Passing now to the third subject, we have an experiment of somewhat shorter duration, viz., of nine months, but sufficiently long to afford ample opportunity for any deleterious effect to manifest itself. The initial weight of the dog, No. 13, was 14.5 kilograms on September 14. The lowest intake of nitrogen was 0.26 gram per kilo of body-weight per day, while the fuel value of the daily food was during one period reduced to 55 calories per kilo. A daily proteid consumption equalling 0.30 gram of nitrogen per kilo, with a total fuel value in the day’s food of 66–70 calories per kilo, was clearly quite sufficient to maintain nitrogen equilibrium and body-weight; indeed, toward the end of the experiment, the animal commenced to gain in weight quite noticeably on the above diet, and was laying by fairly large amounts of nitrogen daily. The accompanying table gives the average daily nitrogen exchange, etc., of the nine balance periods, while the photographs, taken on the dates indicated under each, show the appearance of the animal at various times.

Results of the same general tenor with dogs No. 15 and No. 20 are seen in the appended tables, while the accompanying photographs testify clearly to the general good condition of the animals up to the end of the experiments. In No. 20 particularly, the great gain in body-weight is to be noted, even though the fuel value of the food was reduced as low as 64 calories per kilo, with the nitrogen intake at 0.28 gram per kilo daily. Plainly, the day’s food could have been diminished still more, with perfect safety to both body and nitrogen equilibrium.

The illustrations so far presented, with the general agreement in the character of the results, might perhaps be interpreted as indicating that there is no difficulty whatever in bringing a high proteid consumer, like a dog, down to a low level of proteid consumption. This, however, would be a false impression. Much depends upon the character of the proteid food, at least where any attempt at rapid change is made, for a certain modicum of meat or other animal food seems a necessary part of the daily diet if health and strength are to be maintained. A dog transferred suddenly from a daily ration in which meat and milk are conspicuous elements to a diet in which these are wholly wanting is very liable to show disturbing symptoms almost immediately. One case may be cited in illustration of these statements. On September 29, 1905, dog No. 17, weighing 18.2 kilos, was placed on a daily diet composed of 70 grams of fresh meat, 442 grams of milk, 300 grams of bread, and 28 grams of lard. This ration contained 9.06 grams of nitrogen and had a fuel value of 1465 calories, or 0.5 gram of nitrogen and 80 calories per kilogram of body-weight. On October 11, the animal weighed 18.6 kilograms and was in perfect condition. On the 13th, the meat was reduced to 34 grams per day, but the milk was increased in amount so as to maintain the same nitrogen intake and fuel value as before. This diet was continued until November 3, a balance experiment covering ten days from October 22 to the 31 inclusive, showing that the animal was laying by a little nitrogen. On November 3, the diet was changed to milk, bread, and lard, the fuel value being maintained at 80 calories per kilo daily, while the nitrogen intake was reduced to 0.30 gram per kilo. On this diet, the animal seemed to thrive perfectly, and at the end of two weeks showed a body-weight of 18.2 kilograms. November 19, the milk was withdrawn, the bread being increased so as to keep the daily nitrogen intake and the fuel value unchanged. The day’s food was now composed of bread and lard solely, but, as just stated, the nitrogen and fuel values were unaltered. In four days’ time, however, a change began to creep over the animal; the appetite diminished, and there was apparent a condition of lassitude and general weakness which deterred the animal from moving about as usual.

During the next week the animal grew steadily worse, and would eat only when coaxed with a little milk or with bread softened with milk, the diet of bread and lard being invariably refused. There was marked disturbance of the gastro-intestinal tract; bloody discharges were frequent; the mucous membrane of the mouth was greatly inflamed and very sore; body-weight fell off, and the animal was in a very enfeebled condition. This continued until December 4, with every indication that the animal would not long survive, but by feeding carefully with a little milk and occasionally some meat, improvement finally manifested itself, and by December 18 there was good appetite, provided bread was not conspicuous in the food. Body-weight, which had fallen to 15.5 kilos, was being slowly regained, and on December 30 the animal was again placed on a weighed diet, consisting of 70 grams of meat, 442 grams of milk, 210 grams of cracker dust, and 10 grams of lard. This diet contained 8.26 grams of nitrogen and had a fuel value of 1330 calories, equivalent to 0.5 gram nitrogen and 80 calories per kilogram of body-weight. On January 12, 1906, the weight of the animal was 16.7 kilos, while in general condition there was nothing to be desired. The food was then modified by diminishing the amounts of meat and milk fed daily by one-half, thus reducing the nitrogen intake to 0.35 gram per kilo of body-weight, but maintaining the fuel value of the food at 80 calories per kilo. Under this régime, body-weight still increased, and on January 27 was 17.5 kilograms. A balance period, shown in the accompanying table, extending from January 30 to February 8, affords ample evidence that the body was laying by nitrogen.

In all of the subsequent months, a small amount of meat was a part of the daily food, but as is seen from the table of balance periods, the total nitrogen intake and the fuel value of the food were reduced to even lower levels per kilogram of body-weight. Yet the animal gained steadily, until at the latter part of June the weight was considerably above that noted at the commencement of the experiment in the preceding October. Further, the animal was in nitrogen equilibrium or even gaining nitrogen, and in perfect condition of health and vigor, as is indicated by the accompanying photographs taken at the different periods stated. Especially to be emphasized is the fact that during the last six months of the experiment, the daily intake of nitrogen and the fuel value of the food were as low or even lower than in November, when the daily diet was limited to bread and lard. The disastrous result which showed itself at once on this latter diet, with all animal food excluded, was not due to low proteid or to deficiency in fuel value, but simply to the fact that the animal for some reason could not adjust itself to a simple dietary of bread and fat, although there was ample available nitrogen and fuel value for the body’s needs. Something was lacking, which meat or milk could supply, and this something was indispensable for the maintenance of the normal nutritional rhythm.

This is by no means an exceptional case, but we can cite many other examples of like results where the animal when restricted to a purely vegetable diet, such as bread, pea-soup, bean soup, etc., reinforced by an animal fat, quickly passed from a condition of health into a state of utter wretchedness, with serious gastro-intestinal disturbance. The results are not to be attributed to the lower utilization of the vegetable food, for the disastrous effect is too quickly manifest, and further, often shows itself when the animal plainly has a large store of available nutriment in its own tissues.

This experiment with dog No. 17 has been dwelt upon at some length, because it illustrates a very important principle in the nutrition of a high proteid and carnivorous animal. As before stated, it is not a question of high or low proteid simply, but involves possibly the more subtle question of the relative value of specific forms of proteid food. It will be noted that this statement is made somewhat guardedly, in harmony with the caution necessarily called for in view of our lack of knowledge regarding the possible need of the animal’s body for extraneous principles which only meat, milk, or other animal products can supply. Inorganic salts, nitrogenous extractives, and other substances without any appreciable fuel value, are quite likely to be of primary importance in controlling and regulating the various processes of the body, which combine to maintain the condition of normal nutrition. With a diet restricted to one or two vegetable products, it is quite conceivable that something may be lacking which the system demands, though it cannot be measured in terms of nitrogen or calories. It may be said that man thrives on a purely vegetable diet, but while this is unquestionably true, it must be remembered that man with his free choice of food has recourse, as a rule, to a large variety of vegetable products from many sources, and consequently there is great likelihood of his absorbing from these varied products such supplementary matters as may be needed. On this question, we are in a realm of doubt and uncertainty, but the possibilities suggested must not be ignored, for they may contain a germ of truth of the utmost importance. The fact remains, however, that a dog when restricted to a purely vegetable dietary does not thrive; a little animal food seems necessary to keep up health and strength, and this suffices even though the daily nitrogen intake and fuel value of the food are restricted to a level below that of the vegetable dietary.

With these facts before us, it is difficult to avoid the conclusion that some significance may attach to the specific nature of the proteid. Of course, we must not overlook the radical difference in dietary habits of man and dog. Man as an omnivorous creature has for generations been accustomed to partake largely of vegetable foods, and as a result his digestive tract and his system as a whole has become acclimated, as it were, to the nutritive effects of vegetable matter. Dogs, on the other hand, are typical carnivores, and their habits for generations have led in an opposite direction, so that their gastro-intestinal tracts and their systems have become accustomed to the effects of a diet in which animal food largely predominates. Whether these deeply ingrained characteristics are responsible in any large measure for the difference in behavior of man, on a purely vegetable diet, and dogs is open to question. It would certainly not be strange if such were the case, but as we look at the facts collected in our study of this subject, it is somewhat impressive to note how well dogs thrive on a relatively large amount of vegetable food, provided there is a modicum of animal food added thereto. In other words, these high proteid consumers are apparently quite able to utilize the vegetable foods, but there is something lacking in such a dietary which the body has great need of. Is it not quite possible, as already suggested, that the specific nature of the proteid counts for something in nutrition? The question cannot be answered definitely at present, but there are certain facts slowly accumulating which make the question a pertinent one in this connection.

Thus, it is becoming evident, as was pointed out in an earlier chapter, that the many proteid substances occurring in the animal and vegetable kingdoms are more or less unlike each other in their chemical make-up. They yield different decomposition products, or the same products in widely different proportion, when broken down by the action of hydrolyzing agents; and when we recall that the digestive enzymes of the body convert the proteids of the food into these same end-products, it is plain that in the assimilation and utilization of the proteid foodstuffs the body has to deal with these various chemical units. Hence, an animal suddenly restricted to a dietary in which all of the proteid is furnished by bread might be seriously incommoded, either by the excess of certain amino-acids resulting therefrom, or by a lack of certain other end-products to which its body is accustomed. As an example, we may take the three typical proteids of the wheat kernel, gliadin, glutenin, and leucosin, and note the very striking difference in the proportion of certain of the decomposition products of each, as reported by Osborne and Clapp.73

It is obvious from these figures that the three proteids of the wheat kernel are radically different from each other. Contrast, for example, the content of glutaminic acid in gliadin with the amount in leucosin. With such striking differences in chemical make-up, it is reasonable to assume that corresponding differences in physiological action or food values may exist. Further, “in respect to the amount of these amino-acids, leucosin more nearly resembles the animal proteins than the seed proteins thus far examined, and in this connection it is interesting to note that leucosin occurs chiefly if not wholly in the embryo of this seed and is probably one of its ‘tissue’ proteins, in contrast to the ‘reserve’ proteins of the endosperm of which gliadin and glutenin form the chief part” (Osborne and Clapp). In other words, animal proteids, such as those of meat, are characterized like leucosin by a small content of glutaminic acid and ammonia; while leucin, lysin, aspartic acid, and arginin are relatively more abundant. Until we know more on this subject, however, any broad generalization would be out of place, but certainly there is justification for the supposition that in these differences in chemical constitution are to be found explanation of some of the peculiarities common to certain varieties of proteid food. Wheat flour, aside from its starch, is composed mainly of glutenin and gliadin with their large content of glutaminic acid. Meat proteids, on the other hand, like leucosin, contain only a small fraction of this acid, and, with the other differences indicated, meat proteid and wheat proteid as food for dogs or other high proteid consumers may reasonably be expected to have at the least very unequal values. And if we go a step beyond this and suppose that in the formation of true tissue proteid or the living protoplasm of the cell, certain of these end-products of proteid decomposition are absolutely indispensable, we can easily picture for ourselves a dearth of such building stones in the long-continued use of a diet which lacks that particular proteid from which the necessary building stones can be split off in adequate number.

It has been said, notably by Munk, that in dogs fed for some time on a low proteid diet there is a diminished power of absorption from the intestinal tract, associated with weakened digestion. If it is true that a lowered proteid intake results in a diminished utilization of the ingested food, that efficiency in the digestion and absorption of foodstuffs is impaired, it can only be interpreted as meaning that some injurious influence has been exerted on the epithelial cells of the intestine or the adjacent gland cells. We have, however, failed to find any evidence of deleterious action in the dogs that we have experimented with, where due regard was paid to maintaining a diet suitable for the physiological needs of the body. In the experiments that we have cited, both nitrogen intake and the fuel value of the food per day were lower than in Munk’s experiments, but the utilization of fat and proteid was not sensibly affected. The following tables give the results with ten dogs (including the six dogs already described) for lengths of time ranging from seven to twelve months, the periods indicated being each of ten days’ duration and occurring once each month. In the first table, the utilization of fat is shown, the figures given being based on determinations of the amount of fat contained in the excrement. Knowing the amount of fat in the daily food and the amount which passed through the intestine, it is easy to calculate the percentage of fat utilized.

It is perfectly plain from these results that there was no falling off in the utilization of fat; the percentage amount digested and absorbed, as in dogs 3 and 4, was just as large at the end of the twelve months’ experiment as at the beginning. Clearly, a so-called low nitrogen intake with dogs does not lead to any loss of power in the utilization of the fat of the food. This being so, it is equally clear that the arguments based on Munk’s results in this direction, and applied to man, are without adequate foundation.

The figures in the above table were obtained by determining the amount of nitrogen in the dried excrement from the animals, i. e. the amount that passed through the intestine unchanged;74 and knowing the content of nitrogen in the daily food, the percentage of unabsorbed nitrogen was then easily calculated, after which by simple subtraction the percentage of utilized nitrogen was found. At first glance, it would appear that as the experiments proceeded utilization of nitrogen was less complete. In a sense, this was true, but it was not connected with any impairment of the digestive or absorptive powers of the intestine. It must be remembered that in the earlier periods a larger proportion of the ingested nitrogen was in the form of readily digestible meat, but as the latter was reduced in amount larger proportions of vegetable food were introduced in order to maintain the desired fuel value, and consequently the percentage of non-absorbable nitrogen was increased. The well-known difference in the availability of animal and vegetable proteid has already been referred to in other connections; a difference due not so much to any inherent quality in the digestibility of the two forms of proteid as to the presence of cellulose and other material in the vegetable food which retards in some measure the action of the digestive juices. To this cause must be ascribed the slight falling off in the utilization of nitrogen noticeable in most of the experiments. If, however, the figures are compared with those usually obtained on a diet largely vegetable in nature, it will be seen that the utilization of nitrogen by these dogs was in no sense abnormal.

These experiments on the influence of a low proteid diet on dogs, as a type of high proteid consumers, taken in their entirety, afford convincing proof that such animals can live and thrive on amounts of proteid and non-nitrogenous food far below the standards set by Munk and Rosenheim. The deleterious results reported by these investigators were not due to the effects of low proteid or to diminished consumption of non-nitrogenous foods, but are to be ascribed mainly to non-hygienic conditions, or to a lack of care and physiological good sense in the prescription of a narrow dietary not suited to the habits and needs of this class of animals. Further, it is obvious that the more or less broad deductions so frequently drawn from the experiments of Munk and Rosenheim, especially in their application to mankind, are entirely unwarranted and without foundation in fact. Our experiments offer satisfying proof that not only can dogs live on quantities of proteid food per day smaller than these investigators deemed necessary, and with a fuel value far below the standard adopted by them; but, in addition, that these animals are quite able on such a diet to gain in body-weight and to lay by nitrogen, thereby indicating that even smaller quantities of food might suffice to meet their true physiological requirements.

The results of these experiments with dogs, which we have recorded in such detail, are in perfect harmony with the conclusions arrived at by our experiments and observations with man, and serve to strengthen the opinion, so many times expressed, that the dietary habits of mankind and the dietary standards based thereon are not always in accord with the true physiological requirements of the body. If these views are correct, and the facts presented seemingly indicate that they are, it is time for enlightened people to give heed to such suggestions, that their lives may be ordered more nearly in accord with the best interests of the body. Physiological economy in nutrition is not a myth, but a reality full of promise for the welfare of the individual and of the community in general. Ignorance on dietary matters should give place to an intelligent comprehension of the body’s needs, and an adequate understanding of how best to meet the legitimate demands of the system for nourishment under given conditions of life. It is said that more than half the earnings of the working people of this country is spent for food. Here, we have suggested another form of economy as worthy of consideration; less important perhaps than that which relates to health and strength, but still calling for thoughtful attention. We cannot afford to be ignorant of these things; we must have definite knowledge of the actual facts, and these can only be obtained by careful research and investigation.

As a prominent writer on nutrition has well said, “The health and strength of all are intimately dependent upon their diet. Yet most people understand very little about what their food contains, how it nourishes them, whether they are economical or wasteful in buying and preparing it for use, and whether or not the food they eat is rightly fitted to the demands of their bodies. The result of this ignorance is great waste in the purchase and use of food, loss of money, and injury to health” (Atwater). We all recognize the general force and truth of this statement, but there is a surprising lack of appreciation of the full significance of what is involved thereby. If it is true that the demands of the body for proteid food—which of all foods is the most expensive—are fully met by an amount equal to one-half that ordinarily consumed, and that health and strength are more satisfactorily maintained thereby, it is easy to see how the acquisition of dietary habits leading to consumption of food in harmony with physiological needs will result in a fruitful twofold economy; viz., economy in expenditure, and of still greater moment, economy in the activities of the body by which food and its waste products are cared for.