Context: The sensation of thirst is different from the complex behavior of drinking ad libitum. Rehydration recommendations to athletes differ, depending on the source, yet no previous researchers have systematically compared drinking to thirst (D(TT)) versus ad libitum drinking behavior (D(AL)).
Objective: To compare 2 groups of trained cyclists (D(TT) and D(AL)) who had similar physical characteristics and training programs (P > .05). The D(TT) group (n = 12, age = 47 ± 7 years) drank only when thirsty, whereas the D(AL) group (n = 12, age = 44 ± 7 years) consumed fluid ad libitum (ie, whenever and in whatever volume desired).
Design: Cohort study.
Setting: Road cycling (164 km) in the heat (36.1 °C ± 6.5 °C).
Patients or other participants: Ultraendurance cyclists (4 women, 20 men).
Intervention(s): We recorded measurements 1 day before the event, on event day before the start, at 3 roadside aid stations, at the finish line, and 1 day after the event.
Main outcome measure(s): Body mass, urinary hydration indices, and food and fluids consumed.
Results: No between-groups differences were seen on event day for total exercise time (DTT = 6.69 ± 0.89 hours, DAL = 6.66 ± 0.77 hours), urinary indices (specific gravity, color), body mass change (D(TT) = -2.22% ± 1.73%, DAL = -2.29% ± 1.62%), fluid intake (D(TT) = 5.63 ± 2.59 L/6.7 h, D(AL) = 6.04 ± 2.37 L/6.7 h), dietary energy intake, macronutrient intake, ratings of thirst (D(TT) start = 2 ± 1, D(TT) finish = 6 ± 1, DAL start = 2 ± 1, D(AL) finish = 6 ± 1), pain, perceived exertion, or thermal sensation. Total fluid intake on recovery day +1 was the primary significant difference (D(AL) = 5.13 ± 1.87 L/24 h, D(TT) = 3.13 ± 1.53 L/24 h, t18 = 2.59, P = .02).
Conclusions: Observations on event day indicated that drinking to thirst and drinking ad libitum resulted in similar physiologic and perceptual outcomes. This suggests that specific instructions to "drink to thirst" were unnecessary. Indeed, if athletes drink ad libitum, they can focus on training and competition rather than being distracted by ongoing evaluation of thirst sensations.
Keywords: electrolytes; fluids; rehydration; sport nutrition; urine.