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 less than 0.05 or less) immediately after the marathon, from 2.13 +/- 0.16 to 2.02 +/- 0.18 mmol.l-1 cells and from 0.88 +/- 0.06 to 0.81 +/- 0.07 mmol.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 less than 0.001) from 29 +/- 13 to 5 +/- 3 mumol.min-1 during the marathon and increased (p less than 0.05) 12 h after the race to 38 +/- 18 mumol.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.