Factors Affecting Fluid Intake

The two main factors influencing fluid intake are thirst and taste. Thirst is a sensation of dryness in the mouth and throat related to the body's need for additional fluids. Taste is the response humans have (either good or bad) to substances in the mouth. Humans are more likely to consume more of what they like, or more of what tastes good to them. Most athletes induce "voluntary dehydration" because they don't drink enough despite having plenty of fluids readily available. Insufficient consumption of fluids by athletes is probably due to a lack of the thirst sensation.

The onset of thirst may be the result of habit, ritual, or the need for a warming (hot fluids) or cooling (cold fluids) effect.¹¹ A rise in plasma osmolality of between 2 and 3 percent is needed to produce the sensation of thirst, and sensitivity to a reduction in fluid volume is even less responsive, requiring nearly a 10 percent decrease in blood volume to stimulate thirst.^{12, 13} The thirst sensation, often considered delayed in athletes because it doesn't appear until an athlete has already lost 1.5 to 2.0 liters of body water, is therefore a poor indicator of fluid needs in athletes.¹⁴ There is no hope that an athlete can return to an adequately hydrated state during exercise if fluid consumption begins at the same time the thirst sensation occurs. This apparent delay in the thirst mechanism is a primary reason for athletes to train themselves to consume fluids on a schedule, whether they feel thirsty or not.

Color, taste, odor, temperature, and texture all play important roles in determining if the beverage will be considered desirable and whether it will be consumed. It appears that athletes prefer cool beverages with a slightly sweet flavor. Heavily sweetened beverages (around a 12 percent carbohydrate solution) are not as widely tolerated during exercise as beverages with a 6 or 7 percent carbohydrate solution.^{15, 16} When not exercising, however, the reverse may be true, pointing to an interesting phenomenon of exercise: Food and drink taste differently while exercising than when not exercising. Therefore, athletes are wise to determine the fluids they find most desirable for exercise while they are exercising.