As one of the most important functions of food is the maintenance of animal heat, it can easily be demonstrated that the amount of food required is directly proportional to the superficial area of the skin. Rubner formulated the law that metabolism is proportional to the superficial area of an animal, or, in other words, that it depends upon the amount of heat lost at the surface. The means for the regulation of the temperature of the body are twofold: (1) external cold stimulates an increased production of heat; (2) heat is lost from the surface of the body by a variation in the calibre of the cutaneous blood-vessels, and probably in addition an increase in the evaporation of water. Warming the food ingested and the air inspired, and the losses through the heating of the urine and faeces, are factors of very little significance compared with radiation and conduction in the manner just mentioned, and in a less degree by evaporation of water from the lungs and skin. Of the total amount of heat lost by the body, the skin is approximately responsible for 87.5 per cent., the lungs for 10.7 per cent., and the excreta for 1.8 per cent.

The larger the superficies of the body relative to its bulk, the greater is the amount of heat lost by radiation, and the greater in proportion is the metabolism. An infant manufactures 90 calories per kilogram in twenty-four hours, whereas an adult produces only 32 calories. It is obvious that an increase in the food is necessary to engender the extra quantity of heat, and herein lies the explanation of the apparent paradox, that a thin man eats more food than a fat man and yet is not always successful in losing his thinness. When two objects have the same bulk, yet different superficial areas, the one with the greater superficial area will lose the greater quantity of heat, and therefore a greater quantity of fuel must be supplied to it in order that both may possess the same temperature. When two objects differ in bulk, the smaller of the two will part with a greater quantity of heat in proportion to its bulk, and therefore will require a proportionately greater quantity of fuel to enable it to maintain its temperature at the same height as the other.

For this reason it has been found possible to estimate the amount of food required by knowing the amount of skin surface. Children and small people have a much larger skin surface in proportion to their weight than adults or larger people. A child of 10 lbs. weight has a skin superficies of 3 square feet, whereas a man weighing 200 lbs., or twenty times as much, has a skin area of 21 square feet, only seven times as much. A child of 10 lbs. weight, therefore, requires one-seventh as much food as a man weighing 200 lbs., instead of only one-twentieth as much.

Although of very little value from the point of view of practical dietetics, it is of interest to know that one can calculate the amount of food required by different people of varying sizes, weights, and skin surfaces, by multiplying the weight in pounds by the factor 4.25 and the skin area in feet by the factor 80. The sum of these two products will represent the number of calories per day required to balance the loss sustained by the dissipation of heat from the body and the loss of energy expended in vital work. If the subject is doing severe muscular work, then the factor 7 should be used instead of 4.25.

Tables have been constructed showing at a glance the normal height, weight, superficial areas and number of food units or calories required daily in people of both sexes and at all ages, but they are more interesting than practical. It is of more importance to know that a child under two requires three-tenths of the quantity of food of a man doing moderate work; a child from ten to thirteen three-fifths, and a boy of fifteen four-fifths the amount of food of an average man.