Changes in plasma, erythrocyte, and urinary magnesium with prolonged swimming exercise

Erythrocyte, plasma and urinary magnesium were observed in a group of 8 well-trained swimmers and in a group of 10 untrained subjects before, 2 minutes after and 30 minutes after a swimming test. After effort, plasma Mg (pMg) decreased significantly in both groups. Erythrocyte and urinary Mg variations were not significant in this study. PMg decrease after effort was more important in the swimmers group than in the untrained group (12% versus 6% after 2 minutes; 21% versus 4% after 30 minutes). The observed fall in plasma Mg concentration after exercise cannot be explained by a shift into erythrocytes during exercise, since the Mg content of these blood cells did not suffer significant alterations. The possible causes of the observed phenomena are discussed.