levels of nitrogen intake. Differences in nitrogen balance are not accounted for by nitrogen losses in milk; urinary 3-methyl histidine excretion (a measure of muscle protein breakdown) also is lower in lactating women. Some changes in blood concentrations of vitamins and minerals over the course of lactation are unrelated to changes in plasma volume. For example, serum zinc concentration increases while serum copper decreases between weeks 1 to 2 and 19 to 21 of lactation (van der Elst et al., 1986).
In summary, evidence suggests that, in principle, it is likely to be inappropriate to compare blood values of various nutrients or metabolites of lactating women with reference values for nonpregnant women. However, the degree to which this approach misclassifies women's nutritional status depends on the degree to which levels in lactating women differ from those of their nonpregnant, nonlactating counterparts. As will be evident from the data summarized and discussed below, too few lactating women have been studied to make a meaningful evaluation of the validity of this approach at present.
Inferences about maternal nutritional status also can be made from the nutritional status of the infant. For example, infants with evidence of vitamin B12 deficiency (that is, those with increased concentrations of methylmalonic acid in their urine) may have mothers with poor vitamin B12 status (Specker et al., 1988). The reverse is not necessarily true, however. For example, the nutritional status of breastfed infants of mothers with inadequate folacin (Salmenperä et al., 1986) or vitamin C (Salmenperä, 1984) status may remain optimal. This issue is discussed further in Chapter 7.
Tables developed by the Metropolitan Life Insurance Company have generally been used as normative standards for weight, height, and weight for height (MLI, 1959, 1983). For women in the United States, values derived from recent data from the National Center for Health Statistics (NCHS) (NRC, 1989) also could be used. There are drawbacks for both sets of values for studies of lactating women (Rasmussen, 1988). The use of such normative standards is fraught with all the problems discussed in Chapter 4 of Nutrition During Pregnancy, Part I: Weight Gain (IOM, 1990) as they relate to pregnant women. In particular, it is difficult to obtain accurate measurements without extensive training and monitoring, and comparison of a woman's values with either of these standards does not provide the information needed to make inferences about either the risk of adverse outcomes or the potential benefit from a nutritional intervention for the mother or the infant.
Many anthropometric indicators of nutritional status change continuously during lactation (Butte et al., 1984) and, even at 6 months post partum, may still differ from prepregnancy values (Sadurskis et al., 1988). The rate at which a woman (lactating or not) returns to her prepregnancy weight after delivery is