It has been shown in a previous chapter that different normal individuals of similar age and physique are substantially alike in their energy requirement when performing equivalent amounts of muscular work, and that it is primarily the muscular activity, and not personal idiosyncrasy or the amount of food eaten, which determines the amount of energy transformed in the body. A dietary standard of high fuel value, and designed to maintain metabolism on a high energy level, provides, therefore, primarily for a large amount of muscular work. If this work is not performed and the food continues to be eaten and digested, we may expect to find a storage of fuel in the body chiefly in the form of fat, and this is true whether the surplus food eaten is carbohydrate, fat, or protein. Thus the store of body fat which a person carries is the most reliable indication as to whether the amount of food habitually eaten is or is not properly adjusted to the work performed. The storage of fat does, however, in itself modify the food requirement. While it is true, as has been shown, that, as between a lean and a fat man having the same weight, the lean man will have the greater food requirement, yet it is also true that when any given man becomes fat, his increased size of body calls for increased metabolism of energy. The work involved in walking, for example, will increase in proportion to the weight moved (i.e. to the weight of the body as a whole); and the work of respiration will increase about in proportion to the weight of that part of the body which must be moved with the expansion and contraction of the lungs; while, if fat is deposited in such a way as to interfere directly with the free play of the muscles, there may be an actual lowering of muscular efficiency, so that a larger expenditure of energy may be required in order to produce a given amount of work. If the liberal diet is continued and the digestion remains normal, the storage of fat will continue until it raises the energy expenditure of the body to a point where the food is no longer in excess. If the store of fat carried when this point is reached is excessive, the fuel value has been too high; if the store of fat is not excessive, the fuel value of the diet, although greater than would have been necessary to maintain the body at its former weight, has not been too high, and the body has acquired an asset whose utility may not always be recognized in health, but which may be of great value in case of accident, illness, or exposure.

Opinions differ somewhat as to the desirable degree of fatness as indicated by the relation of height to body weight.

Hill* estimates the average height at 25 years of age as 5 feet 3 inches for women and 5 feet 8 inches for men, and the corresponding average weights as 119 and 150 pounds respectively. He considers that variations of 10 to 15 per cent above or below the average should be considered normal. According to this estimate the woman of 5 feet 3 inches should weigh not less than 102-107, nor more than 131-136 pounds, and the man of 5 feet 8 inches not less than 128-135, nor more than 165-173 pounds. These figures are exclusive of clothing. Hill considers as "fat" those persons whose weight exceeds the average by 15 to 30 per cent, and as "over fat" those who exceed by more than 30 per cent, i.e. over 155 pounds for a woman 5 feet 3 inches or over 195 pounds for a man 5 feet 8 inches.

* Recent Advances in Physiology and Biochemistry.

Symonds has published* the average relation of height to weight in both men and women at different ages, as computed from the records of accepted applicants for life insurance in the United States and Canada. The results are found in the following tables; that for men being based on 74,162 and that for women on 58,855 cases. In all these cases the height includes shoes and the weight includes ordinary clothing.

Symonds's Table Of Height And Weight For Men At Different Ages Based On 74,162 Accepted Applicants For Life Insurance

(Medical Record)

* Medical Record, September 5, 1908; and McClure's Magazine, January, 1909.

Symonds's Table Of Height And Weight For Women At Different Ages

Based on 58,855 accepted applicants for life insurance (McClure's Magazine).

From a study of the records of body weight in relation to the mortality records Symonds concludes that among young people the greatest vitality coincides with a weight somewhat above the accepted average, while with middle-aged and elderly people a condition of slightly less than average fatness is most favorable to vitality and longevity. Another way of stating the same facts is: That the average of healthy men and women keep themselves slightly too thin while young, and allow themselves to grow slightly too stout as they grow older.

Evidently, however, the optimum is very near the average of the accepted applicants as shown in the tables, and Symonds uses these figures as standards in his computations and discussions of the influence of overweight and underweight on longevity and on mortality from specific diseases. Symonds's data therefore support the opinion that the average degree of fatness of healthy American people is just about the most advantageous fatness for them to maintain. Whatever we accept as the ideal relation of weight to height, it is obvious that the proper standard for fuel value of the diet is that which will preserve the desired degree of fatness while sustaining the desired amount of activity. If good authorities differ in standards for fuel value, it is because, consciously or unconsciously, they contemplate different amounts of muscular activity or the maintenance of a different physique.

That the amount of food required per day to maintain a healthy adult at the desired body weight will vary considerably with age and size and enormously with extremes of muscular activity has already been explained at some length in Chapter VII (Conditions Governing Energy Metabolism And Total Food Requirement. Basal Metabolism Of The Adult) and need not be discussed further here. Unless it is desired to increase or decrease the body weight, the optimum energy intake of the healthy adult will be that which coincides with the total energy expenditure; in other words the "standard" and the "requirement" will in this case be the same.