the United States, Greer et al. (1982a) followed groups of infants who were fed human milk and exposed on average to 35 minutes/day of sunshine. Those who received either a placebo or 10 µg (400 IU)/day of vitamin D had similar serum 25(OH)D concentrations at 1 year of age. Similarly, in Hong Kong, Leung et al. (1989) followed 150 formula-fed infants who had a mean intake of 8.6, 3.9, and 3.8 µg (345, 154, and 153 IU)/day of vitamin D at 6, 12, and 18 months, respectively. They observed that none of the infants at 18 months had a serum 25(OH)D level less than 25 nmol/liter (10 ng/ml) and that the mean values in May and June were higher than in January through April.

In the absence of any sunlight exposure, an AI of 5 µg (200 IU)/day will result in few infants ages 7 through 12 months with serum 25(OH)D concentrations less than 27.5 nmol/liter (11 ng/ml). This is based on the observation that, in the absence of sun-mediated vitamin D synthesis, approximately 5 µg (200 IU)/day of vitamin D maintained 25(OH)D levels in the normal range, but below circulating concentrations attained by infants in the summer. However, an intake of 10 µg (400 IU), which is supplied by 1 liter of most infant formulas or 1 quart of milk, would not be excessive.

Serum 25(OH)D. Essentially no scientific literature exists that systematically evaluates the influence of different amounts of vitamin D intake on either BMC, bone radiography, or serum 25(OH)D in children aged 1 through 3 and 4 through 8. Although vitamin D intake was not reported, Meulmeester et al. (1990) measured circulating concentrations of 25(OH)D and PTH in 8-year-old children in an observational study and found that the serum PTH increased when the 25(OH)D levels were below 20 nmol/liter (8 ng/ml). Because serum 25(OH)D concentrations in children correlate well with cumulative exposure to sunlight or dietary intake of vitamin D, this biochemical marker is appropriate for assessment of vitamin D needs of growing children. The major limiting factor in interpret-