Effects of hypohydration on thermoregulation during exercise before and after 5-day aerobic training in a warm environment in young men

Ikegawa S, Kamijo Y, Okazaki K, Masuki S, Okada Y, Nose H. Effects of hypohydration on thermoregulation during exercise before and after 5-day aerobic training in a warm environment in young men. J Appl Physiol 110: 972-980, 2011. First published February 10, 2011; doi:10.1152/japplphysiol.01193.2010.-We examined whether enhanced cardiovascular and thermoregulatory responses during exercise after short-term aerobic training in a warm environment were reversed when plasma volume (PV) expansion was reversed by acute isotonic hypohydration. Seven young men performed aerobic training at the 70% peak oxygen consumption rate (V-O2peak) at 30 degrees C atmospheric temperature and 50% relative humidity, 30 min/day for 5 days. Before and after training, we performed the thermoregulatory response test while measuring esophageal temperature (T-es), forearm skin vascular conductance, sweat rate (SR), and PV during 30 min exercise at the metabolic rate equivalent to pretraining 65% V-O2peak in euhydration under the same environment as during training in four trials (euhydration and hypohydration, respectively). Hypohydration targeting 3% body mass was attained by combined treatment with low-salt meals to subjects from similar to 48 h before the test and administration of a diuretic similar to 4 h before the test. After training, the T-es thresholds for cutaneous vasodilation and sweating decreased by 0.3 and 0.2 degrees C (P = 0.008 and 0.012, respectively) when PV increased by similar to 10%. When PV before and after training was reduced to a similar level, similar to 10% reduction from that in euhydration before training, the training-induced reduction in the threshold for cutaneous vasodilation increased to a level similar to hypohydration before training (P = 0.093) while that for sweating remained significantly lower than that before training (P = 0.004). Thus the enhanced cutaneous vasodilation response after aerobic training in a warm environment was reversed when PV expansion was reversed while the enhanced SR response remained partially.