Discussion
This study demonstrates that aerobic exercise performed four to six times per week beginning six to eight weeks post partum has no adverse effect on lactation and significantly improves the cardiovascular fitness of the mother. The results are consistent with those of our earlier cross-sectional study11 and indicate that exercise is safe, even for previously sedentary women who are breast-feeding their babies. In both studies, women in the exercise group compensated for their increased energy expenditure with a higher energy intake. In this study, as a result, the rate of weight loss and the decline in the percentage of body fat after delivery did not differ between the exercise and control groups. Although a longer-term exercise program would be expected to reduce body fat, breast-feeding women should be aware that exercise alone will not increase their rate of weight loss unless their dietary intake is controlled. They should be advised, however, that the consequences of rapid weight loss during lactation are unknown. The Institute of Medicine recommends that beyond the first month after delivery, lactating women should not lose more than 2 kg per month23.