THE EFFECT OF LOW PROTEID DIET ON HIGH PROTEID ANIMALS

Topics: A wide variety of foods quite consistent with temperance in diet. Safety of low proteid standards considered. Arguments based on the alleged effects of low proteid diet on high proteid animals. Experiments of Immanuel Munk with dogs. Experiments of Rosenheim. Experiments of Jägerroos. Comments on the above experiments. The experiments of Watson and Hunter on rats. The writer’s experiments with dogs. Details of the results obtained with six dogs. Comparison of the results with those of previous investigators. Effect of a purely vegetable diet on dogs. Different nutritive value of specific proteids considered. Possible influence of difference in chemical constitution of individual proteids. Effect of low proteid diet on the absorption and utilization of food materials in the intestine of dogs. General conclusions from the results of experiments with animals.

Man is by choice an omnivorous creature; he reaches out ordinarily in all directions for as wide a variety of foods as his circumstances and surroundings will allow. He rightly cultivates a taste for foods that have individuality of flavor, and derives pleasure and satisfaction from the eating of delicacies that appeal to palate and to reason. All this he can do without becoming an epicure or a glutton, and without violation of physiological laws or disregard of the teachings of temperance. As a being endowed with reason and intelligence he is, however, necessarily mindful of the possible deleterious effect of undue quantities of food, as he is likewise mindful of the desirability of avoiding certain varieties of food which personal experience has taught him are fraught with possible danger. Care and prudence in diet are legitimate outcomes of a reasonable interest in the welfare of the body, upon which so largely depend the happiness and working power of the individual.

The adoption of dietary habits that aim to accord with the physiological requirements of the body does not compel a crucifying of the flesh or a disregard of personal likes and dislikes. A reasonable intelligence combined with a disposition to exercise the same degree of judgment and care in the nutrition of the body as is expended on other matters, of no greater importance, pertaining to the individual, to the household, or to business interests, are all that is needed to bring about harmony between every-day dietary habits and the nutritive requirements of the body. There is no occasion, unless one finds pleasure and satisfaction in so doing, to resort to a limited dietary of nuts and fruits, to become an ardent disciple of vegetarianism, to adopt a cereal diet, to abjure meats entirely, or to follow in an intensive fashion any particular dietary hobby, except so far as may be necessary to insure an adequate amount of non-nitrogenous foods to meet the energy requirements of the body without unduly increasing the intake of proteid or nitrogenous food. Naturally, a man leading a life of great physical activity with the consequent demand for a large energy-yielding intake will be compelled to resort largely to vegetable foods, rich in starch and poor in proteid, or to eat largely of fatty foods. Reliance on meats and animal foods in general, under such conditions, would plainly involve a high proteid intake with a consequent high nitrogen metabolism, with the chance that even then the energy requirement would not be fully met.

In view of all that has been said, reinforced by the various facts brought forward as evidence, we must recognize the value of the non-nitrogenous foods as a source of energy, and this means plainly food from the plant kingdom. In any rational diet, vegetable foods of low nitrogen-content must predominate, while animal foods with their higher nitrogen values must be greatly subordinate in amount, if the nitrogen or proteid metabolism of the body is to be maintained at a level commensurate with true physiological requirements. But there comes the ever-recurring question, Are the lower proteid standards quite safe to follow? Are we warranted in turning aside from the teachings based on the habits and customs of mankind? Many reasons have already been presented which seemingly justify an affirmative answer, while the experimental results and the observations on various groups of people, covering years of time, speak with no uncertainty regarding the element of safety, and indicate clearly that the absolute proteid requirement of the body is quite small; much smaller indeed than the amount of proteid food consumed by the average individual would seemingly imply.

Probably the most striking evidence, certainly of an experimental nature, so far presented against the safety of a relatively low proteid diet for man is that based on the results of several studies made to ascertain the effect of a reduced proteid intake on so-called high proteid animals. Animal kind may be divided into three groups according to the nature of their food, viz., high proteid feeders, such as carnivorous animals in general, of which the dog is a good type; omnivorous or moderate proteid consumers, to which class man belongs; and low proteid consumers, such as herbivorous animals. Three series of experiments have been reported by independent workers on the effects of reducing the amount of proteid food in the diet of dogs. The results of these experiments were of such a character that it has come to be understood that animals of this type cannot exist for any great length of time on a low proteid diet. It is affirmed that in a relatively short period the animals reach such a state that they either die, or are in such poor condition that they must be fed a more liberal amount of proteid to maintain them alive. The explanation offered is that the low proteid diet results “in a loss of the power of absorption from the intestinal tract, caused apparently by a change in the condition of the epithelial cells, as well as by a diminished secretion of the digestive juices.”

The argument based on this evidence is that while a high proteid animal feels at once, or almost immediately, the deleterious effect of a reduction in the amount of proteid food, an omnivorous animal may be more tardy in manifesting the injurious action, which, however, is sure to follow sooner or later from any material reduction of proteid below the customary standards. In other words, man as a moderate proteid consumer can endure for a time even large reductions in the amount of proteid food, but eventually there will be manifested some of the disastrous results obtained with dogs. Here, we have a somewhat serious indictment, one that merits careful consideration. To be sure, it may be objected that between dog and man there is a wide gulf, and that there is no justification for assuming that these two types of animal life have anything in common. Still, the experience of many years has taught the physiologist that much light can be thrown upon the processes of higher types of life by a study of what occurs in lower forms, and on the subject of nutrition any one of experience would hesitate to cast out of court the evidence gathered from observation of what occurs among the higher animals. It will be the part of wisdom, therefore, to scrutinize somewhat carefully the character of this evidence obtained from a study of the behavior of dogs toward a low proteid diet.

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 Jägerroos69 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 Hunter70 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.

This difference in the behavior of the animals towards the two forms of proteid food is to be attributed to the fact that ox-flesh contains more fat than horse-flesh, and consequently the diet with this form of meat was less exclusively proteid in character. Further, there were some indications that horse-flesh is less digestible than ox-flesh. Another fact, showing the far-reaching effect of a distinctly unphysiological diet, is the marked influence of pure meat food on the progeny. Thus, of 93 rats born of meat-fed parents only 19 were alive at the end of two months, while of 97 young born of bread and milk-fed rats, 82 were alive and in apparent health at the end of the same period.

As illustrating how foods that have, superficially at least, approximately the same chemical composition may react differently in the animal body we have the observations of Watson on rats fed with porridge, made by boiling oatmeal with water and skim milk, as contrasted with a diet of bread and skim milk, the two diets having essentially the same composition. Of fourteen young rats fed exclusively on porridge, all, with the exception of two that were withdrawn, succumbed within five months, while the bread and milk-fed animals thrived as usual. Adult rats, however, can live for prolonged periods and maintain their weight on a porridge diet. It is believed that the difference in the behavior of young rats to these two closely allied forms of diet, is due to a difference in the digestibility of the food, the porridge being presumably less readily digested by the young animals than bread. With the more fully developed digestive powers of the adult animals, however, this difference in availability practically disappears as a potent factor in their nutrition. Finally, mention may be made of the fact that a pure rice diet, notably deficient in proteid, arrests the growth of young rats and leads to a fatal issue within three months, while adult rats placed on such a diet lose weight rapidly and die in about the same time. All of these facts bearing on the nutrition of animals quite remote from man have significance as showing how any wide departure from a physiological diet, for that particular species or type, may lead to very undesirable results, and they warn us not to be too hasty in drawing far-reaching conclusions and sweeping deductions from a few experiments with a given species of animal.

Recurring now to the experiments made with dogs, there is certainly suggested an element of danger in a low proteid diet, which, if the experiments are taken at their face value and the conclusions derived therefrom applied to man, needs careful consideration. Jägerroos plainly was not inclined toward the belief that a low nitrogen intake was the cause of the unfortunate results that attended his experiments. Still, his animals did die from some cause, and thereby his position was weakened. Munk and Rosenheim, on the other hand, from their experiments were apparently convinced that a low proteid intake was inimical to dogs, and it will be remembered Rosenheim concluded 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.” If this is really true, there is some ground for the arguments advanced by critical writers regarding the general subject of nitrogen requirements of man. The evidence and the arguments, however, have always seemed to the present writer frail and faulty; but recognizing the hold they have taken on physiologists and the way they are usually applied to man, I have attempted to test the matter experimentally under conditions which would yield trustworthy and conclusive results.

The question how far results obtained with dogs can be applied safely to man may be open to discussion, but we must first be sure of our facts before arguments or conclusions of any kind are warranted. It is to be remembered that dogs are as sensitive in many ways as man, and no physiological experiment covering a long period of time can be carried out with any hope of success unless there is due regard for proper hygienic conditions, some degree of variety in diet, and reasonable opportunities for fresh air and occasional exercise. I fancy that even the most vigorous and hardy man, if confined for six consecutive months in a room just large enough to furnish requisite air-space and to permit of extending his body at full length, would find himself at the end of such a period in a condition far from healthful, even though there were perfect freedom of choice in diet. If, however, there were added to the above conditions monotony in diet extending through many months, there would be no occasion for surprise if the individual lost appetite and strength, and showed signs of disturbance of the gastro-intestinal tract.

It is doubtful if there is full appreciation of the possible effect of monotony, in the ordinary dietary experiments on dogs. Man quickly feels the effect; the sportsman camping in the woods by brook or lake enjoys his first meal of speckled trout and has no thought of ever becoming tired of such a delicacy; but as trout cooked in various ways continue to be placed before him three times a day, and with perhaps very little else, he soon passes into a frame of mind where salt pork would be a luxury, and where he would prefer to go hungry rather than eat the delicacy, if indeed he has appetite to eat anything. Is it strange that dogs confined in cages barely large enough to permit of their turning around, and fed day after day and month after month with exactly the same amount of desiccated meat, fat, and rice, should show signs and symptoms, if nothing worse, of disturbed nutrition? It is necessary in experiments of this kind that the animals be confined for given periods, at least, since otherwise it would be impossible to determine the extent of nitrogen excretion and the rate of proteid katabolism, etc. It is possible, however, to limit the time of close confinement to, say, ten consecutive days, this to be followed by a like period of comparative freedom, thus insuring opportunities for an abundance of fresh air and exercise.

The experiments of which I wish to speak, and which had for their object a study of the effect of low proteid diet on dogs, as types of high proteid animals, were carried out at our laboratory in the Sheffield Scientific School and were made possible by liberal grants from the Carnegie Institution of Washington, thus providing means for securing the requisite number of chemical assistants. The experiments were conducted on a somewhat large scale, over twenty dogs being made use of, while many of the experiments extended through a full year. The results in their entirety are not yet ready for publication, but I am able to present in a general way observations on six dogs, which will serve as an ample illustration of what may be expected with high proteid animals when living on a low proteid diet under healthful conditions. All of the six dogs whose cases are here presented were fed on a mixed diet, with some fresh meat each day; bread, cracker dust, milk, lard, and rice being the other foods drawn upon to complete the dietary. The animals were fed twice a day, each meal being accurately weighed and of definite chemical composition. A large, light, and airy room, kept scrupulously clean, and in the winter time properly heated by steam, served as their main abiding place. In this room were a suitable number of smaller compartments, the walls of which were composed of open lattice work (of iron), so as not to interfere with light or air, and yet adequate to keep the dogs apart. These compartments were not cages in the ordinary sense, but were truly large and roomy. The entire floor under the dogs was composed of metal, the joints all soldered, the floor being sloped to a metal gutter in front so that all the compartments could be flushed out each morning and kept sweet and clean. In pleasant weather, immediately after their first meal, the dogs were taken out of doors to a large enclosure near by, where they were allowed perfect freedom until about four o’clock, when they were taken in for their second meal (between four and five o’clock in the afternoon). The outdoor enclosure was inaccessible to every one except the holder of the key, and the dogs while there were wholly free from annoyance. Once every month, during a period of ten consecutive days, each dog was confined in the metabolism cage so as to admit of the collection of all excreta, in order to make a determination of the nitrogen balance. Practically, therefore, each dog was in close confinement only one-third of the month, the remaining two-thirds being spent in much more congenial surroundings. I have entered thus fully into a description of the conditions prevailing, because I deem them exceedingly important, and because therein undoubtedly lies the explanation of the striking contrast between our results and those of the earlier investigators of this subject.

In considering the outcome of our experiments, it may be wise to enter into some detail concerning the first case to be presented. The animal employed in this experiment was designated as No. 5, and weighed on July 27, 1905, 17.2 kilograms; it was apparently full grown, but was thin and had the appearance of being underfed. At first, it was given daily 172 grams of meat, 124 grams of cracker dust, and 72 grams of lard, the day’s ration containing 8.66 grams of nitrogen and having a fuel value of 1389 calories.71 These figures are equivalent to 80 calories, and 0.50 gram of nitrogen, per kilogram of body-weight. The animal took kindly to the diet, but on August 3 it refused to eat and seemed to have a little fever. The next day it was better, but for the three following days its appetite was poor, and only a portion of the daily food was eaten. Body-weight began to fall off, and was soon at 15.5 kilograms. On the 7th of August, a dose of vermifuge was given, after which the appetite returned and the animal appeared in good spirits. From this time forward it seemed in perfect health, with good appetite, and showed the usual vivacity and playfulness of dog-kind. The diet as specified was continued unchanged until August 25, a balance experiment covering a period of ten days, from the 15th to the 24th of August inclusive, being carried out, in which the nitrogen of the intake was compared with the output for each day. From the accompanying table, where are given the average values of all the balance periods of the experiment, it is to be seen that during this first period the animal was laying on or gaining an average of 2 grams of nitrogen per day.

On August 25, a radical change was made in the diet, by reducing the amount of meat to 70 grams daily, thereby lowering the intake of nitrogen to 4.76 grams, or 0.27 gram per kilo of body-weight; the cracker dust and lard being kept at essentially the same levels as before. This diet was continued through the next balance period, the dog in the meantime gaining in body-weight, and showing for the second balance period an average gain by the body of half a gram of nitrogen per day. The food was then altered by substituting bread for the cracker dust, but so adjusted that the nitrogen and fuel values of the day’s food remained practically unchanged. There was still, however, a gain in body-weight and a slight gain in body nitrogen. At the close of the third balance period, the diet was again altered, one-half of the meat being replaced by milk, while cracker dust was substituted for the bread. The morning meal consisted of 170 grams of milk, 86 grams of cracker dust, and 18 grams of lard, while the afternoon meal was composed of 35 grams of meat, 63 grams of cracker, and 35 grams of lard. The day’s ration, however, still contained 4.76 grams of nitrogen and had a fuel value of 1249 calories. This diet was maintained until November 20, when the animal was again placed on a daily ration of meat (69 grams), bread (166 grams), and lard (80 grams), with a total fuel value of 1228 calories and 4.77 grams of nitrogen. This was continued until December 2, the dog still showing a plus nitrogen balance, but with a little loss in body-weight. On December 2, the diet was again changed by substituting milk for a portion of the meat, but the nitrogen and fuel values were maintained at the same level as before. After a week, December 9, the food was modified as follows: the morning meal contained 170 grams of milk, 110 grams of rice, and 11 grams of lard, while the afternoon meal was composed of 35 grams of meat, 81 grams of rice, and 30 grams of lard. The total nitrogen content of the day’s ration was 4.07 grams, while the fuel value was 1255 calories. At this time, the animal weighed 17.1 kilograms, consequently the intake of nitrogen had been reduced to 0.23 gram per kilo of body-weight, while the fuel value stood at 73 calories per kilogram. This diet was continued until February 9, the balance period, between January 2 and 11, showing that the animal was in nitrogen equilibrium, in spite of the material reduction in the intake of proteid, and that body-weight was increasing. The next balance period, January 30 to February 8, showed still further gain in weight with continuance of nitrogen equilibrium. On February 9, the diet was changed by returning to 70 grams of meat, 158 grams of cracker dust, and 60 grams of lard, with a daily intake of 0.28 gram of nitrogen per kilo of body-weight.

In this manner, the experiment was continued with frequent changes in the character of the diet, but always maintaining essentially the same values in nitrogen and calories as shown in the table, until June 27; having extended through just eleven months, with the animal at the close of the experiment still gaining in body-weight, with a steady plus balance of nitrogen, and with every indication of good health and strength. For ten months the animal lived with perfect comfort and in good condition on an average daily intake of 0.26 gram of nitrogen per kilogram of body-weight, and with an average fuel value of 70.3 calories per kilo. Further, it is to be observed that at no time during the ten months did the daily intake of nitrogen rise above 0.28 gram per kilo, while during one month it fell to 0.23 gram per kilo. Similarly, the fuel value of the daily food never exceeded 73 calories per kilo, while at times it dropped as low as 67 and 65 calories per kilo. That this diet was more than sufficient, both in nitrogen and fuel value, is indicated by the steady increase in body-weight and by the plus nitrogen balances observed in most of the periods throughout the experiment. Indeed, with the comparatively low degree of muscular activity which this animal was accustomed to, it would have been unwise to have kept the subject much longer on a diet so rich as the above, since there would have been danger of detriment to its health and good condition. When these results are contrasted with the statements of Munk and Rosenheim, the latter of whom found that even 0.32 gram of nitrogen and 110 calories per kilo were insufficient to maintain dogs in a condition of health, it is plain that for some reason our results are quite at variance with their findings.

The accompanying photographs, taken on August 19, 1905, February 27, April 24, and at the close of the experiment on June 27, 1906, show the appearance of the animal at the respective dates, and indicate more clearly than words can express the actual condition of the animal.

Turning now to a second subject, designated as dog No. 3, the experiment with which lasted for nearly an entire year, the following general statements may be made. The animal was a small black and white fox terrier, weighing on July 6, 1905, 6.5 kilograms. It was a nervous, affectionate little creature, far less phlegmatic than the animal just described, always on the alert for a petting, and unceasingly active. For these reasons, it seemingly required per kilogram of body-weight a little more food than the preceding animal; a fact also in harmony with the general law that small animals, per unit of body-weight, need more food than larger ones. The diet made use of was of the same general character as employed with the preceding animal, and was changed from time to time to give requisite variety and to insure freedom from too great monotony. The accompanying table, showing daily averages during the twelve balance periods, gives all necessary information regarding the outcome of the experiment.

It will be observed that during the first three months the animal showed a tendency to gain in weight slightly, recalling that its initial weight on July 6 was 6.5 kilograms. Later, the weight fell off a little, but in March it showed an upward movement, though very gradual. With the amount of proteid food given, it is evident that the animal needed about 80 calories per kilo to maintain a condition of body-equilibrium. Nitrogen equilibrium was practically maintained throughout the larger portion of the twelve months, but evidently the animal required 0.31–0.33 gram of nitrogen per kilogram of body-weight. Attention may be directed, in view of the results reported by Munk regarding loss of the power of absorption and utilization of proteid food, to the figures showing the average daily output of nitrogen through the excrement. It is plain from the data presented, that this animal was not suffering from any trouble of this order; indeed, the utilization of proteid food throughout the entire experiment was exceedingly complete, as shown by the relatively small loss of nitrogen through the excrement, thus implying vigorous and unimpaired digestion, together with thorough absorption of the products formed.

The accompanying photographs show the appearance of the animal on August 19, 1905, November 18, 1905, April 3 and June 27, 1906, the close of the experiment.