With regard to energy balance of lactating women, the threshold below which energy intake is insufficient to support adequate milk production has not yet been identified. Resolution of this question will probably require supplementation studies of women in developing countries whose diets are chronically energy deficient. Although such deficient diets are not common in the United States, identification of the level of energy intake that is too low to support lactation will be useful in establishing guidelines for women who want to breastfeed but also want to restrict their energy intake to lose weight. Although chronically low energy intakes by women in disadvantaged populations may not be completely analogous to acute energy restriction among otherwise well-nourished women, ethical considerations limit the kinds of investigations that could directly address the influence of energy restriction. In supplementation studies, measurements should be made of lactation performance and of any impact on the mother's nutritional status and health, including the period of lactation amenorrhea.
With regard to specific nutrients, the impact of relatively low intakes of folate, vitamin B6, calcium, zinc, and magnesium during lactation on the mother's nutritional status and health needs to be assessed in more detail. As a part of this assessment, studies of the absorption of calcium, zinc, and magnesium during lactation will be useful. There is also a need to identify a reliable indicator of vitamin B6 status of infants and to document the relationships between this indicator, maternal vitamin B6 intake, and vitamin B6 content in milk. Finally, resolution of the conflicting findings concerning the impact of maternal protein intake on milk volume would be desirable.
The impact of high levels of physical activity on milk volume, milk composition, and duration of lactation requires further study, especially in populations in which energy intake is low relative to total need. Such research should be designed to identify the relative energy deficit imposed by high levels of physical activity.
With greater numbers of women involved in physically demanding work and with increased interest in physical fitness, an increased number of women in the United States may need or want to resume heavy physical activity post partum. The potential impact of such activity on lactation is unknown.
Studies should be conducted to determine relationships, if any, between the nutritional status of the mother, the concentrations and functions of the components of the immunologic system in human milk, and the susceptibility of the recipient infant to common infectious agents.