Magnesium is an important macromineral (here) required in gram amounts per day in the adult human diet. Magnesium plays a role as a structural component of bones, and low intakes of magnesium cause the formation of larger more perfectly formed hydroxyapatite crystals, and this increases the brittleness of bone considerably. Magnesium also aids bone formation because it acts as a buffer to acid in the plasma caused by consumption of the typical Western low quality diet. Further, magnesium is required as a cofactor in around 300 proteins including 1,25 dihydroxyvitamin D and parathyroid hormone and this further expands its role as an important factor in bone health. The magnesium requirements of individuals varies depending on genetic and environmental factors, but these have been less well studied than the requirements for calcium. Generally it is thought that the magnesium intakes provided by the typical Western diet are too low, and these intakes are compromised further by high sodium intakes.
Because magnesium requirements are highest during periods of rapid skeletal growth, studies have investigated magnesium metabolism during adolescence. For example, in one study researchers studied the differences in magnesium metabolism between white and black girls aged 11 to 15 years1. For 3 weeks the subjects consumed a low sodium diet containing 1.3 gram per day sodium, or a high sodium diet containing 3.8 grams per day sodium. During consumption of both diets the magnesium intake was fixed at 230 mg per day but calcium intake varied between 800, 1300 or 1800 mg per day. The results showed that the sodium intake and the calcium intake had no significant effects on the magnesium excretion rates. However, black girls excreted significantly less magnesium compared to white girls. In addition, the low sodium diet caused significantly higher retention of magnesium compared to the higher sodium diets. Plasma concentrations of 25-hydroxyvitamin D were significantly higher in the white girls compared to black girls.
The differences in magnesium metabolism in this study may explain the greater bone densities found in blacks compared to whites. Previous studies have shown that blacks absorb and retain calcium to a greater extent that whites and the results from this study suggest that they may also excrete less magnesium. As magnesium is required for the formation of smaller and more irregular hydroxyapatite crystals, the higher magnesium retention may explain the greater bone densities in blacks. In addition these results confirm that plasma concentrations of the vitamin D metabolite 25-hydroxyvitamin D are higher in whites compared to blacks, and this might relate to the more efficient production of subcutaneous vitamin D production in whites as had been reported previously. The ability of the high sodium diet to decrease magnesium retention suggests that the low intakes of magnesium in the high salt Western diet may be problematic for skeletal health, and this may be exacerbated by the high plasma pH associated with lower magnesium intakes.
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