The first series of experiments was made in 1891 by the late Immanuel Munk of Berlin, privat docent of physiology at the University, followed by further experiments in 1893.[67] Four dogs in all were studied. The diet made use of was “fleischmehl” (dried meat ground to a powder), fat (suet), and rice boiled together with water. We may refer briefly to the details of one experiment. The dog weighed 10.4 kilograms, and at first was given a daily diet composed of 85 grams of rice, 29 grams of fat, and 30 grams of the flesh meal. This ration contained 30.3 grams of proteid, 31 grams of fat, and 66 grams of carbohydrate, with a total fuel value of 663 calories, or 63 calories per kilogram of body-weight. On this diet, there was at the outset a slight loss of body-weight, after which both body equilibrium and nitrogen equilibrium were practically maintained. After this preliminary period of three weeks, the day’s diet was altered by replacing 15 grams of the proteid by 15 grams of rice, so that the daily ration consisted of 15.3 grams of proteid (with 2.42 grams of nitrogen), 31 grams of fat, and 81 grams of carbohydrate, with essentially the same fuel value per kilo of body-weight as before. Later, the fuel value of the food was further increased by raising the amount of rice to 125 grams per day, the day’s ration then consisting of 15.5 grams of proteid, 37 grams of fat, and 96 grams of carbohydrate, with a total fuel value of 780 physiological heat units, or 78 calories per kilo. On this diet, nitrogen equilibrium was maintained and the animal gained somewhat in body-weight. By the seventh week, however, Munk reports that the animal began to show signs of change; there was loss of appetite, absorption of the daily food was impaired, both proteid and fat failing in large degree to be utilized, while nitrogen equilibrium could no longer be maintained. This condition continued during the next week, aggravated by vomiting and accompanied by loss of strength and vigor. At the beginning of the tenth week of this low proteid ration, the animal was in a very poor condition, with complete loss of appetite, little inclination to take food, etc. On feeding a liberal diet of fresh meat, as much as 250 grams per day, with some fat (50 grams a day), the animal speedily recovered its appetite, and in a short time was in normal condition, absorption of food and utilization of the same being as complete as at the beginning of the experiment.
It is not necessary to give further details bearing on the three additional experiments. It will suffice to quote the general conclusions which Munk drew from the various results obtained, viz., that a low proteid intake in the case of dogs causes a loss of appetite, weakness, vomiting, etc., while body-weight and nitrogen equilibrium are difficult or impossible to maintain. More specifically, Munk’s observations led him to state that for dogs of ten kilograms body-weight a daily intake of 0.255 gram of nitrogen per kilo of body-weight is not sufficient to maintain the normal condition of the body, even when the fuel value of the day’s food amounts to more than 100 calories per kilo. In order to have the animal continue in nitrogen and body equilibrium, the daily food must contain at least 0.31 gram of nitrogen per kilogram of body-weight, with sufficient non-nitrogenous food to yield over 100 calories per kilo.
Let us now pass to the experiments made by Rosenheim,[68] which were carried on at about the same date as Munk’s. In the first experiment, the dog weighed 11.3 kilograms, and was fed daily a low proteid ration having a fuel value of 1447 calories and containing 2.825 grams of nitrogen. This ration was reduced in a short time to a still lower plane, viz., to 1066 calories and 2.525 grams of nitrogen daily. The food as then given was composed of 170 grams of rice, 50 grams of fat, and 25 grams of chopped meat, on which the dog gained weight and preserved nitrogen equilibrium. For six weeks, or thereabouts, the animal maintained its normal condition, after which it began to show symptoms of a general disturbance, with lack of appetite and weakness accompanied by a condition of icterus. Addition of meat extract to the diet to improve the flavor was without any appreciable effect. During the next two weeks, the condition of the animal steadily grew worse, although the body-weight remained practically stationary and nitrogen equilibrium was maintained. A week later, the animal died in a condition of exhaustion, without having manifested any symptoms of disturbed metabolism. There was found a marked catarrhal condition of the mucous membrane of the gastro-intestinal tract, with a fatty degeneration or metamorphosis of the glandular apparatus, but nothing sufficiently specific to account for the peculiar manner of death.
A second experiment with a dog of 5.8 kilograms, fed on meat, fat, and rice, led to essentially the same results as the preceding experiment. At the end of the first month, there appeared indications that the animal was not well, loss of appetite being marked, with disturbance of the stomach accompanied by occasional vomiting. These symptoms disappeared quickly when the animal was given for a few days large quantities of meat. On returning to the original low proteid diet, with its large content of rice, the symptoms gradually reappeared. At the end of two months the animal had again lost its appetite, and before the end of the fifth month the subject was dead. Post-mortem examination showed especially a strong fatty degeneration of the epithelial cells of the mucous membrane of the stomach and intestine. Rosenheim concludes that a diet poor in proteid is unhealthful for dogs, and that a daily ration containing even 0.32 gram of nitrogen per kilogram of body-weight, and with a fuel value of 110 calories per kilo, is not sufficient to maintain the animal in a condition of health.
The next series of experiments was made by Jägerroos[69] of Finland. This investigator was evidently impressed by the unfavorable and monotonous character of the diet made use of by the preceding investigators, and sought to introduce a little variety, recognizing also that with a carnivorous animal it is difficult to reduce the proteid to a low level and maintain the necessary fuel value, without introducing foodstuffs to which the animal is wholly unaccustomed. In the first experiment, the dog had a body-weight of 5.77 kilograms, and at the beginning was fed daily 40 grams of meat and 100 grams of sugar, equal to 0.31 gram of nitrogen and 80 calories per kilo of body-weight. The experiment continued for eight months, sugar being replaced in part by butter, and occasionally bread, fat, and wheat meal being used in proper amount to yield the given nitrogen and fuel values. During the last five months, the intake of nitrogen per day averaged 0.29 gram per kilo, with a fuel value amounting to 89 calories daily per kilo of body-weight. During this period, the animal maintained a plus nitrogen balance for a large part of the time. The experiment was then continued for two months longer, with a gradual diminution in the nitrogen of the food and in the fuel value, the animal dying at the end of the tenth month.
In a second experiment, the dog made use of weighed at the beginning 11.97 kilograms. During the first five months, the average intake of nitrogen amounted daily to 0.29 gram per kilo, while the average fuel value of the food (meat, fat, and sugar) was 76 calories per kilo daily. In the middle of the seventh month the animal was quite ill, with poor appetite, vomiting, etc. Body-weight began to fall off, and the animal soon died. With both of these animals, the experiment ended suddenly by a sharp and short illness.
Jägerroos, however, believed that both animals died from a severe case of infection, and not as the result of the diminished intake of proteid. This view was fully substantiated, in his opinion, by the evidence furnished on bacteriological and morphological examination. There was no pathological alteration and no fatty degeneration in the intestinal epithelium; nothing to indicate any connection between the lowered proteid intake and the death of the animal. To be sure, the long-continued diet poor in nitrogen might have diminished the power of resistance of the body, but no proof of this is offered. There was indicated merely a simple infection, as shown by the presence of Streptococcus and Bacterium coli communis in the blood. But, as Jägerroos states, one might well conceive of a lowered power of resistance on the part of the body, due not to any change in diet, but to the long-continued confinement in a cage with the enforced inactivity and lack of freedom. It is to be noted, furthermore, that here there was no sign of a gradual and progressive weakening of the body, no indication of any disturbance of the digestive tract with diminished power of absorption of either fat or proteid. On the contrary, there was a sudden and sharp attack of some infectious disease by which the animals quickly succumbed. Jägerroos was of the opinion that in the absence of this infection the animals would have continued to live for a long period of time.
If a low proteid diet works so inimically on high proteid animals as Munk and Rosenheim thought, it would naturally be expected that the small proteid ration followed so long by Jägerroos would have resulted in the appearance of marked symptoms, at least a gradual and persistent falling off in body-weight, inability to maintain nitrogen equilibrium, etc.; but none of these things occurred. In Munk’s first experiment, the animal was given no fresh meat whatever during four weeks. Is it not quite possible that in the abrupt cutting off of this wonted form of food a disturbance may have been set up in the gastro-intestinal tract, which paved the way for the more serious results that followed? Jägerroos used only fresh, uncooked meat in his experiments, and laid great stress upon the importance of not departing any more than was necessary from the accustomed form of diet. The writer is strongly of the opinion that sufficient stress has not been laid upon this phase of the subject. A satisfactory diet for dog as for man must meet ordinary hygienic requirements; it must not only be sufficient in amount, but it must be easily digestible, of accustomed flavor, appealing to eye, nostrils, and palate, with reasonable variation occasionally and of moderate volume. With due regard to these conditions, I believe with Jägerroos that not much attention need be paid to the proportion of nitrogen therein, for however small the amount it will be found sufficient to meet the needs of the body.
These are the results, collectively, so frequently used to point a moral for man: Beware of the possible danger of reducing the consumption of proteid food below the commonly accepted dietary standards! We must admit, however, that there is a woeful lack of agreement in these results, and it is difficult to prevent a shadow of doubt from creeping over us as we try to depict for ourselves the way in which a low proteid ration exerts its deleterious effect on dogs. I do not believe that radical changes in diet, whether they involve increase or decrease in total quantities, or in specific elements of the diet, can be made suddenly without danger of some disturbance of the gastro-intestinal tract or other parts of the economy, either in dog or man. It is reasonable to believe also that a high proteid feeder, like a dog, with his more limited dietary, will be far more sensitive to great changes than omnivorous man with his wider range of foodstuffs. Moreover, there is just as good ground for believing that in any animal, excess of proteid is as dangerous as a low proteid diet. Too great a disturbance in the nutritive balance, whether it involves excess or reduction in the amount of a given foodstuff, is liable to be attended with serious disturbance in any sensitive organism.
In illustration of these statements, we have some recent results obtained by Watson and Hunter[70] upon the influence of diet on growth and nutrition. These investigators find that young rats—two and a half months old—when fed upon a diet composed exclusively of horse-flesh, which is chiefly proteid matter with some fat, succumb very quickly, for some reason. Of fourteen young rats fed on this meat diet, six died on the third day. On the morning of this day, as the authors state, “the rats appeared to be in their usual health, but an hour after feeding one of them was lying on its side apparently unconscious. In a few minutes others were affected. They appeared to be paralyzed, they felt cold to the touch, exhibited symptoms of tetany, and speedily became unconscious. Six succumbed within half-an-hour. Of the remainder, some showed similar symptoms, although in less degree, and they recovered when the diet was changed to bread and skim milk.” After two days of the so-called normal diet, composed of bread and skim milk, the remaining eight rats were again placed on an exclusive meat diet. They appeared now to have acquired a certain degree of immunity, for although they exhibited symptoms of deranged nutrition, these were gradually recovered from and they gained in weight. At the end of the eighth month, five of the animals were still alive and in apparent good health, but their growth was permanently stunted. With an exclusive diet of ox-flesh, young rats were much more liable to thrive, although their growth was distinctly retarded.