In infancy a moderate degree of obesity is natural, for the food supply is good and bodily activity small in the early months of life. Excessive fatness in the breast-fed babe is almost invariably due to an excess of lactose or fat in the milk supply. Sometimes there is an inherited tendency also. Dr. G. A. Sutherland found, in the case of a baby who weighed 28 lb. at seven months of age, that the mother's milk contained 8.2 per cent of lactose. The excess of fat rapidly disappeared after weaning. In bottle-fed infants the diet is generally too rich in fat, from added cream, or in carbo-hydrates due to the addition of some one or other of the proprietary foods, usually a malted preparation. This excessive fatness is regarded by many mothers as a sign of health and strength. As a matter of fact it is distinctly injurious. It predisposes to eczema and other skin diseases, to bronchitis, and to bowing of the legs from the undue weight of the body. These very fat babies seem to have deficient vital resistance and to stand acute disease badly. Another danger is that of acidosis or acetonaemia on the administration of anaesthetics. Some are anaemic and others may show signs of scurvy.

In youth, especially about puberty, a moderate degree of fatness should be present as an indication that the food supply is sufficient for growth as well as nutrition. Undue fatness is generally combined with the hereditary tendency, excessive greediness, eating between meals and a large consumption of sweets. It is more common in girls than boys because of their more sedentary life and the feminine tendency to a general deposit of fat, aesthetically charming in moderation. Yorke-Davis reported the case of a boy, five years old, weighing 117 lb., who was fairly intelligent and ate all day. His parents and their other children were normal. Many similar cases are on record.

In the animal world it is difficult to produce fatness during the period of growth. Calves and lambs are easily kept fat while taking milk, but when they are turned out to live on the ordinary diet of the fields they do not store up fat as a rule, any excess of food being devoted to growth. After full growth has been attained they can be readily fattened if the diet is suitable; so, too, in the human race. Once full growth is attained, fat will be deposited if any excessive intake of potential energy in the form of carbon-containing foods is not counterbalanced by the output of kinetic energy in the form of exercise or work. Up to middle age life is active and metabolism correspondingly rapid. After forty the adult tends to become less energetic, more placid in his mode of existence, and to put on fat because of the deficient metabolism. This is still more evident in the female than in the male. Increase in age is often accompanied by increase in material prosperity, greater luxury in diet, and reduction in work or exercise. The working-class woman eats more, drinks more, and works less. She is better clothed and takes a tram or 'bus instead of walking. Among the richer classes the tendency of increased prosperity is towards a reduction in the expenditure of energy in moving about, a diet which is certainly no less liberal and nutritious and often more dainty and tempting, and frequently an increased consumption of alcoholic drinks.

People who eat largely and take little exercise without getting fat may be fortunate in that they can indulge freely in the pleasures of the table. The individual with a tendency to obesity must not be tempted to follow such an example and will have to be rigidly careful. Instinct and craving are untrustworthy guides. The digestive, absorptive and assimilative capacities vary very greatly in different subjects, but the final conclusion on the study of obesity is that, with rare exceptions, it is mainly due to an excess in the food supply, and that it is only on these lines it can be successfully treated. The general principles of diet which will now be considered are those appropriate to simple obesity, and must be modified in accordance with the particular patient, the age, the mode of life, and especially in those instances which are pathological in origin or have developed pathological changes. Occasionally it is necessary to reduce the excessive fat quickly. More often it is quite sufficient and much safer to reduce it slowly.

Treatment resolves itself, therefore, into strict limitation of the diet, exclusion or limitation of alcoholic drinks, light clothing and cool rooms, cold bathing, and plenty of exercise. Exercise augments consumption, but it increases appetite and is insufficient, by itself, to counteract excessive eating. The loss of heat from the surface of the body is an adjunct, though not a very powerful one, in the consumption of fat. Finally, having reduced the obesity to reasonable limits, a minimum diet must be established to prevent re-accumulation.

Fat is a carbon mixture. All the food-stuffs, viz., proteins, fats, and carbo-hydrates contain carbon. That fat in the body can be derived from protein in the food is shown by the deposition of fat in carnivorous animals and the fattening properties of nitrogenous foods in the feeding of cattle. In the ripening of cheese some of the protein is converted into fat. Chiefly, however, the fat is derived from the fats and carbo-hydrates in the diet. As a rule, it is the latter rather than the former which are to blame in excessive obesity, but the effects vary in different individuals. The essential feature in the diet is a diminution in the carbon in the food, either in the form of fat or carbo-hydrate. It is not necessary to combine this with an increase in the consumption of proteins. The excess of protein food may lead to ills worse than mere obesity. An excessive protein dietary has been supposed to cause increased oxidation and consumption of fat. There is no reliable proof of this supposition. More probably the wasting of the patient fed on such a diet is due to the deficient supply of carbon-containing food and to disordered metabolism, a pathological rather than a physiological mode of reducing obesity. An increase in the consumption of green vegetable food is advisable, for such foods are comparatively innutritious and by their bulk give a feeling of fullness.