expected to increase until age 25. As discussed in Chapter 6, a low calcium intake will not affect the concentration of calcium in human milk, but its effect on the mother's long-term bone density is uncertain, especially if the duration of breastfeeding is long. The evidence reviewed in Chapter 8 does not indicate a higher prevalence of osteoporosis in women who breastfed their children, but there is little information on the bone health of breastfeeding women with low calcium intakes. Lactating women in Nepal with low calcium intakes maintained milk calcium levels similar to those of U.S. women, but levels of urinary hydroxyproline were more than twice as high, indicating greater bone resorption (Moser et al., 1988). There is some evidence from animal studies that calcium absorption is enhanced during lactation (Halloran and DeLuca, 1980), but the degree to which this can compensate for low intakes is unclear. Although it is evident that calcium status is only one of many possible factors in the etiology of osteoporosis, dietary guidance for lactating women should include recommendations for good sources of calcium.
There is no generally accepted indicator to use for evaluating the adequacy of zinc intakes. The RDA increment for zinc during lactation is 4 to 13 times higher than the estimated zinc secretion in milk to allow for poor absorption of dietary zinc (estimated at 20% for nonpregnant, nonlactating adults). However, stable isotope studies of seven lactating women in Brazil whose zinc intake averaged only 8.4 mg/day indicate that zinc absorption may be as high as 59 to 84% (Jackson et al., 1988).
The difference between the RDA increment and estimated zinc secretion is especially large during the second 6 months of lactation, when zinc concentrations in milk decline substantially, regardless of the woman's zinc intake (Krebs et al., 1985). Low intakes are not generally reflected in low zinc concentrations in milk, and no major health risks have been associated with zinc intakes lower than the RDA. However, maternal zinc status might be jeopardized by low intake: zinc levels in plasma were found to be lower among lactating women than among nonlactating controls in Nigeria (Mbofund and Atinmo, 1985) but not in the United States (Moser and Reynolds, 1983), despite relatively low zinc intakes in both studies. Given the importance of adequate zinc status to immune function and other outcomes, further research on maternal zinc status during lactation is warranted.
Magnesium is the only other mineral for which intake by lactating women may often be marginal, when compared with the RDA. Again, however, the increment recommended during lactation is two to three times the estimated daily secretion in milk, to account for an estimated absorption of 50% (NRC,