Drinking Strategies: Planned Drinking Versus Drinking to Thirst

Abstract

In humans, thirst tends to be alleviated before complete rehydration is achieved. When sweating rates are high and ad libitum fluid consumption is not sufficient to replace sweat losses, a cumulative loss in body water results. Body mass losses of 2% or greater take time to accumulate. Dehydration of ≥ 2% body mass is associated with impaired thermoregulatory function, elevated cardiovascular strain and, in many conditions (e.g., warmer, longer, more intense), impaired aerobic exercise performance. Circumstances where planned drinking is optimal include longer duration activities of > 90 min, particularly in the heat; higher-intensity exercise with high sweat rates; exercise where performance is a concern; and when carbohydrate intake of 1 g/min is desired. Individuals with high sweat rates and/or those concerned with exercise performance should determine sweat rates under conditions (exercise intensity, pace) and environments similar to that anticipated when competing and tailor drinking to prevent body mass losses > 2%. Circumstances where drinking to thirst may be sufficient include short duration exercise of < 1 h to 90 min; exercise in cooler conditions; and lower-intensity exercise. It is recommended to never drink so much that weight is gained.

Fig. 1

Regulation of body water balance in response to body water deficit typical of exercise/fluid restriction for a 70 kg individual. Schematic includes the estimated magnitude of dehydration (2% body mass loss) required to stimulate the osmotic-dependent response for compensatory water conservation and acquisition (thirst). A change in total body water is equated with a change in body mass (1 L = 1 kg), whereby dehydration is then expressed as a percentage of body mass in accordance with: (Δ body mass/initial body mass) × 100 or, for this example, (1.4 kg/70 kg) × 100 = 2%. AVP arginine vasopressin, Posm plasma osmolality, TBW total body water

Fig. 2

Ad libitum fluid intake vs. sweat losses during treadmill walking in cool (24 °C; filled circle) and hot (49 °C; open circle) environments. Ad libitum fluid intake equals ~  50% of fluid losses (adapted from Greenleaf and Sargent [19] with permission)

Fig. 3

Level of post-race dehydration vs. average running speed and finishing time for 42 km when drinking ad libitum. Adapted from Cheuvront et al. [32]

Fig. 4

Review of dehydration effects on performance in 34 endurance exercise/dehydration studies. Fractions above bars represent the number of statistically significant (p < 0.05) observations (numerator) of total observations (denominator) at the specified level of dehydration. 41 of 60 total observations (68%) were significantly (p < 0.05) impaired by dehydration ≥  2% body mass. Adapted from Cheuvront and Kenefick [12])

Fig. 5

Percentage loss in body mass predicted from sweat rate for 60 and 80 kg runners of average ability a during 5 km (25 min), 10 km (60 min), 21 km (130 min), and 42 km (270 min) and competitive ability b during 5 km (21 min), 10 km (43 min), 21 km (95 min), and 42 km (200 min) road races. The dotted line demarks 2% body mass loss. Losses assume no fluid intake. Adapted from Kenefick and Cheuvront [49]