postpartum to 3 mg/L at 4 weeks, 2 mg/L at 8 weeks, 1.5 mg/L at 12 weeks, and 1.2 mg/L at 24 weeks (Krebs et al., 1995; Moser-Veillon and Reynolds, 1990). With use of a standard volume of 0.78 L/day of human milk secreted per day (Chapter 2), calculated zinc losses via the mammary gland are 2.15 mg/day at 4 weeks, 1.56 mg/ day at 8 weeks, 1.17 mg/day at 12 weeks, and 0.94 mg/day at 24 weeks.
Postpartum involution of the uterus and decreased maternal blood volume should release approximately 30 mg of zinc that has been accumulated during pregnancy (King and Turnlund, 1989); that is, an average of approximately 1 mg/day for the first month. It is reasonable to assume that this endogenous zinc is available for reutilization. Thus, 1 mg/day is subtracted from the amount of zinc lost during the first 4 weeks of lactation. The loss of zinc for weeks 8, 12 and 24 are averaged:
Week 4: (2.15–1.0) = 1.15 mg/day of zinc
Week 8: (2.15 + 1.56) ÷ 2 = 1.85 mg/day of zinc
Week 12: (1.56 + 1.17) ÷ 2 = 1.36 mg/day of zinc
Weeks 12–24: (1.17 + 0.94) ÷ 2 = 1.05 mg/day of zinc
The average calculated increased requirement for absorbed zinc during lactation is 1.35 mg/day.
Reported values for fractional absorption of zinc for adult women outside the reproductive cycle averages 27 percent (Fung et al., 1997; Hunt JR et al., 1992, 1998; Sian et al., 1996; Turnlund et al., 1991). If this value were applied to the calculation of increased dietary zinc required during lactation (1.35 ÷ 0.27), the average dietary requirement would increase by 5 mg/day. However, the fractional absorption of zinc increases during lactation by 0.107 (Fung et al., 1997). Therefore, the fractional absorption would be increased to 0.716 (0.27 ÷ 0.377) to give an additional requirement of 3.6 mg/day (5 × 0.716). This value is added to the EAR for adolescent girls and women to set the EAR during lactation.
Other Criteria. Typically, human milk zinc concentrations are not increased by the administration of a daily zinc supplement across lactation (Kirksey et al., 1979; Krebs et al., 1995; Moser-Veillon and Reynolds, 1990). In one study, however, a modest but statistically significant reduced rate of decline in zinc concentrations in milk across lactation was observed with a zinc supplement (Krebs et al., 1985). The dietary zinc in the placebo group averaged 10.7 mg/ day. In a subsequent study, in which the average dietary zinc was