Plasma volume and blood metabolites in middle-aged runners during a warm-weather marathon

Summary

Plasma volume (PV), blood-borne substrate, and electrolyte responses to a warm weather (WBGT 15.5–24.5‡ C) marathon race were observed in four male and two female runners averaging 48 years of age. Additionally, continuous recordings were made of heart rate (HR) in three, and of rectal temperature (T_re) in two of the men. The race was finished by three of the men, with an average time of 3.61 h, a running pace estimated to require from 60–66% of the runner's $\dot V_{O_2 }$ max. Near steady state levels for HR and T_re, ranging from 140–165 beats·min⁻¹ and from 38.5–39.3‡ C, respectively, were reached early in the race. Total and percentage weight losses for the finishers were 2.52 kg and 3.9%, 2.18 kg and 3.4%, 4.77 kg and 6.7%, respectively; corresponding reductions in PV for these runners were 5.4%, 13.2%, and 27.4%. Pre-race control and immediate post-race serum glucose concentrations averaged 109 and 154 mg·dl⁻¹ respectively. Final blood lactate values ranged from 11–42 mg·dl⁻¹. Thus, it may be concluded that middle-aged runners who were successful in completing the marathon in a warm environment did so while maintaining steady state levels of HR and T_re. Although only water was ingested during the race, serum glucose for the finishing runners remained above, while serum [Na⁺] and [Cl⁻] remained within ±8% of, control values throughout the run. Complete data obtained from only one of the runners suggest that the initial exercise-induced reduction in PV is not augmented by subsequent cumulative dehydration provided water intake during the race is sufficient to limit weight loss to <4%.