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 Of fourteen young rats fed exclusively on porridge, all, with the exception of two that were withdrawn, succumbed within five months, while the bread and milk-fed animals thrived as usual.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.
SUBJECT No. 5. DAILY AVERAGES
Date. | Body- | Food. | Output. | Nitro- | ||||
|---|---|---|---|---|---|---|---|---|
| Total Nitro- gen. | Nitro- | Fuel | Nitro- | Nitro- | Nitro- | |||
1905 | kilos | grams | gram | calories | grams | gram | gram | grams |
| Aug. 15–Aug. 24 | 15.8 | 8.66 | 0.54 | 87.3 | 5.44 | 0.70 | 0.52 | +2.00 |
| Sept. 6–Sept. 15 | 17.1 | 4.76 | 0.27 | 72.4 | 3.41 | 0.32 | 0.48 | +0.55 |
| Oct. 8–Oct. 17 | 17.6 | 4.76 | 0.27 | 71.8 | 3.54 | 0.54 | 0.49 | +0.19 |
| Nov. 22–Dec. 1 | 16.9 | 4.77 | 0.28 | 72.0 | 3.76 | 0.39 | 0.32 | +0.30 |
1906 | ||||||||
| Jan. 2–Jan. 11 | 17.2 | 4.07 | 0.23 | 72.0 | 3.19 | 0.54 | 0.35 | –0.01 |
| Jan. 30–Feb. 8 | 18.0 | 4.07 | 0.23 | 69.0 | 2.87 | 0.54 | 0.62 | +0.04 |
| Feb. 27–Mar. 8 | 18.2 | 5.18 | 0.28 | 73.0 | 3.69 | 0.66 | 0.74 | +0.09 |
| Mar. 27–Apr. 5 | 18.3 | 5.23 | 0.28 | 73.0 | 3.66 | 0.84 | 0.48 | +0.25 |
| Apr. 24–May 3 | 19.1 | 5.22 | 0.27 | 68.0 | 3.76 | 0.38 | 0.48 | +0.60 |
| May 22–May 31 | 19.4 | 5.22 | 0.26 | 65.0 | 3.44 | 0.31 | 0.48 | +0.99 |
| June 17–June 26 | 20.0 | 5.24 | 0.26 | 67.0 | 3.50 | 0.71 | 0.48 | +0.55 |
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.