interaction can be inferred from studies of women with presumably heavy work loads and marginal or adequate nutritional status. Physical activity appears to exacerbate the negative effects of poor nutritional status on milk volume. Data from studies done in The Gambia illustrate this point in comparisons of milk volume under conditions of work and maternal planes of nutrition (Prentice et al., 1982). The physiologic basis for this view and representative data that examine potential relationships between physical activity and lactation performance are considered in more detail in subsequent sections of this chapter.
Because the nutrient demands of physical activity are closely associated with energy, concern arises for the sufficiency of maternal energy stores at the end of a physically active woman's pregnancy. As reviewed earlier in this report, a significant proportion of weight gained during pregnancy represents maternal adipose tissue. One possible functional role of fat stores accumulated during pregnancy may be to buffer the need for additional dietary energy imposed by lactation. If physical activity during pregnancy interferes with the accumulation of normal quantities of fat, lactation performance may be affected adversely. Two mechanisms that can be postulated for those adverse influences are 1) a putative dependence on minimal fat stores to produce milk of normal volume and composition for an acceptable period of time and 2) the likelihood that increased demands for dietary energy for lactation will not be met if the mother has insufficient stores of energy.
Women who gain 11 to 13 kg body weight during pregnancy should have stored 2 to 4 kg of fat. Those energy stores may be assumed to have been accumulated in a physiologic anticipation of lactation. On the basis of an estimated mean milk output during the first 4 to 6 months of lactation of approximately 750 ml/day and a gross caloric concentration in milk of approximately 0.64 kcal/ml, the lactating woman loses approximately 500 kcal/day in milk (Butte, et al., 1984). Estimates of the efficiency of milk production range from 80 percent to 95 percent. The energy required to synthesize 750 ml of milk, therefore, is approximately 25 to 125 kcal/day plus 500 kcal accounted for by the gross energy content of the milk. If a conservative estimate of 500 kcal/day (i.e., 100-percent efficiency) for the cost of milk production is used, 2 to 4 kg of fat (i.e., 17,000 to 34,000 kcal) would theoretically represent sufficient energy for approximately 1.2 to 2.5 months of milk production. If an estimate of 625 kcal/day (i.e., 80-percent