The purpose of this investigation was to determine whether the timing of fluid ingestion affects thermoregulation during exercise-heat stress. On four occasions, seven endurance-trained cyclists [age 25 ± 2 (SE) yr, body weight 70.5 ± 3.3 kg, maximal O2 uptake (VO2max) 4.69 ± 0.11 1/min] performed 140 min of cycle ergometer exercise at 62-66% of VO2max in a hot environment (33°C dry bulb, 51% relative humidity, wind speed 2.5 m/s). The subjects drank 1,173 ± 44 ml of a carbohydrate-electrolyte beverage after 0 min (D0), 40 min (D40), or 80 min (D80) of exercise or consumed the same total volume in small aliquots throughout exercise (DT). The exercise-heat stress resulted in calculated sweating rates of ~1,200 ml/h and a body weight loss of 2.9 ± 0.1% after 140 min of exercise. After fluid intake in the D0, D40, and D80 trials, there was a time period (~40 min) in which the increases in serum osmolality and sodium concentration and the reduction in blood volume were attenuated. During that same time period, there was an attenuated rise in esophageal temperature (Tes; P<0.05). As a result, Tes in the D0 trial was lower than that in the D40 trial from 30 to 60 min of exercise (P < 0.05) and less than that in the D80 trial from 60 to 90 min of exercise (P<0.05), but Tes at the end of exercise was similar among the trials (D0, 38.53 ± 0.11°C; D40, 38.57 ± 0.10°C; D80, 38.56 ± 0.13°C; DT, 38.43 ± 0.09°C). The rise in heart rate (HR) was also attenuated in the D0 trial compared with the D40 and D80 trials (P<0.05), but final exercise HR averaged 165 ± 3, 167 ± 3, 168 ± 3, and 166 ± 3 beats/min in the D0, D40, D80, and the DT trials, respectively (P<0.05). In summary, drinking at the onset of exercise transiently attenuated the rise in Tes and HR at the end of exercise, when the magnitude was similar among trials.
J Appl Physiol. 75(2):688-695.