Sj. Montain et Ef. Coyle, INFLUENCE OF THE TIMING OF FLUID INGESTION ON TEMPERATURE REGULATION DURING EXERCISE, Journal of applied physiology, 75(2), 1993, pp. 688-695
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.