Changes in serum electrolyte levels during marathon running.

Serum electrolytes were measured in 18 well-trained, experienced long-distance runners before and after a standard marathon run (42 km), during which they ingested no electrolytes. Their sweat losses and water deficits after completion of the marathon were also measured. In 12 of the subjects, the percentage change in plasma volume and in total circulating plasma electrolytes was determined. There was a highly significant fall in serum magnesium concentration, with an increase in both potassium and sodium levels. Changes in total circulating plasma sodium and chloride were closely correlated with alterations in plasma volume. On the basis of these observations, it is recommended that athletes drink an augmented volume of fluid during marathon running, irrespective of the prevailing weather conditions. Supplementation of potassium and magnesium is contraindicated during long-distance running. Salt intake is unnecessary during races over the standard marathon distance. Subjective evidence for glucose supplementation is presented.