23c.1 Introduction

The human body contains about 760mg of magne­sium at birth, 5g at age 4‑5 months, and 25g when adult (World Health Organization and Food and Agriculture Organization of the United Nations, 2004). There are three body pools of magne­sium (Rude & Shils, 2006). One pool has a turnover rate of less than 28hours. This pool of extracellular magne­sium, which includes blood, contains 0.8‑1.0% of body magne­sium. The second pool has a turn­over rate of about 11days. This pool is primarily intra­cellular magne­sium, of which about 40% of the magne­sium is found in soft tissues where it performs most of its essential functions. Skeletal magne­sium is the third pool with a slow turnover rate meas­ured in months or years in normal adults. Skeletal magne­sium contains about 60% of body magne­sium of which two-thirds is found within the hydration shell (bone mineral or poorly crystal­lized hydroxy­apatite) and one-third on the crystal surface of the skeleton (Rude & Shils, 2006). Surface magne­sium is readily exchangeable with serum and serves as a reservoir for the maintenance of magne­sium for essential functions when intakes of magne­sium are deficient.

23c.2 Functions of magne­sium

Divalent magne­sium (Mg2⁺) is the fourth most abundant cation in the body and is second to potassium as the most abundant intra­cellular cation. Magnesium is a cofactor for over 600 enzymatic reactions vital to pathways including DNA, RNA, protein and adenosine 5'-tri­phos­phate (ATP) synthesis; cellular energy production and storage; glycol­ysis; and cellular second messenger systems. The predominant enzymatic role of magne­sium in these pathways involves an enzyme inter­acting with MgATP to complete a reaction. Magnesium also binds directly to enzymes to cause conform­ational change (allosteric activation) that gives a catalysis center for reaction to occur. Stabil­ization of ribo­nucleo­tides and deoxy­ribo­nucleo­tides by magne­sium allow for DNA duplication, trans­cription, and maintenance, and transfer RNA function (Rude & Shils, 2006). Magnesium reacts with phosphates and carboxy­lates to stabilize membranes, affecting their fluidity and permeability which influences cellular ion channels, trans­porters, and signaling (Rude & Shils, 2006). Through regulating Ca2⁺ and K⁺ movement in and out of the cell, magne­sium is a controlling factor in nerve transmission, skeletal and smooth muscle contraction, cardiac excitability, vasomotor tone, blood pressure, and bone turnover.

23c.3 Absorption and metabolism

The jejunum and ileum are the primary sites of magne­sium absorption; lesser amounts are absorbed by the cecum and colon. When dietary intakes of magne­sium are near require­ment, 45‑55% is absorbed. With intakes less than 50% of require­ment, the per­cent­age of magne­sium absorbed may increase to 65‑70%. As dietary intake increases over require­ment the per­cent­age of magne­sium absorbed decreases. For example, with an intake of 972mg, fractional absorption of magne­sium was only 11% (Fine et al., 1991).

Magnesium is absorbed via both active trans­port (trans­cellular) and passive diffusion (para­cellular). Active trans­port occurs in the ileum, although apparently also in the cecum and colon through the action of magne­sium trans­porters: transient receptor potential melastatin (TRPM) type 6 (TRPM6) and TRPM7 (de Baaij et al., 2015). Passive paracellular diffusion accounts for about 90% magne­sium absorption and is dependent on a high concen­tration in the jejunum and ileum and a transcellular potential difference generated by sodium trans­port (Romani, 2013).

Magnesium absorption can also be affected by other dietary components, including calcium, phosphorus, fiber, phytate, protein, and oxalates, all of which can decrease absorption, whereas vitamin D and its active metabolites, carbo­hydrates fer­ment­able by bacteria, and oligo­saccharides increase absorption (Nielsen, 2017).

The primary organ regulating magne­sium homeostasis is the kidney where about 10% of total body magne­sium is filtered daily through the glomeruli with the nephron recovering 95‑99% of filtered magne­sium. Passive diffusion associated with calcium, sodium, and water trans­port results in the reabsorption of about 10‑25% of the magne­sium (de Baaij et al., 2015). Passive diffusion in the thick ascending limb of the loop of Henle accounts for about 50-70% of the magne­sium reabsorption. The remaining 5‑10% of filtered magne­sium is reabsorbed in the distal convoluted tubule via an active transcellular mechanism. The active trans­port channel in the kidney apparently involves TRPM6. When magne­sium intakes are deficient, excre­tion of magne­sium by the kidney decreases to 12‑24mg/day to conserve body magne­sium. Diets high in sodium, calcium and protein, and caffeine and alcohol consumption all enhance magne­sium excre­tion (Costello & Rosanoff, 2020).

23c.4 Magnesium deficiency in humans

Signs and symptoms of severe magne­sium deficiency include positive Trousseau's and Chvostek's signs; muscle spasms, fasciculations, and tremor; and personality changes such as apathy, depression, nervous­ness, delirium, and hallucinations (Rude, 1998). However, severe magne­sium deficiency involving these signs and symptoms is rare and usually associated with predisposing and complicating disease states that cause low magne­sium intakes or impaired intestinal or renal absorption.

In conditions of long term moderate or mildly deficient intakes of magne­sium or excessive excre­tion, magne­sium is slowly released from bone to assure that vital essential functions are fulfilled. Nevertheless, there may still be a lack of magne­sium required for some functions, but the signs associated with severe magne­sium deficiency outlined above do not occur. This lack has been called chronic latent magne­sium deficiency (CLMD) (Elin, 2010).

Chronic latent magne­sium deficiency is considered a factor contributing to several chronic diseases, especially cardiovascular disease, including ischemic heart disease, sudden cardiac death, heart failure, atrial fibrillation, and ischemic stroke (Costello & Rosanoff, 2020). The associated risk of CLMD with cardiovascular disease is through impaired glucose metabolism and insulin action, dyslipidemia, hyper­tension, chronic inflammation, and impaired vasomotor tone (Costello & Rosanoff, 2020).

Chronic latent magne­sium deficiency also may result in an increased risk for bone fractures and osteo­porosis (Costello & Rosanoff, 2020). Apatite crystals formed in magne­sium-deficient bone are increased in size, which makes bone more brittle, increasing the fracture risk. Magnesium deficiency can also reduce parathyroid hormone and 1,25-dihydroxycholecalciferol levels, which affect the regulation of magne­sium-dependent calcium channels and changes in the release of inflam­matory cytokines. Such effects enhance osteo­clastic action and inhibit osteo­blastic action, resulting in bone loss and an increased risk for osteo­porosis.

Other chronic diseases with which CLMD has been associated include osteo­arthritis, chondro­calcinosis, depression, and migraine headaches (Costello & Rosanoff, 2020). In addition, CLMD has been found to cause chronic inflam­matory & oxidative stress, indicating that through these effects, CLMD may contribute to chronic diseases, other than those noted above, in which inflam­matory or oxidative stress facil­itates their occurrence.

23c.5 Food sources and dietary Intakes

Foods of plant origin provide about 50% of the magne­sium intake for adults on mixed diets (Hunt & Meacham, 2001). Meats supply about 14‑16% and dairy products (including milk) supply about 34% of the magne­sium intakes in U.S. adults (Hunt & Meacham, 2001). Rich food sources of magne­sium include whole grains / cereal, nuts, pulses, green leafy vege­tables, and dark chocolate. Inter­mediate sources of magne­sium include cheese, meats, and most sea­foods. Refined grains are poor sources of magne­sium.

23c.6 Effects of high magne­sium intakes

Because the kidneys are very effective in the excre­tion of excessive magne­sium, severe magne­sium toxicity rarely occurs. When severe toxicity resulting in high blood magne­sium occurs, the condition arises almost exclusively in individuals with kidney dysfunction or failure who are given magne­sium salts or drugs such as laxatives or antacids. Symptoms of toxicity include lethargy, confusion, nausea, diarrhea, impaired breathing, hypotension, muscle weakness, and heart arrhyth­mias (Romani, 2013). Signs of hyper­magnesemia also include low blood calcium and high blood potassium. The heart rhythm changes in magne­sium toxicity are caused by abnormal electrical conduction which occurs at the nervous, muscular, and cardiac level.

Severe magne­sium toxicity caused by excessive intake through food has not been reported. High intakes of supplemental magne­sium available in forms such as aspartate, citrate, chloride, gluconate, lactate, and oxide can cause adverse effects such as diarrhea, abdominal cramping, and nausea. A tolerable upper intake level (UL), defined as the highest level of daily intake that is likely to pose no risks of adverse health effects to almost all individuals in the general population, was set at 350mg of supplemental magne­sium in the United States and Canada for children older than 8 years and adults by the (Institute of Medicine. 1997),

23c.7 Occurrence of dietary deficiency

In the official Dietary Reference Intakes (DRIs) estab­lished for the United States and Canada (Institute of Medicine. 1997), the Estimated Average Requirements, (EAR, defined as average daily intake estimated to meet the daily require­ments of 50% of healthy individuals) were set for individuals from aged 19 to 70 years and over at 330 to 350mg/day for males and at 255 to 265mg/day for females. These EAR values for adults are consistent with those set for adults in several other countries, as shown in Table 23c.1.

Table 23c.1 Comparison of dietary require­ments of magne­sium for adults (mg/day) set by four different authorities. A panel from the European Food Safety Authority (2015). considered that Average Requirements and Population Reference Intakes for magnesium cannot be derived for adults, infants or children, and therefore defined Adequate Intakes (AIs), based on observed intakes in healthy populations in the European Union (EU).

EAR, Estimated Average Requirement; RDA, Recommended Dietary Allowance; RNI, Reference Nutrient Intake; RDI, Recommended Dietary Intake; RNI*, Recommended Nutrient Intake; NA, Not Available.

US/Canada Institute of Medicine (1997).	Age(y)	EAR	RDA
Males	19-30	330	400
	31-70+	350	420
Females	19-30	255	310
	31-70+	265	320
United Kingdom Dietary Reference Values (1991).	Age	EAR	RNI
Males	19-75+	250	300
Females	19-75+	200	270
Australia/New Zealand NZ Ministry of Health (2006).	Age	EAR	RDI
Males	19-30	330	400
	31-75+	350	420
Females	19-30	255	310
	31-75+	265	320
WHO (2004).	Age		RNI
Males	19-65	NA	260
	65+	NA	224
Females	19-65	NA	220
	65+	NA	190

In a United States survey performed in 2005‑2006, magne­sium intakes less than the EAR were reported in 48% of females in ages 31‑50y, 55% in ages 51‑70y, and 70% in ages 71y, and over; for males, intakes less than the EAR were 45% in ages 31‑50y, 58% in ages 51‑70y and 80% in ages 71y and over (Moshfegh et al., 2009).

Recently, it has been suggested that the DRIs for magne­sium estab­lished for the United States and Canada should be revised and lowered to reflect findings of the improved balance study data (Nielsen, 2016). These balance study data indicated that the EAR and the Recommended Dietary Allowance (RDA, defined as average daily intake sufficient to meet the daily require­ments for almost all (about 97%) of healthy individuals) for 70kg healthy adults should be 175 and 250mg/d for 70kg healthy male and female individuals, and increase or decrease based on body weight, independent of age. The balance data found neutral balance (i.e., when intake and loss of magne­sium from the body were equal) at 2.36mg/kg/day, the value indicating the amount of decrease or increase in the DRIs with each change in kg of body weight. Even with the lower DRIs, many individuals weigh more than 70kg and/or have health problems which will result in an increase in the need for magne­sium. As a consequence, it is likely that over 25% of the United States population have an intake of magne­sium less than the EAR. Thus, because mild or moderate magne­sium deficiency can increase the risk for numerous chronic disorders, it is apparent that magne­sium is a nutrient of concern requiring a clinically useful and sound indicator of magne­sium status.

23c.8 Indices of magne­sium status

Because magne­sium has so many critical functions, the body has strong mechan­isms, such as reducing urinary excre­tion and releasing skeletal stores of magne­sium, to assure that magne­sium is readily available to fulfill those functions. In addition, mild or moderate magne­sium deficiency is mostly asymptomatic. As a result of these character­istics, success in finding a cost-effective, time-efficient, and reliable indicator of magne­sium status is lacking. Among the status indicators that have been evaluated are serum or plasma total magne­sium, serum ionized magne­sium, erythro­cyte magne­sium, erythro­cyte membrane magne­sium, blood mono­nuclear cell magne­sium, urinary magne­sium excre­tion, magne­sium load test, sublingual magne­sium, muscle magne­sium, and fractional excre­tion of magne­sium. An additional factor that may be useful in evaluating exposure to magne­sium is dietary intake.

23c.8.1 Serum/plasma magne­sium

At present, total serum magne­sium is the predominant clinical test to assess magne­sium status, although plasma total magne­sium is sometimes meas­ured instead of serum magne­sium. However, serum is preferred because anti-coagulants might be contam­inated with magne­sium or affect assessment methods. Serum magne­sium determination is inexpensive and easily performed using atomic absorption spectrophotometry or colorimetric methods. The normal range for serum magne­sium is 0.75‑0.95mmol/L. (Figure 23c.1). Hence, theoretically, serum concen­trations above 0.75mmol/L should be considered magne­sium sufficient, and those below 0.75mmol/L should be considered magne­sium deficient, although, as indicated below, perhaps not rightly so (Costello et al., 2016). Serum magne­sium concen­trations are reportedly acceptable for determining a severe lack of magne­sium intake or an elevated urinary excre­tion of magne­sium resulting in a depletion of body stores. This finding is based on the results of a study in which exposure to extreme dietary magne­sium deprivation (less than 12mg/day) resulted in a decline in serum magne­sium from 0.80mmol/L to 0.61mmol/L in three weeks (Fatemi et al., 1991). However, in mild or moderate dietary magne­sium deprivation, serum magne­sium concen­trations can remain in the normal range after several weeks, based on controlled metabolic unit studies of postmenopausal women whose dietary intakes were changed from adequacy (250mg/day) to moderate deficiency (80‑160mg/day) (Nielsen & Johnson, 2017). Nevertheless, the magne­sium-deprived women showed signs indicative of magne­sium deficiency that responded to magne­sium supplementation, indicating their body tissues were becoming magne­sium deficient. Results of these metabolic unit studies are consistent with clinical studies of patients with diabetes mellitus, alcoholism, and malabsorption syndromes, all of whom had serum magne­sium concen­trations in the sufficient range, yet had low magne­sium concen­trations in erythro­cytes, mono­nuclear cells, and muscle (Ryzen et al., 1986; Nadler et al., 1992; Abbott et al., 1994; Rude & Olerich, 1996). These findings indicate that individuals with serum/plasma magne­sium concen­trations within the current reference range might be deficient enough to have a bearing on the risk for chronic disease.

Recently, it has been suggested that the reference range for serum magne­sium should indicate that some individuals with serum magne­sium concen­trations between 0.75mmol/L and 0.85mmol/L might have CLMD (Costello et al., 2016). A review (Elin, 2010) has indicated that magne­sium deficiency is present in 90% of individuals with serum magne­sium concen­trations of 0.70mmol/L or less, in 50% of individuals with concen­trations of 0.75mmol/L, and in 10% of individuals with concen­trations of 0.85mmol/L, emphasizing that serum magne­sium alone lacks reliability as a status indicator of CLMD. Moreover, it also indicates that individuals with low serum magne­sium concen­trations might have adequate magne­sium status.

The reliability of serum magne­sium to indicate mild or moderate magne­sium deficiency may be improved by also assessing urinary magne­sium excre­tion and dietary magne­sium intake. Metabolic unit findings suggest that individuals with serum magne­sium concen­trations >0.75mmol/L or as high as 0.85mmol/L could have a magne­sium deficit such that they respond to magne­sium supplementation if they have a dietary history showing a magne­sium intake <250mg/day and a urinary excre­tion of magne­sium of <80mg/day (Nielsen & Johnson, 2017).

Factors affecting serum magne­sium

There also is a need to appreciate the numerous factors affecting serum magne­sium concen­trations when considering the reliability of this as a measure of magne­sium status.

Diurnal variation in serum magne­sium concen­trations occurs: values are lower in the morning than in the evening (Wilimzig et al., 1996).

Strenuous exercise reduces serum magne­sium through a shift from plasma into erythro­cytes and an increase in excre­tion through urinary excre­tion and sweat loss (Lukaski, 2000).

Various drugs, such as diuretics, proton pump inhibitors, cancer therapies, and antibiotics may decrease serum magne­sium concen­trations (Rude, 1998), whereas antacids and cathartics may elevate concen­trations (Arnaud MJ, 2008).

Disease states such as osteo­porosis may decrease concen­trations (Stendig-Lindberg et al., 1993). Chronic renal failure may increase concen­trations by impairing magne­sium homeo­stasis by the kidney (Elin, 1991).

Serum albumin concen­trations are related to serum magne­sium in a linear fashion (Kroll & Elin, 1985). Thus, hypo­albumin­emia may result in low serum magne­sium concen­trations, and hyper­albuminemia in high serum magne­sium concen­trations.

23c.8.2 Serum, plasma, and blood ionized magne­sium

Ionized magne­sium is the physio­logically active form of magne­sium that is trans­ported across cell membranes and participates in meta­bolic inter­actions and enzymatic reactions. Thus, it has received attention as a better measure than serum total magne­sium for indic­ating the status of magne­sium for essential cellular functions in the body. Ionized magne­sium is deter­mined in serum, plasma, or whole blood samples by using equipment with ion-selective electrodes. The first ion-selective electrodes were challenging to use, cost-intensive, and had poor ability to discrim­inate ionized magne­sium from ionized calcium; this inhibited the use of ionized magne­sium as a status-assessment measure. However, recent models of ion-selective electrodes are cost-effective and have been shown to be acceptably accurate and precise in measuring only ionized magne­sium.

Originally, serum ionized magne­sium was often deter­mined, but at present, it is plasma magne­sium that is mostly meas­ured in whole blood; the ionized magne­sium in erythro­cytes is not meas­ured. An advantage of using whole blood is that less blood and fewer procedures are needed for analyses. High pH and high concen­trations of ionized calcium, lipo­philic cations, and thiocyanate (caused by smoking tobacco) can interfere with the deter­mination of plasma ionized magne­sium by ion-selective electrodes (Ansu Baidoo et al., 2023).

Plasma ionized magne­sium as a status assess­ment measure has the same problems as those outlined for plasma total magne­sium; the body also attempts to maintain magne­sium concen­trations in plasma at a level needed for vital functions through reduced urinary excre­tion and bone resorption. Factors that affect plasma total magne­sium, such as diuretics, proton pump inhibitors, and various disease states, also affect plasma ionized magne­sium. An additional issue is that a refer­ence range for plasma ionized magne­sium indicative of adequate magne­sium status has not been firmly estab­lished and appears to vary depending upon the instrument used for determining a range. A systematic review and meta-analysis involving all types of instruments used found a 95% confidence interval for ionized magne­sium in healthy adults to be 0.40 to 0.68mmol/L (Ansu Baidoo et al., 2023). Using this as a reference range indicates plasma ionized magne­sium concen­trations below 0.40mmol/L are most likely indicative of magne­sium deficiency and those above 0.68mmol/L are hyper­magnesemic. However, the meta-analysis also found that in patients with chronic diseases such as cardio­vascular disease, type 2 diabetes, and hyper­tension associated with magne­sium deficiency, the 95% confidence intervals for plasma ionized magne­sium concen­trations included those found for healthy individuals. This indicates, as noted for plasma total magne­sium, that individuals in the desig­nated adequate range for plasma ionized magne­sium may also be magne­sium deficient. Thus, additional measures of a dietary magne­sium intake <250mg/day and a urinary magne­sium excre­tion <80mg/day might help in identifying individuals who are very magne­sium deficient even though they have plasma ionized magne­sium concen­trations in the low healthy reference range.

23c.8.3 Erythrocyte magne­sium

Magnesium exists in erythro­cytes in free, complex, and protein-bound forms. Magnesium concen­trations in erythro­cytes are about three times greater than those in plasma and reflect chronic rather than acute magne­sium status because of the long lifespan of erythro­cytes (120 days). Cellular magne­sium is considered more reflective of magne­sium status than circulating magne­sium. However, erythro­cyte magne­sium concen­trations have been reported as not correlating well with other tissue pools such as mono­nuclear blood cells or muscle (Ryzen et al., 1986; Elin, 1987). Thus, it is uncertain how well erythro­cyte magne­sium concen­trations reflect body magne­sium stores.

Determination of cellular magne­sium concen­trations takes special procedures including centri­fug­ation to separate blood into an erythro­cyte bottom layer, an inter­mediate buffy-coat layer of white blood cells, and a top layer of plasma. Erythro­cytes are then acid digested and the magne­sium content deter­mined by atomic absorption spectrometry. Care must be taken to avoid hemol­yzing the blood sample when obtaining the erythro­cytes as it would result in loss of magne­sium to the plasma portion. A reference range for the concen­tration of magne­sium in erythro­cytes of 1.65 to 2.65mmol/L has been reported (Costello & Nielsen, 2017). In depletion-repletion meta­bolic unit studies, a change in magne­sium status deter­mined by expressing erythro­cyte magne­sium as mmol/mg erythro­cyte membrane protein appeared to be better than when expressed per erythro­cyte cells, packed cells, or hemoglobin (Nielsen et al., 2003, 2007a, 2007b). The range in values found in these studies for erythro­cyte membrane magne­sium was 2.26 to 3.25 nmol/mg protein. However, this method of measuring status requires additional steps involving hemol­yzing collected erythro­cytes followed by washing and collecting the erythro­cyte membranes after isolating them from the erythro­cytes (Dodge et al., 1963).

The studies of Dodge and co-workers (Dodge et al., 1963) also deter­mined erythro­cyte ionized magne­sium by hemol­yzing the erythro­cytes to release ionized magne­sium for measurement by an ion selective electrode. However, determining the erythro­cyte ionized magne­sium content or expressing magne­sium per cell or g hemo­globin did not appear to be any better for assessing cellular magne­sium status than determining erythro­cyte magne­sium mmol/L packed cells. Further studies might find measuring red blood cell membrane magne­sium useful in the research setting.

Even though the assessment of body magnesium stores or CLMD may be slightly better using erythrocyte magnesium, the method of measurement is laborious and prone to error, that coupled with lack of a well validated reference range, preclude measurements of erythro­cyte magne­sium from being a useful clinical method for determining magne­sium status. Erythrocyte membrane or packed cell magne­sium determin­ation may be useful in determining changes in magne­sium status in long-term research studies of magne­sium depletion and repletion.

23c.8.4 Magnesium in mono­nuclear and other cell types

Experimental findings from rats showing that changes in magne­sium concen­trations in mono­nuclear cells (lympho­cytes and mono­cytes) reflected those in cardiac and skeletal muscle (Ryan & Ryan, 1979) was a stimulus to examine whether these cells could be used for magne­sium status assess­ment in humans. Between 1980‑2000, magne­sium measurement of mono­nuclear cells was examined as indicators of magne­sium status in a small number of studies involving human diseases associated with magne­sium deficiency. Results from notable studies have been reviewed (Arnaud, 2008). Among the findings were the following: a significant corre­lation between magne­sium concen­trations in lymphocytes and muscle in patients with type 1 diabetes; lower lymphocyte ionized magne­sium concen­trations in migraine patients than in controls; lower lymphocyte total magne­sium content in hyper­tensive patients than in controls; and a decrease in lymphocyte magne­sium content in patients with congestive heart failure experiencing arrhythmias because of digitalis toxicity. Magnesium concen­trations in platelets were also examined in patients with diabetes, obesity, and hyper­tension but were found not to be a more sensitive nor a more specific marker of magne­sium status than other cellular magne­sium measurements (Arnaud, 2008).

There is no reference range for cellular mono­nuclear magne­sium. Only a few studies have indicated the normal concen­tration of magne­sium in mono­nuclear cells. For example, a mean of 2.91±0.6fmol Mg/cell was obtained from 20 apparently healthy adults (Elin & Hosseini, 1985), whereas expressed as μmol/mg protein, mean cellular mono­nuclear magne­sium concen­trations ranged from 0.052 to 0.073μmol/mg protein (Elin & Johnson, 1982; Hosseini et al., 1983; Sjögren et al., 1986).

Although cellular mono­nuclear magne­sium appears to be a better indicator of magne­sium status or stores than serum total or ionized magne­sium, several drawbacks preclude the use of cellular mono­nuclear magne­sium as a routine clinical measure. For example, large blood samples are required for fractionating cellular components. Moreover, the fractionation method used to obtain the specific cell types is difficult, time consuming, and prone to errors. Analysis of magne­sium concen­trations in the specific cell types is usually performed by atomic absorption spectrometry.

23c.8.5 Urinary magne­sium

Urinary magne­sium excre­tion is an excellent indicator of magne­sium intake and is usually meas­ured by atomic absorption spectrometry. Urinary magne­sium excre­tion data obtained from 27 metabolic unit studies have shown that when intakes were 200mg/day or lower, urinary magne­sium excre­tion generally ranged from 40‑80mg/day (1.65‑3.29mmol/day). When dietary intakes were greater than 250mg/day, urinary magne­sium excre­tion ranged from 80‑160mg/day (3.29‑6.58mmol/day) (Nielsen & Johnson, 2017).

Four depletion-repletion experiments involving metabolic unit studies with postmenopausal women found that urinary magne­sium excre­tion was between 40‑80mg/day (1.65‑2.88mmol/day) with dietary magne­sium intakes less than the EAR suggested for the United States and Canada of 175mg/day (Nielsen & Johnson, 2017). With magne­sium intakes above 300mg/day, urinary magne­sium excre­tion ranged from 100‑140mg/day (4.11‑5.76mmol/day) The four depletion-repletion experiments also found that urinary magne­sium excre­tion responded quickly to changes in dietary magne­sium. For example, in one study, women who consumed a mean magne­sium intake of 390mg/day had a mean daily urinary excre­tion of 120‑130mg/day) (4.93‑5.35mmol/day). When the women were switched to a diet supplying a mean magne­sium intake of 120mg/day, mean urinary magne­sium excre­tion decreased to 63mg (2.59mmol) per day within 6 days. A switch from a magne­sium deficient intake to an adequate intake resulted in a similar change in urinary magne­sium excre­tion within 6 days. These findings indicate that a single 24‑hour urine sample or short-term determination of 24‑hour urinary magne­sium excre­tion alone is not useful as an indicator of current magne­sium status at the individual level because urinary magne­sium excre­tion could be low while an individual still has an adequate magne­sium status and vice versa. However, at the population level, urinary magne­sium excre­tion could be an indicator of the amount of inadequate magne­sium status in a population. This suggestion is supported by the findings that low urinary magne­sium excre­tion has been associated with the risk of cardiovascular disease (Joosten et al., 2013a; Yamori et al., 2015) and hyper­tension (Joosten et al., 2013b).

Urinary magne­sium excre­tion can also be useful in magne­sium status assessment when combined with other measures of magne­sium status. As indicated above, using a urinary magne­sium excre­tion of < 80mg/day combined with a dietary magne­sium intake < 250mg/day could increase the likelihood of identifying individuals who are magne­sium-deficient even though serum magne­sium concen­trations are within the low healthy reference range. This suggestion is supported by a recent finding that young women with a mean urinary magne­sium excre­tion of 58.2mg/day, a dietary magne­sium intake of 145mg/day, and a normal serum magne­sium responded to magne­sium supplementation by increasing urinary magne­sium excre­tion (Okamoto et al., 2023).

23c.8.6 Magnesium load test

The magne­sium load test is considered by many to be the best method for determining total body magne­sium status or general intra­cellular magne­sium content. The basis for the magne­sium load test is that the kidney regulates the body stores of magne­sium by excreting excess magne­sium when stores are adequate and retaining magne­sium when stores are deficient. This test deter­mines the per­cent­age of magne­sium retained over a given period time after parenteral admin­istration of a magne­sium load (Elin, 2010; Rude & Shils, 2006). Retention of a greater per­cent­age of magne­sium than that retained by individuals with adequate magne­sium status indicates some depletion of body magne­sium. The test depends upon normal renal function; it cannot be used on individuals with renal diseases or who are taking drugs that affect renal function. The magne­sium load test has been used to identify magne­sium deficiency in elderly individuals, and in conditions of chronic alcoholism, ischemic heart disease, hyper­tension, and Crohn's disease (Waters et al., 2008) where hypomagnesemia was not always found.

To perform this test properly, a basal urinary magne­sium excre­tion is deter­mined. Three con­sec­utive 24‑hour urine samples should be collected to yield the most accurate basal magne­sium excre­tion and counteract the effects of both diurnal and random day-to-day variations in magne­sium excre­tion. Never­the­less, measurement of a single 24‑hour basal urinary magne­sium excre­tion collected just before the magne­sium load is most often used because it has been found to give acceptable results. After the determination of the basal urinary excre­tion, magne­sium is admin­istered parentally instead of orally to eliminate variability that would occur with intestinal absorption. A slow infusion, usually done over a period of 4 to 8 hours, is needed because plasma magne­sium concen­tration deter­mines the renal reabsorption threshold. A sudden increase in plasma magne­sium above the normal level would increase urinary excre­tion and thus reduce magne­sium retention, which could result in an erroneous assessment of status. Following the load, urinary excre­tion of magne­sium is meas­ured over the subsequent 24 to 48‑hour period. Although a 48‑hour urine collection gives a slightly more accurate measure of magne­sium retention, 24‑hour urine collections have been found to correlate well with 48‑hour urine collections (Gullestad et al., 1994) so are often used. Urine specimens should be collected with an acidifying agent to prevent precipitation of magne­sium compounds at high pH. The net retention of magne­sium is calculated by comparing basal magne­sium excre­tion with net magne­sium excre­tion after the load.

A standard protocol for admin­istering the magne­sium load has not been estab­lished. Examples of protocols reported include a 30mmol (720mg) of magne­sium sulfate in 1000ml of isotonic saline for 8 hours at a rate of 125ml/hour (Gullestad et al., 1994); an intra­venous infusion of 30 mmol (720mg) of magne­sium in 500ml 5% dextrose over a period of 8‑12 hours (Ismail et al., 2012) and an intra­venous infusion of 0.2mmol/kg body weight (340mg for a 70kg individual) of magne­sium sulfate in 250ml of 5% dextrose over 4 hours (Ozono et al., 1995). The percentage magnesium retention is calculated as:

[(Amount of Mg infused)
    − (post-infusion urinary Mg)
       − (baseline urinary Mg)] x 100%
divided by
    (Amount of Mg infused)

(DiNicolantonio et al., 2018). The infusion protocols outlined above gave similar results in terms of suggesting the magne­sium retention indicating magne­sium adequacy and deficiency. A retention reference range providing the values indicating deficiency and adequacy has not been officially estab­lished. However, it has been suggested that a retention of < 20% indicates adequate magne­sium status; retention between > 20% to > 50% indicates borderline deficient magne­sium status; and retention > 50% indicates magne­sium deficiency (Costello & Nielsen, 2017). The borderline deficiency values, which would indicate CLMD, are apparently appropriate because individuals with values within this range reportedly had chronic diseases associated with a low magne­sium status but did not exhibit hypomagnesemia (i.e., low serum magne­sium concen­trations).

The magne­sium load test is invasive, time-consuming, and cumbersome; it requires close supervision for at least 24 hours after magne­sium infusion to ensure oversight of both the total amount of magne­sium infused and the urine collection. These drawbacks prevent the magne­sium load test being a routine clinical measure of magne­sium status. It has been used most often as a research tool to deter­mine whether individuals with chronic disease are magne­sium deficient.

23c.8.7 Muscle magne­sium

Because muscle contains about 30% of the total body magne­sium, muscle magne­sium has been evaluated as a possible indicator of magne­sium status. Very few studies have found a corre­lation between concen­trations of magne­sium in muscle and serum or erythro­cytes (Elin, 1991). However, some significant corre­lations with mono­nuclear blood cells have been found in some disease states such as type 1 diabetes (Sjögren et al., 1986) and mild hyper­tension (Dyckner & Wester, 1985), which suggests that muscle magne­sium may be a reliable indicator of intra­cellular magne­sium status. This suggestion is supported by a metabolic unit study with postmenopausal women in which magne­sium depletion by dietary means significantly decreased muscle magne­sium, whereas repletion restored muscle magne­sium levels (Lukaski & Nielsen, 2002).

In the magne­sium depletion-repletion metabolic study outlined above, 40mg of muscle tissue was obtained by needle biopsy. After adipose and connective tissue were dissected from each specimen, the specimen was rinsed with distilled-deionized water and dried and digested with nitric acid. Magnesium was deter­mined by inductively coupled plasma emission spectrometry. Muscle magne­sium concen­trations were 53.4, 48.1, and 51.6mmol/kg dry weight during the control, depletion, and repletion periods, respectively. However, the procedure to obtain muscle samples is extremely invasive and requires skill to obtain a sample with minimal fat and without causing a hematoma.

Proton nuclear magnetic resonance spectroscopy has also been used to measure ionized magne­sium in skeletal muscle (Reyngoudt et al., 2019). However, this method of measurement is very expensive and requires the use of complex instruments and sophisticated software, which precludes its use in specialized laboratories only. Because other methods, such as the magne­sium load test, and erythro­cyte membrane and mono­nuclear cell magne­sium determinations give status information of similar validity, using muscle magne­sium to assess status in either the clinical or research setting is not useful.

23c.8.8 Buccal call magne­sium

intra­cellular magne­sium in sublingual epithelial cells was found to correlate with magne­sium in atrial cell biopsies from open heart surgery patients (Haigney et al., 1995). Hence, the measurement of magne­sium in sublingual epithelial cells has been suggested as a method that can assess CLMD (Silver, 2004). The cells are collected by gentle scratching of sublingual tissue and then fixed on a carbon slide with a cytology fixative. The magne­sium in the fixed cells is deter­mined by using energy-dispersive X-ray analysis. However, the test of magne­sium in sublingual epithelial cells as a measure of magne­sium status needs to be validated by using another test such as the magne­sium-load test. Furthermore, the method is expensive and requires skill in collecting suitable sublingual epithelial cell samples for analysis, which has limited its use mainly to research laboratories.

23c.8.9 Magnesium status assessment methods of limited use

Magnesium balance (intake minus loss) is the principal measure used to deter­mine dietary magne­sium require­ments. Most balance studies have been performed in clinical research centers. The method requires an accurate determination of magne­sium from dietary intake and from the complete collection of urine and stool over at least 12 days. It also should include the collection and measurement of sweat, dermal, and menstrual losses of magne­sium, although generally these losses have not been meas­ured in reported balance studies. A negative balance (i.e., magne­sium losses exceed magne­sium intake) indicates a magne­sium deficiency, and a positive balance (i.e., magne­sium intake exceeds magne­sium losses) indicates adequacy. Most reported balance studies prior to 1990 did not have well-controlled dietary intakes of magne­sium, used short periods of data collection, and/or used less accurate methods to measure magne­sium. A poorly controlled magne­sium balance study was used to set the DRIs for magne­sium in the United States and Canada (IOM, 1997). The most valid balance studies have involved individuals in a controlled metabolic unit environment who have been provided with a controlled and consistent diet (Hunt & Johnson, 2006; Nielsen, 2016). Such studies have indicated an EAR of 175mg/day for healthy 70kg individuals.

Fractional excre­tion of magne­sium (FEMg) distinguishes between gastrointestinal and renal magne­sium loss. This method has received some brief mentions as a measure of magne­sium status (Ayuk & Gittoes, 2014; Costello & Nielsen, 2017). The following formula is used to deter­mine this measure (Ayuk & Gittoes, 2014):

    FEMg= [(uMg × sCr)/(sMg × uCr × 0.7)] × 100%

where uMg = urinary Mg concen­tration; sMg = serum Mg concen­tration; sCr= serum creatinine; uCr = urine creatinine

Fractional excre­tion of magne­sium (FEMg) greater than 2% is considered an indication of renal wasting which implicates extra-renal magne­sium losses, decreased intake, or decreased absorption of magne­sium. As this method of assessment requires the determination of three variables, it provides no advantage to assess magne­sium status over the use of serum magne­sium, ionized magne­sium, or the magne­sium load test and hence has received limited application as a measure of magne­sium status.

Magnesium depletion score (MDS) is an aggregate of several risk factors affecting the absorption and excre­tion of magne­sium (Fan et al., 2021; Wang et al., 2022). The MDS is used to identify individuals with abnormal magne­sium absorption and/or excre­tion that may result in a deficient magne­sium status. The MDS is calculated by aggregating the factors:

1. Current use of diuretics counted as 1 point;
2. Current use of proton pump inhibitor counted as 1 point;
3. Heavy drinker (defined as > 1 drink/day for women and > 2 drinks/day for men) counted as 1 point;
4. Mildly decreased kidney function defined as estimated glomerular filtration rate (eGFR) 60mL/(min × 1.73m²) < eGFR 90ml/min × 1.73m² counted as 1 point;
5. Chronic kidney disease defined as eGFR < 60ml/min × 1.73m² counted as 2 points.

A score > 2 has been used to indicate magne­sium deficiency associated with increased risk for systemic inflammation and cardiovascular mortality in adults (Fan et al., 2021). A score of > 3 combined with a dietary magne­sium intake below the US RDA has been used to indicate magne­sium deficiency associated with osteo­porosis (Wang et al., 2022). This suggests that sample size or disease entity may influence the cut-point indicative of magne­sium deficiency. This new suggested method of magne­sium status assessment, especially for individuals with diseases associated with magne­sium deficiency, needs further evaluation and validation before being accepted for general use.