Obesity is a diseased condition of the body, depending upon alteration in the proper balance of nutrition, with an accumulation of fat in and between the tissues.

It may affect either sex, but is much more common among women, in whom it is particularly noticed during and after the climacteric.

It may develop at any age after early childhood. When appearing early it is usually the immediate result of heredity. It is, however, most common after middle age, when the functional activity of the body is diminished and when habits of systematic exercise are replaced by a sedentary life of luxury and ease, combined with overeating.

Aside from heredity, an important cause of obesity is excessive eating, which is a fault among the majority of persons who have passed middle life. When the growth of the body has been completed there is a stage of equilibrium in weight which lasts for several years, and the waste matter of any excessive food taken is eliminated through the emunctories, which are kept active by exercise and work. After middle age, activity is considerably diminished, while the habit of fully satisfying a vigorous appetite may be continued, resulting in the consumption of a larger amount of food than is required for maintaining the vital functions. It is extremely difficult for people who feel in robust health and whose mental is proportionately greater than their physical activity, to realise that they not only require less food than they did ten, fifteen, or twenty years earlier in life, but that eating as they do they may be producing positive harm by overloading the excretory organs. It is for this reason that many persons accumulate weight after reaching a certain advanced period of life.

According to Sir Henry Thompson's view, overeating in the first half or so of life may be relieved by occasional bilious attacks, which enable the system to cast off accumulated waste, but later "the unemployed material may be relegated in the form of fat to be stored on the external surface of the body or be packed among the internal organs, and thus he or she may become corpulent and heavy".

The constant use of alcoholic stimulation in its milder forms, such as beer and ale, favours obesity, and in many individuals seems to be a direct cause of it. There are other cases in which it is difficult to attribute the accumulation of fat to any faults in either eating or drinking. Very stout persons are occasionally extremely moderate eaters, and in these cases the difficulty lies apparently in poor oxidation and insufficient exercise.

The condition of obesity is somewhat modified by climate and race. It is less common in the United States than it is among elderly women in England and Germany.

Not infrequently obesity occurs in connection with pronounced anaemia and hysteria. It prevents such patients from taking an ordinary amount of exercise. This is owing to the increased weight and difficulty of moving about, as well as to the lassitude and positive dislike for muscular exertion which is felt by them. Obesity once established is likely to increase unless properly treated.

So long as the fat is simply stored up as adipose tissue, and between and not within the cellular elements of the body, it is comparatively harmless, excepting as it indicates a disproportion between functional activity and the amount of fuel taken in to develop force, and excepting also as it may interfere with personal convenience in movement and cause greater muscular fatigue from the increased work thrown upon the muscles in supporting so heavy a body. In other cases (and this may be the outcome of any advanced case of polysarcia) the fat may invade the cellular elements of the body, particularly the muscles, and fatty degeneration is produced in the muscular tissues, which very seriously weakens and impairs their functional activity. The special danger in all cases of this class is that the heart muscle may become affected in this manner, which renders it liable to cease beating whenever the slightest additional strain is thrown upon the circulation. It follows that it is most desirable in the dietetic treatment of obesity to determine the original cause of fat accumulation, and to ascertain to what extent it is accompanied by fatty degeneration.

Bauer says: "The fat stored up in the body acts in like manner with the fat contained in the food, since it likewise lessens the waste of tissue and secondarily the oxidation. Thus we understand why abstinence can be longer borne by organisms rich in fat than by those poorly furnished with it, the former consuming less of the albumin of their organs. The stock of fat stored up in the body is moreover the cause why corpulent individuals frequently continue to gain in bulk although they are not in the habit of indulging in food immoderately.

"If an increase of albumin be desired without a considerable addition to the store of fat, a liberal allowance of albumin with relatively small quantities of carbohydrates must be provided. If, on the other hand, a substantial addition to the fat appear desirable, the food must contain less albumin and more carbohydrates, with a fair proportion of fats".

Voit claims that albumin circulating in the blood is more readily oxidised than either fats or carbohydrates, and that the fat existing in the tissues acts like free circulating fat, not primarily by saving proteids from oxidation, but by causing a larger proportion of them to enter into the organised tissues, thus saving tissue waste. The tissues use up the circulating albumin, and, not being exhausted thereby, go on and oxidise the fat besides, and in this manner the store of fat in the body is reduced on a meat diet.

An exclusive lean-meat diet increases the circulating albumin, and, the nitrogenous waste being correspondingly increased, fat does not accumulate in the body. If now fat be added to the diet, then fat is deposited in the body.