proximately 19 percent of total water intake. Canadian survey data indicated somewhat lower levels of total water intake. As with AIs for other nutrients, for a healthy person, daily consumption below the AI may not confer additional risk because a wide range of intakes is compatible with normal hydration. In this setting, the AI should not be interpreted as a specific requirement. Higher intakes of total water will be required for those who are physically active or who are exposed to hot environments.
Over the course of a few hours, body water deficits can occur due to reduced intake or increased water losses from physical activity and environmental (e.g., heat) exposure. However, on a day-to-day basis, fluid intake, driven by the combination of thirst and the consumption of beverages at meals, allows maintenance of hydration status and total body water at normal levels.
Because healthy individuals have considerable ability to excrete excess water and thereby maintain water balance, a Tolerable Upper Intake Level (UL) was not set for water. However, acute water toxicity has been reported due to rapid consumption of large quantities of fluids that greatly exceeded the kidney’s maximal excretion rate of approximately 0.7 to 1.0 L/hour.
Water, which is the solvent for biochemical reactions, has unique physical properties (e.g., high specific heat) to absorb metabolic heat within the body. Water is also essential for maintaining vascular volume and serves as the medium for transport within the body by supplying nutrients and removing waste. In addition, cell hydration has been has been suggested to be an important signal to regulate cell metabolism and gene expression (Haussinger et al., 1994). Daily water intake must be balanced with losses in order to maintain total body water. Body water deficits challenge the ability to maintain homeostasis during perturbations (e.g., sickness, physical exercise, and environmental exposure) and can affect function and health. In very unusual circumstances, excess consumption of hypotonic fluids and low sodium intake may lead to excess body water, resulting in hyponatremia and cellular edema.
Despite the importance of adequate water intake, there is confusion among the general public and health care providers on the amount of water that should be consumed (Valtin, 2002), in part because of misinterpretation of previous recommendations (NRC, 1989).