The incidence and duration of breastfeeding changed markedly during the twentieth century—first declining, then rising, and, from the early 1980s, declining once again. Currently, women who choose to breastfeed tend to be well educated, older, and white. Data on the incidence and duration of breastfeeding in the United States are especially limited for mothers who are economically disadvantaged and for those who are members of ethnic minority groups. The best data for any minority groups are for black women. Their rates of breastfeeding are substantially lower than those for white women, but factors that distinguish breastfeeding from nonbreastfeeding women tend to be similar among black and white women. Social, cultural, economic, and psychological factors that influence infant feeding choices by adolescent mothers are not well understood. In the United States, where few employers provide paid maternity leave, return to work outside the home is associated with a shorter duration of breastfeeding, but little else is known about when mothers discontinue either exclusive or partial breastfeeding. Such data are needed to estimate the total nutrient demands of lactation.
The few lactating women who have been studied in the United States have been characterized as well nourished, but this observation cannot be generalized since these subjects were principally white women with some college education. Women from less advantaged, less well studied populations may be at higher risk of nutritional problems but tend not to breastfeed.
To determine whether women are adequately nourished, investigators use biochemical or anthropometric methods, or both. For lactating women, however, there are serious gaps and limitations in the data collected with these methods. Consequently, there is no scientific basis for determining whether poor nutritional status is a problem among certain groups of these women. To identify the nutrients likely to be consumed in inadequate amounts by lactating women, the subcommittee used an approach involving nutrient densities (nutrient intakes per 1,000 kcal) calculated from typical diets of nonlactating U.S. women. That is, they made the assumption that the average nutrient densities of the diets of lactating women would be the same as those of nonlactating women but that lactating women would have higher total energy intake (and therefore higher nutrient intake). Using this approach, the nutrients most likely to be consumed in amounts lower than the RDAs for lactating women are calcium, zinc, magnesium, vitamin B6, and folate.