends that the replicate measurements should be independent and made on non-consecutive days 3–10d after the first recall or record. However, if the data can be collected only on consecutive days, then three daily measurements should be used. The random subsample should consist of at least 50 indi­viduals per demographic group. Where possible, the demographic groups to be sampled should be defined according to the sex and life-stage groups used for the chosen Nutrient Reference Values Deitchier M, Arimond M, Carriquiry A, Hotz C, Tooze J. (2020a) Washington DC. Intake Center for Dietary Assessment/FHI solutions. Available at www. intake.org.

Note that it is more important to have a minimum number of replicate observations in the subsample than a minimum proportion of replicate observations. Once the required number of replicate observations have been obtained, methods can be applied to correct for the measurement error associated with both day-to-day variability in intakes for a single person (termed within-person variation) and the variation in intakes between indi­viduals (termed between-person variation) (Chapter 6).

Four statistical methods are now available for estimating the distribution of usual intake. For more details, the reader is referred to (Tooze, 2020). Of the methods, the first was outlined by the National Research Council and later refined by Iowa State University (ISU) with software developed to use the ISU method. Note, this method does allow for a random sub-sample of repeated 24h recalls or food records, and has the capacity to adjust for season, day of the week, and/or sequence effects, but is not recommended for general use with episodically consumed foods, food groups, or nutrients (Tooze, 2020). The adjustment process provides estimates of the usual nutrient intakes for each specified age and gender-specific subgroup. An example comparing the adjusted distributions of usual zinc intakes (using the refined NRC approach of with the observed zinc intakes for New Zealand adult females aged 19–50y is shown in Figure 3.6. The adjustment process used yields a distribution with reduced variability (sometimes referred to as “shrinking”) because the within person variation has been removed, while preserving the shape of the original observed distribution

Later, the National Cancer Institute (NCI) developed a method which also adjusts for the same covariates (season, day of the week, and/or sequence), but also has the capability of incorporating covariates to identify estimates for sub-populations in the survey. In addition, the NCI method, unlike the ISU, can be used with episodically consumed foods, food groups, or nutrients.

The European Food Consumption Validation Project have developed the Multiple Source Method (MSM) that can also be used for episodically consumed foods, food groups, or nutrients, although caution must be used when employing the program with models containing covariates. The Dutch National Food Consumption Survey 2007-2010 have developed a program called the Statistical Program to Assess Dietary Exposure (SPADE) , but this program currently requires 24h recalls on at least two-days for all respondents in the sample and is not recommended for use with episodically consumed foods, food groups, or nutrients.

All the methods described briefly above use varying approaches to yield an adjusted distribution of “usual" nutrient intakes that can then be used to predict the proportion of the population at risk of nutrient inadequacy using either the full probability approach, or the Estimated Average Requirement (EAR) cutpoint method; details are given in Chapter 8. However, the choice of which “usual intake method” method to use depends on whether foods, food groups, or nutrients are episodically consumed, whether the probability of consumption and the consumption-day amount are correlated or not, and whether replicates of the 24h recalls or food records are available on all the population groups to be surveyed or only on a sub-sample

Figure 3.6 shows that, in the example, adjusting the distribution significantly using the ISU method reduces the proportion of indi­viduals consid­ered to have intakes below the EAR for zinc. Comparable data are essential for national food policy development and food fortification planning. Food patterns associated with inadequate nutrient intakes can also be identified using this approach, enabling food assistance programs to be designed and improvements in nutrition education made.

Note that a method exists for estimating usual intakes through using only one 24h recall provided a suitable external within-person to between-person variance ratio is available, although the collection of repeated recalls on non-consecutive days remains the recommended method Luo H, Dodd KW, Arnold CD, Engle-Stone R (2019. A new statistical method for estimating usual intakes of nearly-daily consumed foods and nutrients through the use of one 24-hour dietary recall. Journal of Nutrition 149: 1667-1673. Within-person variation can also have a significant effect on estimates of the prevalence of abnormally high nutrient intakes. Table 3.7
Table 3.7: “True” and observed prevalence estimates and the number of repeated measurements needed to reduce the observed prevalence to within 5% of the “true” prevalence. Data for women aged 45–54y from NHANES II (1976–1980). From: Sempos CT, Looker AC, Johnson CL, Woteki CE. (1991). The importance of within-person variability in estimating prevalence. In: Macdonald I (ed.), Monitoring Dietary Intakes, pp. 99–109. Springer-Verlag, Berlin..
Variable Prevalence (%)
"True" Observed
No. of repeated
measurements
needed
Cholesterol
> 300mg
15 37 39
Calcium
> 800mg
12 21 9
shows data from NHANES II. The large differences between the observed prevalence and the calculated “true” prevalence represent the effect of removing the within-person variation by calculation. In this case, very large numbers of repeated measurements on each indi­vidual are required to reduce the observed prevalence of abnormally high intakes of cholesterol and calcium to within 5% of the true prevalence

### 3.3.3 Ranking indi­viduals by food or nutrient intake: level three

When the study objective is at level three and involves ranking indi­viduals within a group, often for the purpose of linking dietary intakes with risk of chronic disease, the preferred approach is to obtain multiple observations on each indi­vidual. The number of days required to achieve the level three objective can be calculated from the ratio of the within- to the between-person variation in nutrient intakes (often termed the “variance ratio"); for more details see Chapter 6 . Sometimes an estimate of the variance ratio can be obtained from the literature, again preferably from an earlier study on a comparable group. Alternatively, a pilot study may be necessary to obtain this inform­ation.

Several authors have developed equations for calculating the number of replicate days required to meet level three objectives Black et al., ( 1983). suggest using the following formula for the number of days (n) of diet records needed:

$\small n = (r^{2} /(1 − r^{2})) × (s^{2}_{w}/s^{2}_{b})$

In this equation, r is the unobservable correlation between the observed and true mean intakes of indi­viduals over the period of observation, and s2
w
and s2
b
are the observed within- and between-person variances, respe­ctively. This equation should be used in association with Table 3.8 which shows the proportion of indi­viduals correctly and incorrectly classified in the extreme fractions for different values of the corrlation coefficient between the observed and true intakes (r). The value of r chosen will depend on the degree of misclassification that the investigator is prepared to accept (Table 3.8).

Table 3.8: Proportion of individuals correctly and incorrectly classified in the extreme fractions for different values of the correlation coefficient (r) . (a), correctly classified in the extreme thirds, fourths, or fifths of the distribution of intakes; (b), misclassified into the opposite extreme fraction. From Nelson et al., (1989).
Correctly and incorrectly
classified into extreme fraction
rThirds Fourths Fifths
0.75 a
b
0.69
0.049
0.63
0.013
0.59
0.004
0.80 a
b
0.72
0.033
0.68
0.006
0.65
0.002
0.85 a
b
0.76
0.018
0.72
0.002
0.69
<0.001
0.90 a
b
0.80
0.006
0.77
<0.001
0.75
<0.001
0.95 a
b
0.86
<0.001
0.84
<0.001
0.83
<0.001

As an example, assume that the investigator requires that when the indi­viduals are divided into terciles, fewer than 5% (< 0.05) of the indi­viduals are grossly misclassified into the opposite tercile. This will require an r value of 0.75 (Table 3.8). Assuming s2
w
/ s2
b
= 1.7, then the number of days (n) $\small n = (r^{2}/(1–r^{2}))× 1.7$ $\small n = 0.75^{2}/(1–0.75^{2}) × 1.7$ $\small n = \mbox{3 days}$

The number of days needed to generate a given r increases as the chosen r increases. If the size of the within-person variation (s2
w
) in nutrient intake is small compared with the size of the between-person (s2
b
) variation, then fewer replicate days are needed to meet level three objectives.

The U.S. subcommittee on criteria for dietary evaluation recommended using independent days for replicating the measurements of one day nutrient intakes to reduce any effect of autocorrelation between intakes on adjacent days.

An alternative approach to achieving level three objectives is to use a semi-quantitative food frequency question­naire. This approach is often used in epidemiological investigations to study associations between intakes and risk of disease and does not require a measurement of absolute nutrient intakes. Although this approach is much simpler, involving only a single inter­view with each subject, it is difficult to quantify the errors involved and to separate the effects of within- and between-person variance.

### 3.3.3 Determining usual intakes of nutrients of indi­viduals: level four

Reliable estimates of usual food or nutrient intakes of indi­viduals that can be used with confidence to meet a level four objective, involving correlation or regression analysis with indi­vidual biochemical measures, are the most difficult to obtain. Large numbers of measurement days for each indi­vidual are required using 24h recalls or estimated or weighed food records.

An estimate of the within-person variation for each nutrient of inter­est should be obtained from the literature, preferably from an earlier study on a comparable group or a pilot study, as noted earlier. This estimate may be expressed as the variance, s2
w
; stan­dard deviation, sw; or as the coefficient of variation (CVw) expressed as a percentage: $\small CV_{w}= s_{w}/ (\mbox{mean intake}) × \mbox{100%}$

This estimate can be used in the following equation to determine the number of days required per indi­vidual to estimate an indi­vidual's nutrient intake to within 20% of their true mean 95% of the time

$\small n = (Z_{α}CV_{w} / D_{0})^{2}$

where n = the number of days needed per indi­vidual, Zα = the normal deviate for the percentage of times the measured value should be within a specified limit (i.e., 1.96 in the example below), CVw = the within-person coefficient of variation (as a percentage), and D0 = the specified limit (as a percentage of long-term true usual intake) (i.e., 20% in the example given below).

The following example calculates the number of days required to estimate a Malawian woman's zinc intake using 24h recalls to within 20% of the true mean, 95% of the time. In this example, the CVw (i.e., 34%) for zinc intakes on Malawian women via 24h recalls is taken from the literature (IZiNCG, 2004). Thus if Zα = 1.96 and CVw = 34%. then:

$\small n = (1.96 × 34\% / 20\%)^{2} = 11 d$

If a pilot study is undertaken in which replicate 24h recalls are conducted, then the actual CVw for each nutrient of inter­est can be calculated. In this way, the estimate of the number of days required to measure the usual intake of each of the nutrients of inter­est in an indi­vidual, with a required degree of precision, can be defined. In general, consid­erably more days are required to obtain reliable estimates of intakes of indi­viduals to meet the level four objective, compared with level three (i.e., relative ranking of subjects into groups)

Sometimes, dietary histories or semi-quantitative food frequency question­naires are used to obtain this level four data on usual nutrient intakes for correlation with biomarkers Some investigators emphasize, however, that the accuracy of a semi-quantitative food frequency question­naire is only equivalent to two to three repeat 24h recalls

In some experimentally controlled studies such as balance studies, inform­ation on the actual nutrient intakes of an indi­vidual over a finite time period are required. For such data, weighed food records (Section3. 1.4), completed for the duration of the study period, are the recommended method. Nutrient intakes can then be calculated using food composition data. Alternatively, duplicate meals can be collected throughout the period for later analysis; details are given in Chapter 4.

### 3.3.4 Combining dietary instruments

Recognition of the limitation of even two 24hr recalls to adequately measure, at the indi­vidual level, usual intake of foods or nutrients that are not consumed daily (i.e., episodically consumed foods) has led to the use of more than one dietary instrument in a given study. In this way, the advantages of each method can be exploited and their weaknesses minimized. Consequently, in large scale epidemiological studies, a common approach is the admin­istration of a food frequency question­naire to all respondents, and 24h recalls or food records in a subsample. The food frequency question­naire provides inform­ation on foods that are consumed less frequently (often, “in the past year") not given by the 24h recalls while the 24h recalls yield richer detail by attempting to correctly quantify portion size for each eating occasion Alternatively, a food frequency question­naire could be used as a supplement to one or more quantitative 24h recalls admin­istered to the full sample, providing richer detail and less systematic bias. This approach appears to have the most added value when the research question of inter­est is to estimate diet-health relationships, espe­cially when the estimation of usual dietary intake at the indi­vidual level involves a food, food group, or nutrient that is consumed episodically (Tooze, 2020).

Statistical methods have been developed to combine the dietary data from repeated 24h recalls and food frequency question­naires. Conceptually, these statistical methods presume that the usual food intake of an indi­vidual equals the probability of consuming a food on a given day (queried as frequency of usual intake over a specified time period), multiplied by the average amount of intake of that food on a typical consumption day. Repeated 24h recalls from the same indi­vidual yield inform­ation on consumption probability and amount, while the food frequency question­naire provides inform­ation on intake frequency of rarely consumed foods The reported food frequencies can be used as a covariate in both steps of the NCI method and with MSM to enhance the estimation of usual intakes from the 24h recall data, espe­cially from foods that are not consumed every day. Note, however, that in low income countries where context-specific food frequency question­naires are often not available or inappropriate in the study setting, it may be preferable to increase the number of repeat 24h recalls per respondent rather than embarking on the development, validation, and collection of dietary data using a food frequency question­naire in addition to using the 24h recall method. For more details, the reader is advised to consult (Tooze, 2020). at: Intake.org

## Acknowledgements

The assistance of Nutrition International in the preparation of this web-page is gratefully acknowledged. RSG would like to thank past zinc collaborators, particularly my former graduate students, and is grateful to Michael Jory for the HTML design and his tireless work in directing the trans­ition to this HTML version.