Accumulation of 2H2O in plasma and eccrine sweat during exercise-heat stress

The purpose of this research was to characterize the movement of ingested water through body fluids, during exercise-heat stress. Deuterium oxide (2H2O) accumulation in plasma and eccrine sweat was measured at two sites (back and forehead). The exercise of 14 males was controlled via cycle ergometry in a warm environment (60 min; 28.7°C, 51%rh). Subjects consumed 2H2O (0.15 mg kg−1, 99.9% purity) mixed in flavored, non-caloric, colored water before exercise, then consumed 3.0 ml kg−1 containing no 2H2O every 15 min during exercise. We hypothesized that water transit from mouth to skin would occur before 15 min. 2H2O appeared rapidly in both plasma and sweat (P < 0.05), within 10 min of water consumption. The ratio 2H2O/H2O (D:H) was 47.3–55.0 times greater in plasma than in back sweat at minutes 10, 20, and 30 (ΔD:H relative to baseline). At elapsed minute 20, the mean rate of deuterium accumulation (ΔD:H min−1) in plasma was 14.9 and 23.7 times greater than in forehead and back sweat samples, respectively. Mean (±SE) whole-body sweat rate was 1.04 ± 0.05 L h−1 and subjects with the greatest whole-body sweat rate exhibited the greatest peak deuterium enrichment in sweat (r2 = 0.87, exponential function); the peak 2H2O enrichment in sweat was not proportional (P > 0.05) to body mass, volume of the deuterium dose, or total volume of fluid consumed. These findings clarify the time course of fluid movement from mouth to eccrine sweat glands, and demonstrate considerable differences of 2H2O enrichment in plasma versus sweat.