160. Protein Used In A Variety Of Ways

The functions of the proteins are not restricted, however, to the use already described. According to existing views, they are utilized in more ways than any other class of nutrients. It was held at one time by prominent scientists that outside the vegetable fats the proteins are the sole source of animal fats, and this view was, not so very long ago, to some extent accepted. Indisputable proof to the contrary is now in our possession, and some investigators even go so far as to deny the possibility of the formation of fat from protein. On this point, opinion is divided. Certainly we must be convinced that nitrogen compounds of the food are, with some species, not the most important source of animal fat, for various investigators, such as Lawes and Gilbert, Soxhlet, and others, have shown upon the basis of searching experiments that sometimes over four-fifths of the fat stored by pigs must have had its origin outside the food protein and fat. Besides all this, the common experience of feeders of animals that foods highly non-nitrogenous are often the most efficient for fattening purposes is good evidence that fat formation is not greatly dependent upon the protein supply. Nevertheless, the possibility of producing animal fat from protein is not disproved, and there are several considerations which make it seem probable that under certain conditions, this does occur.

1 Loc. cit., p. 69.

Protein can unquestionably serve as fuel, or, in other words, as a source of energy. (See p. 162.) The amount so used depends much upon the subject and character of the ration. In the case of mature carnivorous animals and adult persons who neither gain nor lose in flesh, the protein eaten serves as a source of energy rather than of constructive material. When a portion of the protein is required for milk formation or for building tissue in the growing body, a varying quantity escapes oxidation.

The formation in the animal of carbohydrates from protein appears t6 be proven. We have seen that the sugar formed during digestion may be stored as glycogen, and that this glycogen in the liver and muscles is a reserve store of fuel to maintain the activities of the animal body, and it is important to know whether the contribution of energy from the digested protein is through the same avenue. There are now good grounds for believing that this may be the case.

In the plant, the amino acids which are the building stones of the proteins are synthesized from the carbohydrates and lower nitrogen compounds, and there seems to be no reason why a reverse process may not take place. Much investigation has been directed at this problem that did not furnish a conclusive answer, although in some experiments the sugar formed when a pure protein diet was fed, can be accounted for only by deciding that it was due to protein cleavage. Nevertheless, the question will bear further study.

161. Functions Of Carbohydrates

Carbohydrates are usually characterized as the fuel portion of the food, or that part which is burned to produce the various forms of energy. This conception of the function of these bodies is correct in the sense that in many dietaries they constitute the larger part of the fuel, although not the whole of it, and in some dietaries less than half. For instance, the diet of a certain ball team contained on the average daily 181 grams of protein, 557 grams of carbohydrates, and 292 grams of fat. If the young men neither lost nor gained in flesh, the protein of their diet supplied about one-eighth of the fuel energy that they expended, the remainder coming about equally from the carbohydrates and fats.

In contrast to this, Japanese students in the cadet school at Tokyo were found to eat 83 grams of protein, 631 grams of carbohydrates, and 14 grams of fat. In this case, over 80 per cent of the fuel energy came from the carbohydrates. As we shall see later (pp. 161-162) all the nutrients may serve as fuel in the body, but in the dietaries of many classes of people the carbohydrates are the chief source of the energy that comes from food oxidation.

162. Carbohydrates A Source Of Animal Fats

Contrary to views that held for a time, it is now well established that the animal fats may have their source in the carbohydrates; in other words, starch and sugar and related bodies may serve the main purpose in producing body fat. In many experiments, notably those with swine, the protein and fat of the food have fallen far short of accounting for the fat in the body increase, sometimes much the greater part of the latter having no possible source other than the carbohydrates. A practical expression of this general conclusion concerning the fat-forming function of carbohydrates is seen in the well-recognized value of corn meal as a fattening food, a feeding stuff nearly seven-tenths of which consists of starch. Recent experiments with milch cows leave no doubt that milk fat may also be derived from carbohydrates.

163. Functions Of The Fats And Oils

So far as is at present known, the possible uses of the food fats and oils and of the carbohydrates are similar. In other words, both may serve as fuel, and both may be a source of animal fat. The differences are that the supply of carbohydrates is much the larger, and the fuel value of a unit weight of fat much the greater. Moreover, it seems possible for a vegetable fat to become deposited in the animal without essential change, whereas fat formation from carbohydrates involves complex chemical transformations.