many vitamins they plateau and do not respond further to supplementation through diet or pharmaceutical preparations. Although the milk concentrations of water-soluble vitamins are generally more responsive to maternal dietary intake than are concentrations of fat-soluble vitamins, there are important exceptions. These are discussed below for specific vitamins.
Vitamin A. The vitamin A content of human milk comprises principally retinyl esters (96%). The concentration of this vitamin in human milk decreases over the course of lactation from approximately 2,000 to 600 µg/liter (Chappell et al., 1986; Cumming and Briggs, 1983). Concentrations of carotene, a precursor of vitamin A, are reported to differ from 0 to 320 µg/liter (Butte and Calloway, 1981; Chappell et al., 1986; Department of Health and Social Security, 1977). This wide range may reflect mainly analytic difficulties and sampling errors (Jensen, 1989). β-Carotene is stored in the mammary gland during pregnancy and is rapidly secreted into milk during the first few days of lactation (Patton et al., 1990). Several reports indicate that the amount of vitamin A in human milk decreases with maternal deficiency of the vitamin and increases with excessive intake (Ajans et al., 1965; Butte and Calloway, 1981; Hrubetz et al., 1945).
Results of supplementation trials are equivocal. In vitamin A-depleted mothers, supplementation was found to increase the concentration of vitamin A in milk in some studies (e.g., Venkatachalam et al., 1962) but to have no effect in others (Belavady and Gopalan, 1960; Villard and Bates, 1987). Chappell et al. (1985b) noted no association between reported maternal intake of vitamin A and carotene with corresponding values in the milk of well-nourished Canadian women. In contrast, Gebre-Mehdin and coworkers (1976) reported that the concentration of retinyl esters was low in milk from disadvantaged Ethiopian women compared with that in milk from Ethiopian women of higher socioeconomic status and from Swedish women.
Vitamin D. Human milk normally contains 0.5 to 1.5 µg (20 to 60 IU) of vitamin D per liter (Greer et al., 1984a). Several studies indicate that the vitamin D activity of human milk is directly related to maternal vitamin D status. Hollis et al. (1983) reported that the vitamin D concentrations in human milk drop to undetectable levels during maternal deficiency and increase following supplementation and exposure to ultraviolet light. Potentially toxic amounts (175 µg, or >7,000 IU, per liter) of vitamin D could occur in human milk following daily administration of pharmacologic doses (2,500 µg, or 100,000 IU) of vitamin D2 (ergocalciferol) to the mother (Greer et al., 1984b). The vitamin D activity of human milk is accounted for principally by vitamin D metabolites but also by vitamin D2 and vitamin D3 (cholecalciferol).