ERYTHROCYTE, PLASMA AND URINARY MAGNESIUM IN MEN BEFORE AND AFTER A MARATHON

Erythrocyte, plasma and urinary magnesium (Mg2+) concentration was measured in 23 runners before and after a marathon race. Blood samples were drawn from an antecubital vein the morning before the race (baseline), at 3 p.m. (2 h before the start), upon finishing and 12 h later. Compared with the baseline values, the intra-erythrocyte and plasma Mg2+ were decreased (p < 0.05 or less) immediately after the marathon, from 2.13 .+-. 0.16 to 2.02 .+-. 0.18 mmol .cntdot. l-1 cells and from 0.88 .+-. 0.06 to 0.81 .+-. 0.07 mmol .cntdot. l-1 respectively. The Mg2+ concentration returned to pre-race values 12 h after completion of the marathon. The urinary Mg2+ excretion rate decreased (p < 0.001) from 29 .+-. 13 to 5 .+-. 3 .mu.mol .cntdot. min-1 during the marathon and increased (p < 0.05) 12 h after the race to 38 .+-. 18 .mu.mol .cntdot. min-1. It is concluded that the reduction in plasma Mg2+ ion concentration during the marathon cannot be attributed to erythrocyte uptake, urinary excretion or loss in sweat. It is suggested that Mg2+ may be released from erythrocytes into the extracellular fluids during sustained exercise and taken up from these fluids by the adipose cells.