Magnesium absorption has been studied in both humans and animals under diverse experimental conditions. As a result, the data often appear confusing and conflicting. In this review we attempt to summarize information concerning Mg absorption and, where possible, to reconcile apparently conflicting observations. Most studies suggest that Mg is absorbed predominantly in the distal intestine. At usual Mg intakes, Mg absorption occurs primarily by intercellular diffusional and solvent drag mechanisms. There is evidence for a saturable component of Mg absorption in the small intestine and the descending colon that is important at low dietary Mg intakes. Pharmacological doses of vitamin D increase Mg absorption in both vitamin D-deficient and vitamin D-replete animals. A substantial amount of Mg absorption, however, occurs independent of vitamin D. In addition, vitamin D may reduce Mg retention through increases in urinary Mg excretion. Intestinal interactions between Mg and calcium or phosphate have been demonstrated in both humans and animals. The nature of these interactions cannot be readily explained by data currently available.

Transport characteristics of Ca and Mg were compared at three different levels of the absorptive process in male (120-170 g) Wistar rats. Balance studies in intact rats revealed that fractional Ca absorption decreased with increased Ca intake so that net Ca absorption remained constant. Fractional Mg absorption decreased modestly with increased Mg intake so that net Mg absorption increased proportionately with increased dietary Mg. Everted duodenal sacs demonstrated the presence of active Ca absorption with serosal-to-mucosal (S/M) ratio of 2.65 +/- 0.20 (n = 6), which was greater than unity (P less than 0.001). In contrast, the S/M for Mg did not exceed unity. Mucosal duodenal Ca uptake exhibited a large saturable (Michaelis constant of 4.80 +/- 0.34 mM, maximal velocity of 4.71 +/- 0.13 nmol.min-1.mg-1) and a small nonsaturable component (0.12 +/- 0.01 nmol.min-1.mg-1) in 5-wk-old rats (120 g). In 72-wk-old rats (600 g) the diffusional component of Ca uptake became predominant, and the slope increased significantly to 0.32 +/- 0.01 (P less than 0.05). Duodenal Mg uptake was completely concentration dependent and exhibited no age-related changes.

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