marker of vitamin D status, functional indicators of bone length and reduced bone mass (rickets in the extreme form) serve as useful evaluative outcomes of deficiency. Vitamin D intakes between 8.5 and 15 µg (340 and 600 IU)/day would have the maximum effect on linear growth (Feliciano et al., 1994; Fomon et al., 1966; Jeans and Stearns, 1938; Stearns, 1968). At intakes greater than 45 µg (1,800 IU)/day, linear growth may be reduced (Jeans and Stearns, 1938).
A recent study in Chinese infants (Specker et al., 1992) demonstrated that both latitude and intake of vitamin D over a relatively narrow range affect infant vitamin D status. Although there was no evidence of rickets in any of the infants from northern China (40 to 47° N), vitamin D supplements of 2.5 or 5 µg (100 or 200 IU)/day resulted in 17 of 47 and 11 of 37 infants, respectively, having serum 25(OH)D concentrations less than 27.5 nmol/liter (11 ng/ml) by 6 months of age. For infants supplemented with 10 µg (400 IU)/day, only 2 of 33 had deficient vitamin D status. In contrast, Chinese infants from two southern cities (22° N and 30° N) maintained normal vitamin D status even on 2.5 µg (100 IU)/day of vitamin D.
Seasonal variation in vitamin D status of infants is also apparent. In studies from the United States (Greer et al., 1982a; Specker and Tsang, 1987) and Norway (Markestad and Elzouki, 1991), serum 25(OH)D concentrations in human milk-fed infants not receiving vitamin D supplements decreased in winter due to less sunlight exposure. However, this decrease did not occur in infants receiving a vitamin D supplement of 10 µg (400 IU)/day beginning at 3 weeks of age (Greer et al., 1982a). The impact of the seasonal reduction in vitamin D status on bone mineral mass has not been clearly delineated. In Greer's (1982) study, BMC of the placebo group was significantly less than the vitamin D-supplemented group at 12 weeks, but this difference was no longer significant by 26 weeks of age. However, there were no differences in the mean serum calcium, alkaline phosphatase, or PTH levels between the placebo and vitamin D-supplemented groups.
With habitual small doses of sunshine, breast- or formula-fed infants do not require supplemental vitamin D. For infants who live in far northern latitudes or who are restricted in exposure to sunlight, a minimal intake of 2.5 µg (100 IU)/day of vitamin D will likely prevent rickets (Glaser et al., 1949; Specker et al., 1992). However, at this intake and in the absence of sunlight, many infants will have