I've included some abstracts found on Mednets using Medline searches that discuss exercise, dieting, and lactation. 386S-389S Effects of maternal caloric restriction and exercise during lactation. In affluent populations, breast- feeding women often wish to return to their prepregnancy weight as soon as possible postpartum and may restrict energy intake or increase exercise to achieve this goal. Aerobic exercise improves cardiovascular fitness and does not affect milk energy transfer to the infant, but exercise alone is not likely to increase the rate of weight loss unless dietary intake is controlled.
Less information is available on the effect of rapid weight loss.
Exercise enhances maintenance of lean body mass and is therefore a recommended component of any weight loss program. Maternal plasma prolactin concentration generally increases under conditions of negative energy balance, which may serve to protect lactation. Further research is required on the longer-term effect of energy restriction and on the effects of energy balance in lactating women with low fat reserves. A randomized study of the effects of aerobic exercise by lactating women on breast-milk volume and composition. The potential risks and benefits of regular exercise during lactation have not been adequately evaluated.