Whether sex shall be said to influence the energy requirement will depend upon our use of terms. Boys spend on the average more energy than girls, and men more than women, but it is doubtful if the differences are due to other causes than have been considered above. In experiments in which children were allowed to move about in a small respiration room, boys were found to expend decidedly more energy than girls of the same age and weight; but this was probably due to the greater restlessness and muscular tension of the boys, for in another series in which both boys and girls were kept motionless and relaxed during the observations the difference was not found. Benedict and Emmes found, as noted above, a slightly higher basal metabolism in men than in women of the same height and weight, but attribute this to a difference in the average composition of the body.

While sex alone seems not to be a measurable factor in energy metabolism, the performance of the reproductive func-tions may make large demands upon the maternal organism. As weight increases during pregnancy energy metabolism increases in at least equal proportion. In the last two weeks of human pregnancy Murlin finds the energy metabolism per unit of weight about 4 per cent higher than for non-pregnant women. During lactation, when the entire nutritive requirement of the nursing infant is being met through the mother, the energy needs of the latter are greatly increased. Production of milk involves an extra energy requirement much beyond the actual energy value of the milk secreted. While accurate determinations are not at hand, it seems safe to conclude that the nursing mother taking only moderate exercise may need as much food as a man at muscular work. Liberal feeding of the nursing mother (e.g. up to 2800 to 3000 Calories for a woman with moderate muscular exercise) is not only important for the conservation of her own bodily resources but may prolong the period of lactation and thus be of great value to the child as well.1