Total body water (TBW) was determined on 35 subjects with a tritium (HTO) and an ethanol (ETH) dilution method, the latter using breath analyses for blood ethanol content. Lean body mass (LBM) was estimated by hydrostatic weighing. Mean values for water fraction (TBW/wt) were 0.618 +/- 0.05 with HTO and 0.603 +/- 0.06 with ETH. The difference was not significant. The correlation between the two methods was highly significant (r = 0.90, SEE = 3.5 liters, P less than 0.0001). High correlations were also found between TBW and LBM with either method for TBW (r = 0.94 with HTO, r = 0.91 with ETH). Mean values for TBW/LBM were 0.735 with HTO and 0.717 with ETH. The ETH method compares favorably with the HTO, it has the advantage that it is nonradioactive and can be repeated daily if necessary.
Circulatory fluid shifts were studied in middle-aged runners (6 males and 5 females, ages 32-58 yr) during a 42.2-km marathon race run in mild weather (dry-bulb temperature = 17.5-20.4 degrees C). Running times for the subjects were 3:12-4:40 (mean values were 3:34 for males and 4:10 for females). Venous blood samples were taken without stasis in all subjects seated at rest before the start of the race and within 3 min of finishing; eight of the subjects also paused for samples at 6 and 27 km during the race. At 6 km, body weight loss averaged less than 1%, whereas plasma volume (PV) had decreased by 6.5% in male subjects and 8.6% in female subjects. By the end of the race, hypohydration had reached 3.2% in male subjects and 2.9% in female subjects, but PV in both groups remained stable. Sweat rates during the race averaged 545 and 429 g X m-2 X h-1 for male and female subjects, respectively, with ad lib. water intake replacing 21-72% of fluid loss. Increases in plasma protein concentration throughout the race reflected the observed initial decrease in PV. The interpretation of PV responses to exercise and/or hypohydration is critically dependent on selection of base-line conditions; we were able to control for posture-exercise effects by treating the early exercise (6 km) sample as the base line for examining the effects of later fluid loss. Under these conditions, the vascular compartment resisted volume depletion. The ability to maintain stable PV can be explained in part by relationships among oncotic and hydrostatic pressures in the intra- and extravascular fluid compartments.
Plasma volume (PV), blood-borne substrate, and electrolyte responses to a warm weather (WBGT 15.5-24.5 degrees 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 (Tre) 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 VO2 max. Near steady state levels for HR and Tre, ranging from 140-165 beats . min-1 and from 38.5-39.3 degrees 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 Tre. 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 less than 4%.
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