Changes in the body and its organs during lactation: nutritional implications.

Lactation makes considerably greater demands on the mother's body than pregnancy does in species where the young are helpless at birth and depend on mother's milk for a comparatively long time. These species include many rodents and carnivores and also man. The requirements for the production of milk are met partly from within, by the mobilization of the mother's body tissues, and partly from without, by an increased food intake. Both ways lead to alterations in the composition of the mother's body which may or may not be permanent. Fat laid down in the body during pregnancy is used and there may also be losses of calcium from the skeleton, especially if the calcium intake is low. Whether extra protein is deposited during human pregnancy and lost again during lactation still remains an open question. In rats, the answer depends partly on the intake of protein and energy. The increase in food intake during lactation in some species is so great that the gastrointestinal tract grows considerably to deal with it. In the rat, the small intestine more than doubles its weight. The intestinal wall dilates and the area of mucosa almost doubles. The liver also increases considerably in size owing to an increase in both number and size of cells. After lactation, the liver becomes smaller again, and so does the amount of protein in it, but there is no loss of cells, so that rats that have reared litters have more cellular livers than those that have not. There is no indication whether similar changes occur in women.