Activity, age, and size are the most important factors affecting the total food requirement of the body, but several other conditions, such as bodily constitution and environment, may have measurable influence. Since the food requirement of the adult is more accurately known than that of the growing organism, it will be best to consider the conditions affecting the energy metabolism of the adult first and the demands of growth later.

Basal Metabolism Of The Adult

The basal rate of energy metabolism, as shown by the heat production (determined either by direct or indirect calorimetry) at complete rest and at a sufficiently long time after the last meal to eliminate the direct effects of food, has now been studied in considerable detail. In the healthy adult this basal metabolism depends chiefly upon the size, shape, and composition of the body and the activity of certain internal processes. It may or may not be appreciably influenced by the temperature of the surroundings.

Influence Of The Size, Shape, And Composition Of The Body

For different adults of the same species the energy metabolism and therefore the total food requirement as a rule increases with the size, but not to the same extent that the body weight increases; so that the requirement, though greater in absolute amount, is less per unit of body weight in the larger individual than in the smaller. The energy metabolism increases in proportion to the surface rather than the weight. Thus, Rubner collected the following data from experiments upon seven different dogs, all full grown but differing greatly in size.

Here the heat production in calories per kilogram was over twice as great in the smallest as in the largest dog, but the total metabolism was nearly proportional to the surface area throughout.

That the relationship of energy metabolism to body surface is not due simply to loss of heat through the cooling effect of the environment will be apparent from the observations upon the regulation of body temperature.

Armsby, in his Principles of Animal Nutrition, quotes the explanation offered by von Hosslin - that the internal work and the consequent heat production in the body are substantially proportional to the two thirds power of its volume, and since the external surface bears the same ratio to the volume, a proportionality necessarily exists between heat production and surface.

Largely as the result of Rubner's work, it became customary to express energy requirements in terms of surface; but, on account of the difficulties involved in actual measurements, the surface was customarily computed from the weight, usually by Meeh's formula S = W⅔ X C or S = 12.33√W2, in which S represents the surface and W the weight, the constant 12.3 having been found by Meeh in a series of measurements of men.

Benedict found that the basal metabolism of normal men and women per unit of surface as computed from the weight by the Meeh formula is by no means constant, varying from 29 to 40 Calories per square meter per hour among 89 men, and from 26 to 38 Calories per square meter per hour among 68 women.

Recently DuBois and DuBois have made a new series of measurements of body surface in which they find that Meeh's formula gives results which are much too high, probably because Meeh's measurements were made on thin men. Tabulating the results of other measurements with their own, they find that among the 20 cases of direct measurements of body surface which had been reported up to 1915, the errors in results computed by Meeh's formula range from -7 to +36 percent. Differently stated, if the principle of Meeh's formula be employed it would be necessary to vary the "constant" from 9.06 to 13.17 in order to express the relationships of weight and surface actually found among these 20 individuals.

The errors involved in computing the surface from the weight alone are therefore much greater than were formerly supposed. DuBois and DuBois have devised two new methods by which the surface may be computed with much greater accuracy : (1) from a series of nineteen measurements of different parts of the body, the surface of each part being computed and the results added together ("linear formula"), and (2) a "height-weight formula" which these authors have derived mathematically from the data of all available measurements of height, weight, and surface.

The height-weight formula may be written thus:

A= W0.425 x H0.725 x C or in the form:

Log A = (Log W x 0.425) + (Log H x 0.725) + 1.8564 in either of which A = Surface area in square centimeters H = Height in centimeters W = Weight in kilograms C = A constant (71.84)

In connection with this formula the authors give also a chart * from which the approximate surface area may be obtained at a glance if height and weight are known. The data given in the accompanying table have been taken from the DuBois chart.

Table Showing Surface Area In Square Meters For Different Heights And Weights According To The Height-Weight Formula Of Dubois And Dubois

* Reproduction of the chart may be found on page 126 of the third edition of Lusk's Science of Nutrition.

On applying the "height-weight formula" to the recorded energy metabolism of the large number of men studied in Benedict's laboratory, as well as in his own, DuBois finds that all the data for men under 50 years of age are within 15 per cent of the average basal heat production of 39.7 Calories per hour per square meter of surface area properly computed, and that 86 per cent of all the cases are within 10 per cent of the average. Means, using the more accurate "linear formula," finds all of his 16 normal cases (9 men and 7 women) and also most of his obesity cases to fall within DuBois' "normal limits" (i.e. within 10 per cent of DuBois' average of 39.7 Calories per square meter per hour). DuBois1 believes that one may "feel certain that with men between the ages of 20 and 50 the (basal) metabolism of each individual is proportional to his surface area whether he be short or tall, fat or thin."

Differences of build (shape of body) are associated not only with varying ratios of weight to surface but also with differences of fatness, i.e. of body composition. The thin man, besides having a greater surface in proportion to his weight, differs also from the stout man in that a larger percentage of his body is actual protoplasm. Since the metabolism of the body depends more upon its weight of protoplasm (active tissue) than upon its total weight, we have here an important reason for believing that the food requirement will be greater in a tall, thin man than in a shorter and fatter man of the same weight.

Von Noorden tested this question by observing the metabolism (for one day without food) of two men of different build but nearly the same weight. The results were as follows: 1st man, thin and muscular, weight 71.1 kilograms - 2392 Calories = 33.6 Calories per kilogram; 2d man, stout, weight 73.6 kilograms - 2136 Calories = 29.0 Calories per kilogram. These two men had nearly the same weight but differed in height, in body composition, and in energy expenditure.

1 American Journal of the Medical Sciences, June, 1916, page 786.

Even with the same height and weight there may be differences in the composition of the body. Thus a man of average height and weight but large-boned and loosely built will be of less than average fatness; a man of the same height but less broad-shouldered must be somewhat fatter in order to weigh the same. Hence equality of height and weight does not necessarily imply the same shape and composition of body. Benedict finds among normal adults of like height and weight the basal metabolism of athletes about five per cent higher, and that of women about five per cent lower, than that of average non-athletic men. He attributes these divergencies to differences in body composition, holding that women have somewhat more fat, and athletes somewhat less, than non-athletic men of the same weight and height.