INFLUENCE OF THE TIMING OF FLUID INGESTION ON TEMPERATURE REGULATION DURING EXERCISE

The purpose of this investigation was to determine whether the timing of fluid ingestion affects thermoregulation during exercise-heat stres s. 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 l/min] performed 140 min of cycle ergometer exercise at 62-66% of VO2max in a hot environment (33-degrees-C dry bulb, 51% relative h umidity, wind speed 2.5 m/s). The subjects drank 1,173 +/- 44 ml of a carbohydrate-electrolyte beverage after 0 min (DO), 40 min (D40), or 8 0 min (D80) of exercise or consumed the same total volume in small ali quots throughout exercise (DT). The exercise-heat stress resulted in c alculated sweating rates of approximately 1,200 ml/h and a body weight loss of 2.9 +/-0.1% after 140 min of exercise. After fluid intake in the DO, D40, and D80 trials, there was a time period (approximately 40 min) in which the increases in serum osmolality and sodium concentrat ion and the reduction in blood volume were attenuated. During that sam e time period, there was an attenuated rise in esophageal temperature (T(es); P < 0.05). As a result, T(es) in the DO trial was lower than t hat 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 T(es) at the end of exercise was similar among the trials (DO, 38. 53 +/- 0.11-degrees-C; D40, 38.57 +/-0.10-degrees-C; D80, 38.56 +/- 0. 13-degrees-C; DT, 38.43 +/- 0.09-degrees-C). The rise in heart rate (H R) was also attenuated in the DO trial compared with the D40 and D80 t rials (P < 0.05), but final exercise HR averaged 165 +/- 3, 167 +/- 3, 168 +/- 3, and 166 +/- 3 beats /min in the DO, D40, D80, and DT trial s, respectively (P > 0.05). In summary, drinking at the onset of exerc ise transiently attenuated the rise in T(es) and HR that occurred if r ehydration was begun later in exercise. However, altering the time of fluid ingestion did not affect T(es) or HR at the end of exercise, whe n the magnitude of dehydration was similar among trials.