Present views concerning the role of fats, carbohydrates, and proteins in the animal organism are not essentially different from views that prevailed a generation ago. An earlier theory, brilliant but untenable in the light of later more exact experiments, was advanced by Liebig about the middle of the 19th century. This theory held that the protein taken with the food constitutes the sole source of muscular energy and that fats and carbohydrates serve only to maintain the body temperature.
Voit, in the course of experiments undertaken to test the validity of Liebig's theory, established the remarkable fact that severe physical work is not accompanied by any material increase in the destruction of protein within the animal organism, as of course would be the case if protein were the sole or even the chief source of muscular energy. The destruction of Liebig's erroneous but definite theory of metabolism naturally led to renewed investigations concerning the function of fats, carbohydrates, and protein; and in this necessary constructive work, Voit became the recognized leader. From his laboratory came investigations and deductions which have since been almost universally accepted as final.
*Paper read at the Lake Placid Conference on Home Economics, June, 1905.
Voit's dietary standards, the practical outcome of all this work, are intended to represent a few fundamental facts. A man of average size gives off in a day a certain quantity of energy (in the form of work and heat). This energy can be measured and often has been measured,"with a fair degree of accuracy. The more physical work a man does the more energy both in the form of work and of heat is given off, and the increase in energy consumption due to work or exercise has also been measured. The daily consumption of energy in the animal organism is obtained at the expense of food. And since it is known just how much energy can be obtained from burning a given quantity of fat, starch, or protein, it becomes theoretically simple, and practically quite possible, to calculate the amount of food that a given individual doing a certain work must consume in order to maintain the equilibrium of the intake and outgo of energy. Such calculations are based on the assumption that food materials when oxidized within the animal organism liberate the same amount of energy as when burned in ordinary air or oxygen, and there is no reason to doubt the correctness of this assumption.
In so far as Voit's dietary standards prescribe the amount of dry food material, of available energy-giving material necessary under given conditions, they have undoubtedly been of very great service. The dietary standards, however, prescribe not only how much available energy the daily food must contain, but also how much of that energy can be most profitably supplied in the form of protein and how much in the form of fats and carbohydrates. Voit's well known average diet, for example, calls for 56 gm. fat, 500 gm. carbohydrates, and 118 gm. protein.
The justification and probable value of such more specific standards of diet is the subject of this paper, and it is a subject on which I think there is room for differences of opinion. It should, however, at once be stated that such differences of opinion do not concern the non-nitrogenous part of the dietary standards. Voit, and with him all competent to have an opinion, are agreed that the fats and carbohydrates need not at all be provided in the ratio of 56 to 500.