Intake from Human Milk. Zinc nutriture in later infancy is quite different from that in the younger infant. It is likely that neonatal hepatic stores, which may contribute to metabolically usable zinc pools in early postnatal life, have been dissipated (Zlotkin and Cherian, 1988). Human milk provides only 0.5 mg/day of zinc by 7 months postpartum (Krebs et al., 1994), and the concentration declines even further by 12 months (Casey et al., 1989). It is apparent, therefore, that human milk alone is an inadequate source of zinc after the first 6 months. As a result, extrapolation from human milk intake during the 0 through 6 months postpartum period, which yields 2.4 mg/day, does not reflect adequate zinc intake during the second 6 months.
Intake from Human Milk and Complementary Foods. Data from the Third National Health and Nutrition Examination Survey indicate that the median intake of zinc from complementary foods is 1.48 mg/day (n = 45) for older infants consuming human milk. Thus, the average zinc intake from human milk and complementary foods is estimated to be approximately 2 mg/day (0.5 + 1.48).
Factorial Analysis. Excretion of endogenous zinc is used to estimate the physiological requirement of zinc in older infants and young children. The Estimated Average Requirement (EAR) for zinc is determined by dividing the physiological requirement by the fractional zinc absorption. Apart from some data on excretion of zinc in the urine (Alexander et al., 1974; Cheek et al., 1968; Ziegler et al., 1978), direct measurements of endogenous zinc excretion are not available for older infants, children, or adolescents. These endogenous zinc losses (intestinal, urinary, and integumental), therefore, are estimated by extrapolation from measured values for either adults (see “Adults Ages 19 Years and Older”) or younger infants. These extrapolations have been based on a reference weight.
Intestinal losses vary directly with the quantity of zinc absorbed (see “Adults Ages 19 Years and Older”). The average intestinal excretion of endogenous zinc in infants aged 2 to 4 months who receive human milk is approximately 50 μg/kg/day (Krebs et al., 1996). There is a “critical” level of intestinal excretion of endogenous zinc