Marathon running: physiological and chemical changes accompanying late-race functional deterioration

Summary

Twenty-one experienced runners were studied before, during and immediately after a marathon race to ascertain whether either depletion of energy substrate or rise in body temperature, or both, contribute to laterace slowing of running pace. Seven runners drank a glucose/electrolyte (GE) solution ad libitum (Na⁻ 21 mmol 1⁻¹, K⁺ 2.5 mmol l⁻¹, Cl⁻ 17 mmol l − l, PO₄ ²⁻ 6 mmol 1⁻¹, glucose 28 mmol 1⁻¹) throughout the race; 6 drank water and 8 drank the GE solution diluted 1:1 with water. Although average running speeds for the three groups were not significantly different during the first two-thirds (29 km) of the race, rectal temperature was significantly higher (P < 0.05) and reduction of plasma volume was greater (P <0.05) in runners who replaced sweat losses with water. During the last one-third of the race, the average running pace of the water-replacement group slowed by 37.2%; the pace slowed by 27.9% in the 8 runners who replaced their sweat loss with GE diluted 1:1 with water (1/2 GE) and 18.2% in runners who replaced fluid loss with full-strength solution (GE). Eleven runners (5 in the water group, 4 in the 1/2 GE group and 2 in the GE group) lapsed into a walk/run/walk pace during the last 6 miles of the race. Ten of these had a rectal temperature of 39° C or greater after 29 km of running, and plasma volume in these runners was reduced by more than 10%. Only 1 runner among those who ran steadily throughout the race had such an elevation of temperature and reduction of plasma volume. A significant reduction in plasma glucose concentration (less than 3.3 mmol 1⁻¹) was present in 5 of the 11 walk/run/walk subjects and in none of those who ran steadily. The results are consistent with the suggestion that assumption of the walk/run/walk pace resulted from high body temperature associated with either diminished plasma volume or low blood sugar.