A Comparison of the Physiological Effects of Drinking to Thirst versus Drinking to a Generic Schedule During 2 Hours of Exercise in Thermoneutral Conditions

The 2000 NATA fluid replacement position statement includes guidelines for scheduled drinking to avoid losing greater than 1-2% body weight. A recent consensus statement suggests that drinking to thirst is more appropriate. PURPOSE: Investigate the physiological effects of drinking to thirst versus drinking to a schedule during 2 hr of continuous running in a thermoneutral environment. METHODS: Nine male and seven female trained endurance runners (age=35.4±11.6yrs, height=171.6±8.9cm, weight=69.2±12.4kg, %bodyfat=16.7±6.4%, VO2max=56.7±9.0ml/kg/min) completed two experimental trials (Thirst versus Scheduled). Participants ran on a treadmill at 60% VO2max for 2hr in a climatic chamber (21ºC and 40% RH). Scheduled drinking was as follows: 600mL 2hr pre-trial, 300mL 20min pre-trial, and 300mL every 10min during running. In the Thirst trial subjects drank when they felt a deep-seated desire for water. Outcome measures included blood electrolytes(Na+, Cl-, K+), ∆body weight(kg), fluid intake(L), and sweat loss(L). Correlated t-tests were used and p Pre-trial urine osmolality (Thirst=318.4±191.6 mOsm/kg and Scheduled=260.6±195.4 mOsm/kg) and blood electrolytes were not different. Blood Na+ was different between Thirst and Scheduled mid-trial(141.9±1.4 versus137.8±1.3 mmol/L, P- mid-trial(104.2±1.4 versus 101.6±1.3 mmol/L, P+, whereas drinking to thirst resulted in blood Na+ within the normal range