Because of the deleterious effects observed in case of hypomagnesemia related to digestive illnesses, it was thought that magnesium was a magical supplement with multiple properties: mild laxative, stimulant, muscular cramp soother and intellectual and physical enhancer at pharmacological doses. What is myth and what is reality?
Found in all tissues of plants and animals, magnesium fulfils many functions. It acts as cofactor for numerous enzymes involved in energy metabolism (ATP synthesis, namely muscular, and glycolysis) and in protein synthesis.
It is involved in cytoskeleton protein contraction, neuromuscular transmission and calcium and potassium homoeostasis.
As part of chlorophyll, it plays an important role in plant photosynthesis.

Magnesium is mostly absorbed through the ileum and the colon. In case of deficiency, absorption through the jejunum is possible. Vitamin D favours its absorption. Its absorption coefficient is weak (40 to 60%) and varies with individuals and conditions (increased with deficiency, decreased with excess). Its properties are about the same from one salt to another, except as magnesium hydroxide, which is not very absorbable. Magnesium excretion is 125 to 425 mg/day in adults (5 to 17 mmol/day). Fifty to 80% is excreted in stools, the remainder in urine. Magnesium homoeostasis is done mainly through kidneys: it is reabsorbed in the ascending limb of the loop of Henlé and the early distal tubule. Many factors, including hormones, influence this reabsorption, serum magnesium concentration being the most important one. In case of chronic magnesium deficiency renal losses decrease to 125-150 mg/day (5 to 6 mmol/day) and digestive absorption doubles.

According to experts, because of its low digestive absorption, magnesium recommended daily intake (RDI) should be 300 to 400 mg. Healthy subjects rarely have an intake lower than 250 mg/day (Figure 1).
Mean intake is near or superior to the RDI. There is little or no deficiency in children. Five to 15% of teenagers and adults have intakes lower than recommended.
Systematic magnesium fortification of certain foods such as cereals does not bring unanimity, since magnesium is found in most plant foods, especially in seeds such as almonds, walnut, hazelnuts and cocoa (150 to 250 mg/100g). In a daily diet, fish, legumes, cheese and highly mineralised water are the main sources of magnesium (Table 1)
Table 2 shows RDIs (mg/day) in calcium, phosphorus and magnesium, three essential minerals entering bone composition, also needed for neuromuscular function.

Magnesium deficiency
Several situations may lead to magnesium deficiency:
· Chronic food restriction, as magnesium intake is closely bound to caloric intake.

D'après Potier de Courcy G., Dossier conjoint IFN / ANIA / Alliance 7
Actes des colloques scientifiques, IFN, Octobre 1998

Magnesium is an essential electrolyte. Its wide presence in foods along with the digestive tract's and the kidneys' homeostatic mechanisms make magnesium deficiency barely impossible. Only the synergetic effects of a lower caloric intake along with an increase in magnesium losses can lead to an overt deficiency, such as with chronic intestinal diseases.
To this day, there is no scientific evidence for magnesium supplementation in cases of asthenia, irritability and unspecific functional problems.
 
Dr Isabelle ROYER
Internal Medicine Service
Avicenne Hospital - Bobigny

Dupin H., Abraham J., Giachetti I. In " Apports nutritionnels conseillés.
" Tech. Doc. Lavoisier (Paris) 1992 : 37-38.

Mc Lean RM. Magnesium and its therapeutic uses : A review. Am. J. Med. 1994 ;96 : 63-76.

Schaafsma G. Bioavailability of calcium and magnesium. Eur. J. Clin. Nutr. 1997; 51 (suppl 1) : 13S-16S.