Twenty young men completed three exercise tests on a treadmill: H1 (walk at 60% VO2peak until exhaustion at 42 掳C), N (walk at 22 掳C; duration equal to H1) and H2 (walk until exhaustion at 42 掳C after a 10-day heat acclimation program). Core temperature (Tc) was recorded continuously and venous blood samples were taken before, during and after each exercise test. Exercise-induced sweat production was calculated on the basis of body mass change taking into account water intake and the volume of blood samples drawn.
Lower pre-exercise Tc, lower rate of rise in Tc during exercise, and prolonged time to exhaustion in H2 compared with H1 revealed that the subjects successfully achieved an acclimated state. Overall, serum GH level was higher in H1 compared with both N and H2 (p < 0.001) but did not differ between the two latter trials (p > 0.05). Tc correlated with serum GH concentration (r = 0.615, p < 0.01). Analysis of the individual data revealed a group (n = 9) possessing a threshold-like pattern of the relationship between Tc and blood GH response, whereas a plateau-like pattern was evident in the rest of the subjects (n = 11). Both sweat production (r = 0.596; p < 0.001) and the rate of sweat production (r = 0.457; p < 0.001) correlated with the growth hormone area under the curve.
Heat acclimation decreases the GH response to moderate intensity exhausting exercise in the heat. GH may have a modest stimulating effect on whole-body sweat production during exercise.