It is recommended that nursing mothers do nothing consciously to try to lose weight during the first two months postpartum as this is the time that your body needs to recover from childbirth and establish a healthy milk supply. Crash diets, fad diets, liquid diets, weight loss medications, and any other plan that results in quick weight loss may cause your body to release contaminants that are stored in fat into your bloodstream making their presence more likely in your milk. The best way to lose weight while breastfeeding is to increase your activity level and decrease your caloric intake.

Combining calorie-cutting with exercise will make your weight loss plan more efficient. Exercise has not been shown to interfere with milk production as long as weight loss is slow and not extreme. You will want to take care not to lose more than one pound a week or four pounds a month while you are breastfeeding. While exercising you may find that nursing right before you begin and wearing a support bra during your routine makes you more comfortable, especially if the type of exercise you are doing is jarring, such as running or jumping. If you notice that your baby is fussy at the breast right after you exercise, it may be that he is sensitive to the buildup of lactic acid in your milk; although this is more common in babies whose mothers engage in very strenuous exercise. Again, nursing him immediately before exercising should help you avoid this problem. Occasionally excessive and rigorous upper arm movement can contribute to plugged ducts. If you are prone to plugged ducts, you may want to consider altering your exercise routine if it includes such movement.

Lately there have been some studies that might indicate that the composition of breastmilk is different for a short period following exercise, perhaps making it less antibody-rich. Other studies have not shown this to be a conclusive finding. This possibility should not dissuade you from exercising because even if there is some validity to the study, the relatively short period that your milk would be affected is not significant enough to affect your baby's overall health. And however many anti-infective properties your baby receives during a feeding following your exercise routine is much more beneficial than a formula feeding without antibodies!

Other sources: "Eat Well, Lose Weight While Breastfeeding"