Effect of physical activity on the metabolism of magnesium in the rat.

The influence of exercise and immobilization on magnesium (Mg) balance, as influenced by vitamin D, was evaluated in female Sprague-Dawley rats that were divided into four groups: exercised fed ad libitum, pair-fed exercised, immobilized, and control. After 9 weeks, Mg absorption and retention were higher in the exercised groups than in the control group; however, immobilization resulted in an increase in the urinary excretion of Mg and a negative Mg balance. Serum levels of 1,25-dihydroxyvitamin D [1,25(OH)2D] were higher in the exercised group than in the control group. The tibial content of Mg was higher in both exercise groups and lower in the immobilized than in the control group. In an additional experiment to evaluate the Mg balance during early adaptation to exercise training, no change was seen in Mg absorption and balance during the first 5 weeks of exercise, although serum levels of 1,25(OH)2D increased significantly. Immobilization resulted in a decrease in Mg retention without any effect on intestinal absorption. Urinary Mg was consistently higher in the exercised and immobilized groups than in the control group. Prolonged exercise training, after adaptation, increases and longer immobilization decreases intestinal absorption and retention of Mg. The influence of physical activity on Mg absorption is not necessarily mediated by vitamin D. Physical activity may influence mineral absorption by a mechanism associated with growth.