During the postpartum period many women are eager to lose weight. Exercise is often considered safer than dieting and is beneficial since it maintains lean body tissue and improves cardiovascular fitness. However, a high level of physical activity may create a substantial caloric deficit. Although studies in rats1 and mice2 have shown no adverse effects of regular exercise on the production of milk, forced exercise reduces the milk volume in dairy cows3. On the other hand, some of the physiologic effects of exercise may enhance milk production. In nonlactating women, exercise triggers short-term increases in plasma prolactin levels4-6. Whether these temporary increases in prolactin (a key hormone during lactation) have any effect on lactation is not known. Regular exercise also increases the sensitivity of adipose tissue and muscle cells to insulin7 and enhances fatty-acid use during peak exertion8-10. These effects may facilitate blood glucose homeostasis and the mobilization of fat stores during lactation.
There is very little information available on the effect of exercise on lactation in humans. In an earlier cross-sectional study, we found no differences in milk volume or composition between eight sedentary women and eight highly trained women who exercised11. However, the subjects were self-selected, and the women in the exercise group may have been unusual in their capacity to exercise intensively while lactating. We therefore undertook a randomized study to assess the effect of initiating an exercise program during lactation. In this paper we report the outcomes related to energy expenditure and lactation.