modify the chronic use of diuretics in situations where excess water losses occur (e.g., prolonged environmental heat).

Some medications, such as lithium, may interfere with regulatory systems for the control of arginine vasopressin release and result in a central or nephrogenic diabetes insipidus (Posner and Mokrzycki, 1996; Stone, 1999). In this setting, water losses through the kidney increase dramatically as arginine vasopressin is unavailable to stimulate water reabsorption back into the collecting tubules.

As is described in Chapter 2, unless there is reason to believe that human milk is inadequate in meeting an infant’s need for a nutrient, an adequate intake (AI) is derived for infants based on data regarding human milk consumption for this age group.

Water Production and Losses. Infants ages 0 to 12 months merit special consideration regarding water losses and requirements. Compared with children and adults, infants have a higher total body water content per kg of body mass (Altman, 1961), a higher surface area-to-body mass ratio, a higher rate of water turnover (Fusch et al., 1993), a less-developed sweating apparatus (Kuno, 1956), a limited ability to excrete solutes, and a lower ability to express thirst.

During the first year of life, more than half of daily water losses occur through urine (Goellner et al., 1981; NRC, 1989). Insensible loss accounts for approximately 40 percent and stool for approximately 5 percent. Most studies report daily urine losses of approximately 90 to 110 mL/kg of body weight. Based on periodic monitoring of 15 healthy, full-term infants undergoing metabolic balance studies, daily urine volume was 59 percent of volume intake in the first month of life. It gradually decreased, reaching 47 percent in months 6 to 12, and again rose to just over 50 percent during months 12 to 32 (Goellner et al., 1981).

It is not known what percentage of insensible water loss is respiratory loss versus losses from the skin. Sweating can occur soon after birth, but not in all infants. In one study, 64 percent of full-term newborns excreted sweat within several hours of birth when they were exposed to a warm environment in the nursery (Agren et al., 1997). Some evidence exists, however, that the sweating apparatus is not fully developed before the third year of life (Kuno, 1956).