A RANDOMIZED STUDY OF THE EFFECTS OF AEROBIC EXERCISE BY LACTATING WOMEN ON BREAST-MILK VOLUME AND COMPOSITION

Background. The potential risks and benefits of regular exercise durin g lactation have not been adequately evaluated. We investigated whethe r regular aerobic exercise had any effects on the volume or compositio n of breast milk. Methods. Six to eight weeks post partum, 33 sedentar y women whose infants were being exclusively breastfed were randomly a ssigned to an exercise group (18 women) or a control group (15 women). The exercise program consisted of supervised aerobic exercise (at a l evel of 60 to 70 percent of the heart-rate reserve) for 45 minutes per day, 5 days per week, for 12 weeks. Energy expenditure, dietary intak e, body composition, and the volume and composition of breast milk wer e assessed at 6 to 8, 12 to 14, and 18 to 20 weeks post partum. Maxima l oxygen uptake and the plasma prolactin response to nursing were asse ssed at 6 to 8 and 18 to 20 weeks. Results. The women in the exercise group expended about 400 kcal per day during the exercise sessions but compensated for this energy expenditure with a higher energy intake t han that recorded by the control women (mean [+/-SD] intake, 2497+/-43 6 vs 2168+/-328 kcal per day at 18 to 20 weeks; P < 0.05). Maximal oxy gen uptake increased by 25 percent in the exercising women but by only 5 percent in the control women (P < 0.001). There were no significant differences between the two groups in maternal body weight or fat los s, the volume or composition of the breast milk, the infant's weight g ain, or maternal prolactin levels during the 12-week study. Conclusion s. In this study, aerobic exercise performed four or five times per we ek beginning six to eight weeks post partum had no adverse effect on l actation and significantly improved the cardiovascular fitness of the mothers.