Exercise improves cardiovascular fitness, plasma lipids, and insulin response in lactating women. However, exercise alone, without caloric restriction, does not promote weight loss. Once lactation is established, over-weight women may restrict their energy intake by 500 kcal/day to promote a weight loss of 0.5 kg/week without affecting the growth of their infants. Moderate exercise does not affect maternal immune status or levels of sIgA, lactoferrin, and lysozyme in human milk. Further research is needed to determine the effect of exercise and energy restriction on maternal bone health and immune status and composition of human milk.