In spite of the wide variations observed in the increase of the metabolism with different foodstuffs, there is a distinct uniformity in the majority of experiments which indicates that the act of taking food results in an increased heat production, carbon-dioxide production, and oxygen consumption. With diets predominating in carbohydrates, the quantitative relationship of these increases is more strikingly noticed in the carbon-dioxide production. With the protein diets, the evidence is more pronounced with the oxygen consumption. With the three typical nutrients we may consider as firmly established: (1) that the ingestion of a diet rich in protein results in a marked increase in the total metabolism both for the oxygen consumption and the carbon-dioxide production, this increase being, in general, roughly proportional to the amount of protein ingested; and (2) that with carbohydrate there is almost invariably a marked increase in the excretion of carbon dioxide, and in many instances, especially with sugars other than dextrose, there is likewise an increase in the oxygen consumption. The exact interpretation of the increases with carbohydrate is not so simple as in the case of protein, for there is undoubtedly a formation of fat from carbohydrate. In respiration experiments in which only the carbon-dioxide production is determined, the interpretation of the increase is obviously very difficult. With a fat diet, the evidence is conflicting and little information is obtainable. Pure fat is rarely given in experiments, but is usually combined with other food materials. In those instances in which it has been included in a mixed diet, a small increase has usually been noted. Two of three experiments made by Gigon with pure olive oil implied a distinct lowering of the basal metabolism. In any event, it is safe to conclude that the influence of the ingestion of fat upon metabolism is very small compared with that of sugar and protein.

Although a considerable portion of the literature is devoted to a discussion of the causes of these variations in the metabolism, the two main theories have been (1) the Verdauungsarbeit theory of Zuntz and his scholars, which ascribes the greater proportion of the increased metabolism to the work of digestion, and (2) the specific dynamic action theory of Rubner. Clear-cut evidence for or against these theories is, in spite of the great mass of experimental data, not readily found. Writers are about evenly divided between the two theories. Those upholding the Verdauungsarbeit theory have the distinct advantage of having a definite process to consider. On the other hand, the definition of the specific dynamic action in Rubner's theory, and more particularly the application of the theory, is somewhat obscure and has led to a great deal of confusion. It should be stated, however, that few theories regarding the physiology of digestion have stimulated so much excellent research work as has the specific dynamic action theory.