NRC, 1989). Lower concentrations found in riboflavin-deficient populations can be increased by supplementation (Bates et al., 1981). The average niacin concentration in human milk increases from 0.75 mg/liter in colostrum to approximately 1 mg/liter in mature human milk. Actual niacin levels are largely dependent on maternal intake; an observational study reported levels as high as 6 mg/liter among women who were successfully lactating (Pratt et al., 1951).

Vitamin B6. The vitamin B6 content is low in colostrum and varies between 50 and 250 µg/liter in mature milk. In the United States, levels in mature milk have been reported to be approximately 93 ± 8 (standard deviation [SD]) µg/liter, a value 10 times higher than levels in maternal serum. The vitamin B6 content of milk is directly related (R = .8) to maternal intake (Styslinger and Kirksey, 1985). Roepke and Kirksey (1979) reported drastically reduced vitamin B6 levels in milk from mothers with a long history (4 to 12 years) of oral contraceptive use before conception. Supplements of 20 mg/day were required to increase milk concentrations in those mothers and to reverse neurologic symptoms of deficiency in their infants (Kirksey and Roepke, 1981). However, the contraceptives taken by these women contained higher levels of estrogen than those that are now used in contraceptive formulations; current interrelationships among contraceptive use, vitamin B6 intake, and vitamin B6 concentrations in human milk are unknown. Further discussion of vitamin B6 in human milk is included in Chapter 9.

Folate and Vitamin B12. Folate and vitamin B12 in human milk are bound to whey proteins; therefore, maternal factors regulating protein secretion are more likely to affect milk levels of these vitamins over the short term than are fluctuations in maternal vitamin intake.

Improved methods of analysis have permitted detection of much higher folate levels in human milk than previously reported. Milk folate is quantitatively bound to folate-binding protein; folylpolyglutamates account for a considerable portion of total folate. In the United States, folate concentrations in human milk average 85 ± 37 (SD) µg/liter during the first 3 months of lactation (Brown et al., 1986a); in Japan, they average 141 ± 43 (SD) µg/liter during the first 6 months of lactation (Tamura, 1980). In apparently well-nourished women in industrialized countries, no correlation was found between maternal serum and milk folate levels (before or after maternal supplementation) (Salmenperä et al., 1986b; Smith et al., 1983; Tamura et al., 1980). However, milk folate levels were found to increase from 5 to 60 µg/liter after 4 days of oral folate supplementation of two lactating women with megaloblastic anemia resulting from dietary folate insufficiency (Metz et al., 1968). Folate levels in human milk typically increase with the progression of lactation, even as levels in maternal serum and red blood cells decrease (Smith et al., 1983). There is evidence



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