Although mean energy expenditure was higher in the exercise group than in the control group -- by almost 300 kcal per day at the midpoint of the study -- this difference narrowed by the end of the 12-week program because the exercising women cut back on other activities. This finding suggests that among breast-feeding women, high levels of energy expenditure may be difficult to sustain because of fatigue or time constraints.
Basal prolactin levels did not differ between the two groups, a finding that is consistent with the results of our earlier study11. This finding suggests that the short-term increase in prolactin levels after exercise in nonlactating women that has been reported by others4-6 does not influence basal levels. The prolactin response to nursing was highly variable, a fact that probably reflects differences in the infants' sucking and the mothers' basal prolactin levels. In malnourished women, basal prolactin levels are higher during conditions of greater energetic stress,24 but this effect is probably mediated through the infants' frequency and duration of nursing. In the present study, the frequency and duration of nursing did not differ between the exercise group and the control group