The energy cost of human lactation.

I . The energy values of the diets of twenty-three women who were breast-feeding and thirty-two who were bottle-feeding their babies were determined by 7 d weighed surveys. All the subjects were healthy and living at home, and their babies were thriving. The lactating mothers took, on average, 591 kcal (2.5 MJ)/d more than those who were not lactating. 2. Both groups were losing weight, on average. The estimated contribution of such losses to the total energy supply was added to and the amounts expended on basal metabolism deducted from the dietary energy intakes. Since the activity of each group was fairly similar, it was possible to conclude that the average amount of energy available to support lactation was 618 kcal (2.6 MJ) daily. The average energy value of the milk produced was estimated from the weights of the babies to be 597 kcal (2.5 MJ) daily. 3. Critical evaluation of those averages, and of the assumptions on which they were based, led to the conclusion that the energy exchanges in human lactation have an efficiency of 90 yo or more, with a lower limit of about 80 %. 4. The additional supply of 600 kcal (2.5 MJ) in the daily diet should suffice to support lactation and a ‘round figure’ of 500 kcal (2.1 MJ) daily may be regarded as reasonable in official recommended allowances.