Additional energy needs for an exclusively breastfeeding woman are approximately 670 kcal/day. If one allows for gradual weight loss, the net increment needed is about 500 kcal/day. There is little evidence of energy-sparing adaptations in basal metabolic rate or dietary-induced thermogenesis during lactation, although physical activity may be reduced during the early postpartum period. In women with adequate fat reserves, moderately negative energy balance is not likely to affect lactation. The recommended increment in protein intake during lactation has been estimated to be about 15 g/day, based on a milk protein concentration of 11 g/liter. However, if one takes into account the protein cost of non-protein nitrogen in human milk, the recommended increment in protein is about 20 g/day. The latter value is consistent with data from nitrogen balance studies in lactating women. Low protein intakes are unlikely to affect milk volume but may alter certain fractions of milk nitrogen.