experience catch-up growth during infancy. Disease or injury may adversely affect nutrient utilization either directly by increasing rates of catabolism or urinary and fecal losses or indirectly by sequestrating a nutrient in tissue compartments.
Finally, other environmental influences, such as the degree of exposure to ultraviolet light (ordinarily from sunshine) in the case of the synthesis of vitamin D, may be important determinants of the nutritional status of the infant.
Plasma levels of 25-hydroxycholecalciferol (a vitamin D metabolite used as a measure of vitamin D status) in the mother are positively correlated with those in the neonate, providing evidence that maternal vitamin D status affects the infant's vitamin D stores (Hillman and Haddad, 1974; Hoogenboezem et al., 1989; Markestad, 1983; Markestad et al., 1983). Several investigators have found that plasma concentrations of 25-hydroxycholecalciferol in the neonate are within the normal range for adults (10 to 40 ng/ml) (Markestad, 1983; Roberts et al., 1981), whereas Ala-Houhala (1985) reported that 25-hydroxycholecalciferol plasma levels in Finnish neonates were abnormally low in winter (mean, <10 ng/ml) compared with the range of values found in summer (12 to 18 ng/ml, p < .001). These studies suggest that infants born to mothers with inadequate vitamin D status are highly dependent on a regular supply of vitamin D through diet, supplements, or exposure to ultraviolet light.
Plasma 25-hydroxycholecalciferol levels in unsupplemented breastfed infants have been compared with those of formula-fed infants or breastfed infants receiving approximately 10 µg of supplemental vitamin D per day. In six reports, plasma 25-hydroxycholecalciferol levels were substantially lower in unsupplemented breastfed infants (Ala-Houhala, 1985; Chan et al., 1982; Greer et al., 1982; Lichtenstein et al., 1986; Markestad, 1983; Roberts et al., 1981). Four studies (Ala-Houhala, 1985; Greer et al., 1982; Hoogenboezem et al., 1989; Markestad, 1983) have shown plasma levels of this compound at or below the lower limits of normal (≤10 ng/ml) (Nutrition Foundation, 1984) in the unsupplemented groups. Ala-Houhala (1985) reported that in the winter months, 10 of 18 unsupplemented breastfed infants had plasma levels of less than 5 ng/ml, which may lead to rickets (Nutrition Foundation, 1984). Despite reports of rickets in breastfed infants (Arnaud et al., 1976; O'Connor, 1977; Ozsoylu, 1977) and the low vitamin D content of human milk, breastfeeding has long been considered to be protective against rickets (Belton, 1986; Lakdawala and Widdowson, 1977).
Breastfed infants require approximately 30 minutes of exposure to sunlight per week if wearing only a diaper, or 2 hours per week if fully clothed without a hat, to maintain normal serum 25-hydroxycholecalciferol levels (Specker et