children who live in far northern latitudes are at high risk for developing rickets (Lebrun et al., 1993). At the other end of the age spectrum, older adults are more prone to developing vitamin D deficiency (Holick et al., 1989; Need et al., 1993). Indeed, vitamin D deficiency is now a significant concern in adults over the age of 50 years who live in the northern industrialized cities of the world (Dawson-Hughes et al., 1991; Gloth et al., 1995; Lips et al., 1988).
Human Milk. The vitamin D available to the infant during the first 6 months of life depends initially on the vitamin D status of the mother during pregnancy and later on the infant's exposure to sunlight and diet. Conservative estimates of the length of time a human milk-fed infant in the Midwest must be exposed to sunlight to maintain serum concentrations above the lower limit of normal are 2 hours/week with only the face exposed to sunlight or 30 minutes/week with just a diaper on (Specker et al., 1985b). Human milk contains low amounts of vitamin D, and colostrum averages 397 ± 216 ng (15.9 ± 8.6 IU)/liter of vitamin D (Nakao, 1988). In a population of 25 Caucasian and African American women who had a mean vitamin D intake of 11.4 µg (457 IU)/day, milk concentrations of vitamin D and 25(OH)D were 315 ng (12.6 IU)/liter and 188 ng (37.6 IU)/liter respectively, with a total of 51 IU/liter of biologic activity (1 IU of vitamin D = 25 ng and 1 IU of 25(OH)D = 5 ng). Women consuming 15 to 17.5 µg (600 to 700 IU)/day of vitamin D had total milk vitamin D concentrations ranging from 120 to 3,400 ng (5 to 136 IU)/liter with a predicted mean of 645 ng (26 IU)/liter (Specker et al., 1985b). Although maternal vitamin D intake is associated with the vitamin D content of human milk, the latter is not correlated with the infant 's serum 25(OH)D concentrations, due to the overwhelming effect of sunlight exposure on the infant's vitamin D status (Ala-Houhala, 1985; Ala-Houhala et al., 1986; Feliciano et al., 1994; Hillman, 1990; Markestad and Elzouki, 1991).
Serum 25(OH)D, Linear Growth, and Bone Mass. Although an individual's serum 25(OH)D concentration is the best biochemical