Effects of aerobic fitness on hypohydration-induced physiological strain and exercise impairment

Aim: Hypohydration exacerbates cardiovascular and thermal strain and can impair exercise capacity in temperate and warm conditions. Yet, athletes often dehydrate in exercise, are hypervolaemic and have less cardiovascular sensitivity to acute hypervolaemia. We tested the hypothesis that trained individuals have less cardiovascular, thermoregulatory and performance affect of hypohydration during exercise. Methods: After familiarization, six trained [O_{2 peak} = 64 (SD 8) mL kg⁻¹ min⁻¹] and six untrained [O_{2 peak} = 45 (4) mL kg⁻¹ min⁻¹] males cycled 40 min at 70%O_{2 peak} while euhydrated or hypohydrated by 1.5–2.0% body mass (crossover design), before a 40-min work trial with euhydration or ad libitum drinking (in Hypohydration trial), in temperate conditions (24.3 °C, RH 50%, v_a = 4.5 m s⁻¹). Baseline hydration was by complete or partial rehydration from exercise+heat stress the previous evening. Results: During constant workload, heart rate and its drift were increased in Hypohydration compared with Euhydration for Untrained [drift: 33 (11) vs. 24 beats min⁻¹ h⁻¹ (10), 95% CI 5–11] but not Trained [14 (3) vs. 13 beats min⁻¹ h⁻¹ (3), CI −2 to 3; P = 0.01 vs. Untrained]. Similarly, rectal temperature drift was faster in Hypohydration for Untrained only [by 0.57 °C h⁻¹ (0.25); P = 0.03 vs. Trained], concomitant with their reduced sweat rate (P = 0.05) and its relation to plasma osmolality (P = 0.03). Performance power tended to be reduced for Untrained (−13%, CI −35 to 2) and Trained (−7%, CI: −16 to 1), without an effect of fitness (P = 0.38). Conclusion: Mild hypohydration exacerbated cardiovascular and thermoregulatory strain and tended to impair endurance performance, but aerobic fitness attenuated the physiological effects.