Hydration status, as assessed by plasma or serum osmolality, is the primary indicator used for water. As documented previously, physical activity and environmental conditions have substantial influences on water needs (see later section, “Special Considerations”). Also, because of homeostatic responses, some degree of over- and underhydration can readily be compensated over the short-term. While it might appear useful to estimate an average requirement (an EAR) for water, it is not possible. An EAR is set based on data indicating that about half the individuals in the life stage group would have their needs met, while the other half would be inadequate at a specific intake level. Given the extreme variability in water needs that are not solely based on differences in metabolism, but also in environmental conditions and activity, there is not a single level of water intake that would ensure adequate hydration and optimal health for half of all apparently healthy persons in all environmental conditions. Thus an AI is established in place of the EAR (upon which a Recommended Dietary Allowance could be based).

Based upon a review of water balance studies (Table 4-5) for inactive adults in temperate climates, the minimal water requirement should be approximately 1 to 3.1 L/day to replace respiratory, urinary, fecal, and insensible fluid losses (Table 4-2). Data from NHANES III demonstrate that normal hydration status for all adults, as measured by serum osmolality, can be achieved with a wide range of water intakes (e.g., first through 99th percentile of total water intake) (Appendix Table G-1). Therefore, the AIs for total water are set based on median intakes of total water (drinking water, beverages, and food) from NHANES III (Appendix Table D-1), rounded to the nearest 0.1 L. These AIs cover the minimal losses that routinely occur in temperate climates for somewhat sedentary individuals and are based upon the factors previously discussed.

Individual water requirements can vary greatly, even on a day-to-day basis, because of differences in physical activity and climates. To a lesser extent, dietary factors also influence water requirements, as the osmotic load created by metabolizing dietary protein and or-