When exercising in a cold environment, fluid losses can occur via sweat, cold induced diuresis and respiration but failure to replace lost fluid is common in the cold due to a blunted thirst response (Kenefick et al. Med Sci Sports Exerc 2004;36;1528–34). This study assessed voluntary fluid intake and measures of hydration status following moderate intensity exercise in the cold. Ten healthy males (age 22±2 years, mass 67.8±7.0 kg, height 1.77±0.06 m, VO2peak 60.5±8.9 ml.kg.min-1) completed two trials following familiarisation separated by 7–14 days. In each trial, subjects sat for 30 min before cycling at 70% VO2peak for 60 min in either 25.0±0.1°C, 50.8±1.5% RH (warm) or 0.4±1.0°C, 68.8±7.5% RH (cold). Subjects then sat for 120 min at 22.2±1.2°C, 50.5±8.0% RH. Ad libitum drinking was allowed during the exercise and recovery periods. Urine volume, body mass, serum osmolality and Na and K concentrations and sensations of thirst were measured at baseline, postexercise and after 60 and 120 min of the recovery period. Sweat loss was lower in the cold trial (0.48±0.15 l vs 0.96±0.18 l) (p<0.0001) but body mass losses over the trials were similar: 1.15±0.34% versus 1.03±0.26% for cold and warm trials respectively. More fluid was consumed in the warm trial (0.81±0.42 l) compared to the cold (0.50±0.49 l) (p=0.001) replacing 51±17 and 33±27% of the total fluid lost respectively (p=0.013). Cumulative urine output was greater in the cold trial (0.81±0.46 vs 0.54±0.31 l) (p=0.036). Serum osmolality, Na and K concentrations, thirst sensations and plasma volume changes were not different between trials (p>0.05). Postexercise serum osmolality was higher compared to baseline in the cold (292±2 vs 287±3 mOsm.kg-1, p<0.0001) and warm trials (288±5 vs 285±4 mOsm.kg-1; p=0.048). Voluntary fluid intake was less in the cold environment, however in both the warm and cold environment, ad libitum fluid intake, combined with fluid losses, resulted in similar changes in body mass.
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