We have seen at what low levels of proteid intake, nitrogen equilibrium can be established, and we may well have faith in the conception of an endogenous proteid katabolism which involves only minimal quantities of proteid. Further, we have observed the constant tendency of the body to maintain a condition of nitrogenous equilibrium, even with varying income, and how slow the body is to lay by nitrogen on a rich proteid diet, even when long deprived of proteid food; a fact difficult of explanation except on the assumption that the real need of the body for nitrogen is small, and that the tissues habitually carry a relatively large reserve of nitrogenous material. We may assume with Folin that "all the living protoplasm in the animal organism is suspended in a fluid very rich in proteid, and on account of the habitual use of more nitrogenous food than the tissues can use aB proteid the organism is ordinarily in possession of approximately the maximum amount of reserved proteid in solution that it can advantageously retain. When the supply of food proteid is stopped, the excess of reserve proteid inside the organism is still sufficient to cause a rather large destruction of proteid during the first day or two of proteid starvation, and after that the proteid katabolism is very small, provided sufficient non-nitrogenous food is available. But even then, and for many days thereafter, the protoplasm of the tissues has still an abundant supply of dissolved proteid, and the normal activity of such tissues as the muscles is not at all impaired or diminished. When 30 grams or 40 grams of nitrogen have been lost by an average-sized man during a week or more of abstinence from nirogenous food the living muscle tissues are still well supplied with all the proted they can use That this is so, is indicated on the one hand by the unchanged creatinin elimination, and on the other by the fact that one experiences no feeling of unusual fatigue or of inability to do one's customary work. Because the organism at the end of such an experiment still has an abundance of available proteid in the nutritive fluids, it is at once seemingly wasteful with nitrogen when a return is made to nitrogenous food. This is why it only gradually, and only under the prolonged pressure of an excessive supply of food-proteid again acquires its original maximum store of this reserve material."

We may reasonably suppose that the reserve of proteid present in the body is contained in the fluid media, and not as a part of the living protoplasm. Further, we are apparently justified in the belief that the sole form of proteid katabolism which is vitally important for the welfare of the body is the endogenous katabolism. This must be provided for adequately and indeed liberally, and in addition tbere should be sufficient intake to keep up an abundant supply of reserve proteid, but beyond these necessities there would seem to be no legitimate demand for additional proteid. The voluminous exogenous proteid katabolism so conspicuous in most individuals would seem to have no justification in fact, or in physiological reasoning. What good, for example, can be accomplished by this constant splitting off of nitrogen, with its subsequent speedy removal from the body? The organism can neither use it nor store it up, and why therefore should this daily burden of an excessive and accelerated proteid katabolism be borne? As we have seen, the energy of muscle work is derived mainly, and can come wholly, from the breaking down of non-nitrogenous materials, fats and carbohydrates. The very fact that an intake of say 120 grams of proteid is followed at once by the removal of the larger part of the contained nitrogen, as a result of the exogenous katabolism of the body, would seemingly warrant the view that the proteid so decomposed might advantageously be replaced by a corresponding amount of carbohydrate. In muscle work, as in heat production, carbohydrate and fat are the materials burned up, or oxidized. Proteid, on the other hand, is not so oxidized, at least not the nitrogen-containing portion of the molecule.

There are apparent only two possible reasons for assuming a need on the part of the body for the high exogenous katabolism of proteid so commonly observed. The one is that the carbonaceous residue left after the cleavage of nitrogen from the proteid molecule is better adapted for the needs of the body than either carbohydrate or fat. Although this does not seem very probable, it is of course a possibility and merits consideration. Feeding experiments, with a comparatively small proteid intake, continued over a sufficient length of time, would show conclusively how much weight should be attached to this hypothesis. The other possibility is that the body may derive some advantage from the presence, in the tissues and fluids, of the varied nitrogenous cleavage products split off from proteid so abundantly in exogenous katabolism. These substances are mainly amino-acids on their way to urea, and there is no apparent reason why they should be of service to the organism. Still, the processes going on in the tissues and organs of the body are intricate and not wholly understood, and we can conceive of some useful function of which as yet we have no knowledge. In the construction of tissue proteid, for example, as in a possible synthesis out of the fragments formed by hydrolytic cleavage, it is not impossible that certain comer-stones are needed, and that in order to obtain these there must l>e a more or less wasteful breaking down of food-protcid. However improbable this may seem, it, like the preceding hypothesis, can be tested in a way by adequate feeding experiments, which shall determine the effect on the body of a low proteid intake continued over a long period of time. On the other hand, it is equally plausible, and for some reasons more probable, to assume that this excessive exogenous katabolism may be in a measure prejudicial to the best interests of the body; that the many nitrogenous fragments formed in the efforts of the organism to prevent undue accumulation of reserve proteid may in the long run do as much harm as good.

Further, there is reason in the question whether the continual carrying of excessive amounts of nitrogen reserves in the shape of soluble proteid in the blood and lymph, and iu the meshes of tissue and cell protoplasm, is advantageous for the maintenance of the highest degree of efficiency? We all recognize that an excessive accumulation of fat is distinctly disadvantageous to the welfare of the body, and there is, physiologically speaking, equally good ground for considering that the storage of unorganized proteid ia amounts beyond all possible requirements of the body may be equally undesirable. Because less tangible to the eye, the accumulation of unnecessary proteid is not so easily recognizable, but this fact does not diminish the possible danger which such accumulation may constitute. It must be granted, however, that we are dealing here with hypotheses and not facts, but though hypothetical the suggestions made are of sufficient moment to merit attention and experimental study. In a later chapter, we shall have occasion to present some facts bearing on these questions.

In the meantime, we may lay due stress upon the significance of these views regarding proteid katabolism. We must accept as settled the general idea that there are two distinct forms of proteid katabolism within the body; one form representing the decay of tissue or cell protoplasm, small in amount, with its own particular decomposition products, and absolutely essential for the continuance of life. The other form, the so-called exogenous katabolism, runs a totally different course with distinctive side-products and end-products; it is variable in extent, in harmony with variations in proteid intake, and subject to the suspicion that at the level ordinarily maintained it constitutes a menace to the preservation of that high degree of efficiency which is an attribute of good health.