The resting metabolic rate did not change significantly over time in either group. The mean energy expenditure (not including energy expended in producing milk) was similar between the groups at base line, differed by 281 kcal per day (1.2 MJ per day) (P = 0.09) at the midpoint of the study because of the exercise program, but did not differ significantly at the end of the study even though the exercising women continued to expend about 400 kcal per day (1.7 MJ per day) in exercise. The data on energy expenditure in the active range indicate that the women in the exercise group cut back on other activities during the second half of the study. There was no change in the amount of time spent sleeping by either group.
The intake of breast milk by the infants was slightly lower in the exercise group than in the control group at all three study points when expressed in terms of grams per day (because the infants of the exercising mothers were smaller initially and thus consumed less milk), but not when expressed in terms of grams per kilogram of body weight per day. The breast-milk composition (lipid, protein, and lactose concentrations, and energy density) did not differ between the two groups except for the concentration of protein, which was significantly higher in the exercise group than in the control group at the final measurement. However, the change in the protein concentration did not differ between the groups (P = 0.80); both declined significantly over time (P