Effect of age and gender on sweat lactate and ammonia concentrations during exercise in the heat
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abstract
The dependence of sweat composition and acidity on sweating rate (SR) suggests that the lower SR in children compared to adults may be accompanied by a higher level of sweat lactate (Lac-) and ammonia (NH3) and a lower sweat pH. Four groups (15 girls, 18 boys, 8 women, 8 men) cycled in the heat (42 degrees C, 20% relative humidity) at 50% VO2max for two 20-min bouts with a 10-min rest before bout 1 and between bouts. Sweat was collected into plastic bags attached to the subject's lower back. During bout 1, sweat from girls and boys had higher Lac- concentrations (23.6 +/- 1.2 and 21.2 +/- 1.7 mM; P < 0.05) than sweat from women and men (18.2 +/- 1.9 and 14.8 +/- 1.6 mM, respectively), but Lac- was weakly associated with SR (P > 0.05; r = -0.27). Sweat Lac- concentration dropped during exercise bout 2, reaching similar levels among all groups (overall mean = 13.7 +/- 0.4 mM). Children had a higher sweat NH3 than adults during bout 1 (girls = 4.2 +/- 0.4, boys = 4.6 +/- 0.6, women = 2.7 +/- 0.2, and men = 3.0 +/- 0.2 mM; P < 0.05). This difference persisted through bout 2 only in females. On average, children's sweat pH was lower than that of adults (mean +/- SEM, girls = 5.4 +/- 0.2, boys = 5.0 +/- 0.1, women = 6.2 +/- 0.5, and men = 6.2 +/- 0.4 for bout 1, and girls = 5.4 +/- 0.2, boys = 6.5 +/- 0.5, women = 5.2 +/- 0.2, and men = 6.9 +/- 0.4 for bout 2). This may have favored NH3 transport from plasma to sweat as accounted for by a significant correlation between sweat NH3 and H+ (r = 0.56). Blood pH increased from rest (mean +/- SEM; 7.3 +/- 0.02) to the end of exercise (7.4 +/- 0.01) without differences among groups. These results, however, are representative of sweat induced by moderate exercise in the absence of acidosis.
