Fitness alters fluid regulatory but not behavioural responses to hypohydrated exercise

Dehydration is typical during prolonged exercise. Because training stimulates numerous adaptations, some involving fluid regulation, it is conceivable that training involves adaptations to dehydration. This study tested the hypothesis that trained individuals have altered fluid regulatory, but not behavioural or perceptual responses to exercise when hypohydrated. Six trained (V.O2 peak: 65 ± 8 mL kg− 1 min− 1) and six untrained (V.O2 peak: 45 ± 4 mL kg−1 min−1) males cycled for 40 min at 70%V.O2 peak, once whilst euhydrated (EUH) and once whilst hypohydrated by ~ 2% body mass (HYPO), before a 40-min performance trial with euhydration (in EUH) or ad libitum drinking (in HYPO), in temperate conditions (24.3 °C, 50% rh). Baseline hydration was achieved by complete or partial rehydration from exercise + heat stress on the previous evening. Body mass was reduced (− 1.8 ± 0.1%) and plasma osmolality was increased (5 ± 1 mosmol kg− 1) similarly between fitness groups in HYPO compared to EUH (P < 0.05). During exercise, plasma [AVP] rose more in HYPO than EUH; the elevation was greater in the Untrained (4.1 ± 1.7 vs. 2.0 ± 0.8 pmol L− 1, P < 0.01) than Trained (1.4 ± 0.6 vs. 1.1 ± 0.5 pmol L− 1, P < 0.01; P = 0.02). Increases in plasma [AVP] relative to osmolality were higher in Untrained than Trained (0.47 ± 0.06 vs. 0.025 ± 0.05 pmol mosmol− 1, P = 0.03). Fitness groups had equivalent thirst ratings during fixed exercise but Trained were thirstier than Untrained when self regulating in HYPO (4.0 ± 1.5 vs. 2.7 ± 1.2; P = 0.05); thus Trained tended to consume more fluid (1.20 ± 0.16 vs. 0.88 ± 0.16 L; P = 0.19), but maintained similar hypohydration consistent with their greater sweat rate during HYPO. In conclusion, aerobic fitness attenuates the neuroendocrine ([AVP]) response to hypohydrated exercise, but not perceptual (thirst) or behavioural (ad libitum drinking) responses.