Methods
Study Design
Breast-feeding women were randomly assigned to either the exercise group or the control group. Those in the exercise group undertook a program of aerobic exercise lasting 45 minutes per day, 5 days per week, for 12 weeks beginning 6 to 8 weeks post partum. Those in the control group did not engage in aerobic exercise more than once per week during the same period. The women who were enrolled met the following criteria: they had no chronic illness, used no regular medication, and did not smoke; they had not exercised more than twice per week during the previous 3 months; they planned to breast-feed their infants exclusively for at least 20 weeks; and they had delivered healthy infants at term (37 to 43 weeks' gestation). Measurements were made at base line (6 to 8 weeks post partum), at the midpoint of the study (12 to 14 weeks), and at the end of the study (18 to 20 weeks). The women's energy expenditure, resting metabolic rate, body composition, dietary intake, and breast-milk volume and composition were measured at all three points. Maximal oxygen uptake and plasma prolactin levels were measured at the beginning and end of the study. The protocol was approved by the Human Subjects Review Committee of the University of California, Davis.