It is not certain that sex has any direct influence upon the amount of food needed. Women require less than men because they are commonly smaller, have a larger proportion of fat than the average man, and usually do less muscular work. When these points have been allowed for, there does not seem to be much difference. In a healthy woman in bed in hospital awaiting an operation the amount of food taken, reckoned as calories per kilogramme, was about the same as in men under similar circumstances. The diet in pregnancy should be liberal, but need not be excessive. It has been calculated that the average daily deposition of material in the foetus during the last three months of pregnancy is not more than about 3 grammes of protein and 3 1/2 of fat, with '7 grammes of salts. Material is also required for the development of the uterus and mammae; and there is often a general deposition of fat at this time, to be called on later to meet the demands of lactation. The oxidation processes are increased during pregnancy because the weight is increasing; the excretion of carbo-dioxide per kilogramme remains at the same level.
Observations on animals show that the loss of protein in parturition is often greater than the gain during pregnancy : that is to say, the mother does not store as much protein from the food as is laid down in the foetus and, therefore, supplies some to the foetus from her own tissues. This was not invariably the case, however, and it is possible for the mother with a sufficient supply of suitable food to assimilate and retain sufficient protein for the foetus without any drain upon her own tissues. During and after delivery there is a considerable loss of protein, but with a rich diet nitrogenous equilibrium may be attained towards the end of the first week of the puerperium.
During lactation from 500 to 1,000 calories is given out from the body daily in the form of milk, and an equivalent amount of food should be added to the ordinary diet. It has been shown in animals that the mother may retain sufficient protein from the food to make up that lost in the milk. The diet should include abundance of protein and fat.