The deposition of fat in the body is the result of a food supply in excess of that required to provide for the loss of energy in the production of work, heat, and other metabolic changes constantly taking place in the tissues generally. Obesity, therefore, may depend on an excess of food supply combined with a normal or a diminished energy-loss. Or the food supply may be less than normal, or normal, while the energy-loss is reduced, either in consequence of diminished muscular activity or because of diminished oxidation due to disease. Obesity is spoken of as exogenous when metabolism is normal and the fat deposition depends on the disproportion between food and work. It is called endogenous when there is no such apparent cause.

It is doubtful whether there is a true endogenous type of obesity, that is, one which depends for its causation on an abnormal cellular activity, a defective protoplasmic metabolism, either inherited or acquired. Experimental observations, on the oxygen consumed and on the total exchange of gases in the obese, do not indicate any diminution of cellular activity. Allowing for the fat, some of these experiments appear to indicate that in the obese tissue-metabolism is even more active. But the estimation of the food supply shows that the calorific requirements and, therefore, the food requirements are less for the obese than the normal individual, that is, their metabolism is less active. This deficiency in metabolic activity is due mainly to mechanical causes. The increased weight of the body leads to decrease in active exertion and diminution of muscular activity. The muscles become weaker and their metabolism is reduced. The superincumbent layer of fat in the panniculus adiposus acts as a conservator of body-heat, still further reducing the metabolism of heat-producing materials. Alcohol encourages obesity for, by its ready oxidation, it is a source of energy and a sparer of fat. The thyroid glands and the sexual organs have been thought to have some influence on the energy-exchange and consumption of fat. Thyroid extract stimulates metabolism, increasing the consumption of oxygen and the excretion of carbonic acid and nitrogen. In myxoedema the adiposity may be the result of thyroid deficiency but, more probably, it depends on the inertia and muscular inactivity. Possibly the obesity which follows on convalescence from infectious disease may be due to deficient oxidation, the result of degenerative changes in the thyroid. This idea is based on the assumption that the gland is stimulated during fever to increased activity, causing increased oxidation and wasting. Here, too, we must bear in mind that during convalescence the diet is liberal and muscular activity is limited.

Similarly, the increased development of fat, in animals and birds which have been castrated, has been ascribed to a reduction in the oxidative activity, whereas it is due more probably to diminished muscular activity, a placid existence and a liberal food supply. The fact that obesity is frequently associated with impotence is no proof that it is the result thereof. Other factors come into play, notably age and inactivity. Monks and nuns are often quoted as illustrative of the influence of the sex organs, but, even assuming that their activities are in abeyance, it is probable that obesity in this class is due to their sedentary life and excess of fattening foods.

Certain types of morbid obesity are pathological or associated with degenerative changes. In children we sometimes see obesity, . signs of degeneracy, a tendency to giantism, early menstruation, premature development of the sex organs, hirsuties, and precocious puberty. Boys assume the "John Bull" or "Infant Hercules " type of figure, with a somewhat feminine aspect in the shape of enlarged mammae and deficient hair. Girls have an unduly masculine aspect, blotchy skin, bloated cheeks and pendulous mammae. This blotchy aspect of the skin is absent in simple obesity. Many cases of precocious puberty, obesity and adrenal tumours are associated.

Another type of fatty degeneration has been described under the name of " Lymphatic Infantilism." It is more common in boys than girls. Up to the age of about ten years they develop rapidly and are precocious. Then they grow fat, but not taller. The mammae enlarge, the energies and testicular development are arrested, and the boy becomes a mere bag of fat.

In older subjects pathological adiposity is seen in chlorotic girls and in the fat anaemics of later life.

Thus we may recognize three types of obesity. The first is the plethoric type, characterized by simple obesity, high colour and good circulatory powers. In the second or anaemic type the circulatory powers have failed to a greater or less extent, either as a sequel of the plethoric variety or the result of disease. The third type is purely a pathological and degenerative one which occurs in children. The second type passes into a hydraemic variety in which the circulation still further fails and the tissues are more or less waterlogged.

The influence of heredity is very marked in simple obesity. Just as we find a tendency to the prolongation of life to extreme old age in some families, so we may find a constitutional tendency to obesity in others. In quite half the cases such an inherited tendency is present. Some individuals will get fat on a limited diet, whereas others will eat largely of all kinds of food and yet remain thin. The fattening process is not merely a question of diet. In the animal world these inherited tendencies are developed by selective breeding.

Age, sex, temperament, occupation and climatic conditions are all factors affecting obesity. A dietary which is sufficient for the needs of the body in a cold climate or during vigorous bodily labour, will prove much too liberal during idleness or in hot weather, and the extra food is liable to be deposited in the tissues as fat. It is quite rare to see a fat agricultural labourer, for his diet is limited and his output of energy large. It is quite common to see a fat brewer's drayman, for his work is comparatively light and his consumption of beer is usually excessive.