We must now consider briefly the fate of the food-stuffs after absorption and the way in which they serve to nourish the body. When we attempt to trace the course and the transformations of these materials from the time that they disappear from the intestine to the final reappearance of their elements as urea and water in the urine, and carbon dioxide and water in the expired air, we find ourselves plunged at once into the most difficult problems of physiology, a full discussion of which, however interesting to the student of dietetics, would not in the present state of our knowledge,or rather our ignorance, yield any practical information bearing upon dietetics. We shall therefore confine ourselves to a brief summary of the modern view as to the processes by means of which foods are oxidized to furnish energy, or built up into tissues.
When a meal of protein is taken the major part of the nitrogen which it contains is excreted in the urine in a short time; if protein be introduced directly into the intestines of an animal the same is true; as much as 40-50 per cent of the nitrogen may appear in the urine within a couple of hours. These facts, which were formerly supposed to show that the tissues preferred protein food and therefore used it up at once, are now regarded as evidence that the nitrogenous part only of the protein molecule is thus rapidly passed out, and that the rest of the molecule is kept in the body for immediate or future use (Folin); it is not oxidized simultaneously with the excretion of the nitrogen, for the heat given off is not greater after a meal of protein. Since protein foods are absorbed in the form of aminoacids we must suppose that the nitrogen is split from these bodies as ammonia. It is not known for certain where this splitting takes place but it is probable that it is within the intestinal wall. Nencki has shown that there may be four times as much ammonia in the portal blood during digestion as in the systemic circulation, and this ammonia may be that split off from the aminoacids on its way to the liver to be converted into urea and passed out in that form in the urine. The denitrified remainder of the aminoacid would still possess a considerable caloric value. A gramme molecule of leucin, for instance, furnishes on combustion 130 calories; and the oxyacid which may be supposed to be formed by the denitrification of leucin gives 110 calories. The denitrification of glycocoll, again, would only involve a loss of about 15 per cent of its heat value. The steps in the oxidation of the denitrified oxyacids are not understood but it is theoretically possible that sugar may be elaborated from them. This would explain the formation of the large quantities of sugar that are believed to be derived from protein, for although the carbo-hydrate derivatives, such as glucosamine, obtained from protein by comparatively simple methods, may give rise to some sugar, the total quantity of these glycoproteins is small and is inadequate to furnish the amount which may under special circumstances, in diabetes, for example, be formed from protein. It is also theoretically conceivable that fat may be synthesized from these oxyacids.
The above considerations only deal with that proportion of the nitrogenous food which is not required for building up protein tissues, for which purpose there must be a synthesis of protein from the aminoacids absorbed. Many facts speak in favour of such a synthesis, though it is not known whether it takes place in the intestinal wall, in the liver, or elsewhere. The proteins we eat are different in composition from those of our own tissues; they do not on hydrolysis yield the same proportions of the various aminoacids. The following table (adapted from Aders Plimmer) shows what great differences exist between various proteins. It is reasonable, therefore, to suppose that the body may select in certain proportions the aminoacids presented to it in order to build up any particular tissue. We have already referred to the experiments of Kaufmann and of Willcock and Hopkins upon diets of gelatine and zein, which show that particular aminoacids are needed in the diet. It appears that, whatever is supplied, the body makes its proteins of a particular type. Abderhalden bled a horse severely so that a considerable quantity of the serum protein was lost.
Egg albumin .
Zein (from maize) .
The animal was then fed upon gliadine, which contains four times as much glutamic acid as the serum protein; but in spite of the absorption of gliadine the composition of the serum protein remained the same. Again, when the vegetable protein, zein, is given as food it cannot be recognized in the tissues and is therefore completely broken down, its constituents, or part of them, being built up into other proteins. Hence it is always desirable to supply protein in the diet in excess of the actual requirements, in order that there may be plenty of choice; and especially when the protein of the food is derived from vegetables, for if the aminoacids resulting from the hydrolysis of the protein of the food are not present in the same proportions as are needed to build up tissues, and this is likely to be the case with vegetable proteins, it is clear that in the selection of the suitable groups many will be rejected and can only serve for oxidation purposes.
A diet may therefore be deficient in the quality of its protein constituents whilst sufficient in their quantity. In mothers' milk it is probable that the necessary ingredients are present in exactly the right proportions, in which case the digestive breaking down and the subsequent building up may be supposed to go on with a minimal waste. Abderhalden suggests that in such a disease as rickets it is possible that there is a deficiency of a qualitative nature and that the cells of the growing tissues suffer because they are not supplied with a particular material. Some substances are especially necessary to healthy existence. Adrenalin, for instance, is essential; it is derived from an aromatic precursor, and Hopkins points out that "it is probable that the suprarenal gland requires a constant supply of some one of the aromatic groups of the protein molecule to serve as an indispensable basis for the elaboration of adrenalin." In starvation such a precursor would have to be obtained by tissue breakdown outside the gland.