concentrations of some nutrients, both between women and in the same woman at different stages of lactation, the estimates in Table 9-1 are provided primarily as an illustration of the relative levels of secretion in milk. They reflect the nutrient output by women exclusively breastfeeding a single infant; therefore, they are underestimates for those women breastfeeding twins or triplets and overestimates for those whose infants are given substantial amounts of other milks or solid foods.
Previous editions of Recommended Dietary Allowances (e.g., NRC, 1980) presented RDAs for lactating women as increments to be added to the RDAs for nonpregnant, nonlactating women. In contrast, the 1989 RDAs for lactating women are presented as absolute amounts. This provides convenient numbers that apply to all lactating women regardless of age and reflects the limited precision of the published data. The absolute values in the 1989 RDAs represent the RDAs for nonpregnant, nonlactating women aged 25 to 50 plus increments, which are shown in Table 9-1. For many nutrients, it is evident that the RDA increments were designed to exceed the estimated daily output during lactation. The RDAs for lactating women are the same for mothers of all ages, but some of the RDAs for nonpregnant, nonlactating women differ for women of different age groups. The difference is most notable for vitamin D and calcium. For those two nutrients, no increment for lactation is included for women younger than 25. Their prepregnancy RDAs are considerably higher than those of the older group (10 µg compared with 5 µg for vitamin D, and 1,200 mg with 800 mg for calcium), and they are identical to the older women's RDAs for these nutrients during lactation.
Daily outputs of energy, vitamin A, vitamin B12, iron, and iodine in milk tend to exceed the recommended increments. The allowance for energy is based on the assumption that lactating women can draw on fat stores deposited during pregnancy to help support milk production. Thus, gradual weight loss is expected during lactation. For women who are underweight or whose weight gain during pregnancy was low, a 650-kcal/day increase in energy intake during the first 6 months of lactation is recommended. Total energy needs during lactation depend greatly on the level of physical activity, as described in Chapter 5.
In estimating output for several nutrients, the subcommittee used values for milk concentrations that were higher than from those used in deriving the RDAs. These are identified in Table 9-1, along with other comments regarding the rationale for the RDA increments for lactation.
In addition to examining the daily output of nutrients in milk, it is useful to estimate the overall nutrient outputs resulting from various durations of lactation. Obviously, these estimates depend on the timing and degree to which