Erythrocyte,plasma and urinary magnesium in men before and after a marathon

Summary 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 · 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<0.001) from 29±13 to 5±3 mol · min^–1 during the marathon and increased (p<0.05) 12 h after the race to 38±18 mol · 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.