Effect of hydration status on thirst, drinking, and related hormonal responses during low-intensity exercise in the heat

During exercise-heat stress, ad libitum drinking frequently fails to match sweat output, resulting in deleterious changes in hormonal, circulatory, thermoregulatory, and psychological status. This condition, known as voluntary dehydration, is largely based on perceived thirst. To examine the role of preexercise dehydration on thirst and drinking during exercise-heat stress, 10 healthy men (21 ± 1 yr, 57 ± 1 ml·kg −1 ·min −1 maximal aerobic power) performed four randomized walking trials (90 min, 5.6 km/h, 5% grade) in the heat (33°C, 56% relative humidity). Trials differed in preexercise hydration status [euhydrated (Eu) or hypohydrated to −3.8 ± 0.2% baseline body weight (Hy)] and water intake during exercise [no water (NW) or water ad libitum (W)]. Blood samples taken preexercise and immediately postexercise were analyzed for hematocrit, hemoglobin, serum aldosterone, plasma osmolality (P osm ), plasma vasopressin (P AVP ), and plasma renin activity (PRA). Thirst was evaluated at similar times using a subjective nine-point scale. Subjects were thirstier before (6.65 ± 0.65) and drank more during Hy+W (1.65 ± 0.18 liters) than Eu+W (1.59 ± 0.41 and 0.31 ± 0.11 liters, respectively). Postexercise measures of P osm and P AVP were significantly greater during Hy+NW and plasma volume lower [Hy+NW = −5.5 ± 1.4% vs. Hy+W = +1.0 ± 2.5% ( P = 0.059), Eu+NW = −0.7 ± 0.6% ( P P