Popular Science Monthly/Volume 1/July 1872/On the Digestibility of Vegetable and Animal Foods
AT the session of the Munich Academy of Science, December 4, 1869, Voit detailed the chief results of investigations made in his laboratory by Drs. Bischoff, Forster, Hoffmann, and Meyer, students of medicine, as to the differences which exist between animal and vegetable substances in point of digestibility, both in the carnivora and human beings; and also as to the importance of nutritive salts and condiments. These results have such a bearing upon alimentary hygiene that we have thought it desirable to give an extended analysis of this essay of the learned Bavarian physiologist. As the subject is important, we give a verbatim statement of the results.
Vegetable food, it is well known, contains, as its essential nutritive elements, albuminates, or nitrogenous materials, fatty matter, and hydrates of carbon, with water and nutritive salts. Such food, therefore, has the same elements as animal food; but in point of digestibility the two differ widely. Thus, while the carnivorous animal, when fed with sufficient flesh-meat, passes but little excrementitious matter, while it traverses the entire intestinal tract within eighteen hours after a meal; the herbivorous animal, on the contrary, when fed abundantly with vegetable substances, often retains the food in the intestine a whole week; and a considerable portion of this food remains unused. The proportion which the solid excretions bear to the weight of the animal is, for a dog fed on meat, as 3 to 10,000; for man with mixed food, 5 to 10,000; and for the ox, as 60 to 10,000.
We must observe that this difference is not due solely to the varying digestibility of the substances compared. These substances, as found in the excreta of herbivorous animals, are not such as resist the action of the digestive fluids. Henneberg and Stohmann have shown that the proportion of such substances digested depends upon their respective proportions in the sum of the food consumed. This is one of the most important recent contributions to the physiology of digestion. What renders vegetable food harder to digest is the fact that the albuminoid substances, fatty matters, starch, etc., are there incased in a coating of cellulose, to break up which requires some time. On this account, the intestine of the herbivora is longer and more complicated than that of the carnivora. The latter digest but a small proportion of cellulose. The same holds good for man, except when he consumes young cellulose but little consolidated, as tender pulse, roots, or fruits. The cellulose of hay and grass is of such quality that the human digestive apparatus cannot extract any of their nutritive elements. In order that we may utilize them, they must first be transformed by herbivora into their own substance.
Albuminates, whether derived from the animal or from the vegetable kingdom, leave but little alimentary residue. If fat be added to the albuminates, this residue is increased in proportion to the amount of fatty matter, especially when the latter is in excess. The addition of sugar, no matter in what quantity, has not the same result, provided it does not cause a diarrhoea. Of sugar, but faint traces are to be found in the residuum. With starch it is quite different, even when it is made pulp by boiling.
Adolph Meyer has made some interesting experiments in this matter. A dog was given 1,000 parts of bread per day (536 dry matter) and with such food his excreta amounted to 70 parts of dry substance. The equivalent of the albumen in the bread was then given, in the shape of flesh-meat, its starch being replaced by the respiratory equivalent in the shape of fat (2, 4: 1); consequently the sum must have been 377 meat, 184 fat. The dry excrement was then only 20, with five of fat. More albumen was, therefore, digested in the latter case than in the former, for in the excretory residue from the bread there was found of nitrogen 2.45, while in that from meat and fat it was only 0.97. To show that it is really the starch which yields excessive residue, and not some other constituent part of the bread, Mr. Meyer gave the albumen of 1,000 parts of bread under the form of pure flesh-meat, and also starch in the form of fecula reduced to a pulp. The sum was 377 parts of meat and 522 of starch. This ration yielded 68 parts of excrementitious matter per day, as when the equivalent food was given in the shape of bread. Still these excreta contained less nitrogen than that from bread. According to the investigations made by Dr. Bischoff, more nitrogen was assimilated out of 302 parts of meat and 354 of starch, than out of 800 of bread, although in each case the quantity of albumen was equal. It is hence seen that it is the starch which gives the greater part of the rejected residue. It follows that carnivora and man living on vegetable food ought to evacuate the bowels twice a day; whereas, on an exclusive meat-diet, they might retain the intestinal contents for at least four days.
Bischoff thinks that, as the starch must first be transformed into sugar before it can be absorbed, there is not sufficient time for this change to be thoroughly brought about, before the starch is carried along by the general action of the intestine. At first, excrementary matter has a very strong acid reaction, which, as Bischoff supposes, is owing to the presence of a great quantity of organic acids, especially butyric acid. Pettenkofer and Voit have found in the gases exhaled by animals consuming starchy food, chiefly hydrogen and carburetted hydrogen. These same gases are also found, in the intestine of the herbivora. The expulsion of the starch is probably due to the generation of these gases, which excites the peristaltic movement of the intestine. If the chyme were permitted to remain longer in the intestine, the starch would be completely transformed into sugar and entirely absorbed. The obstinate diarrhoeas of infants are doubtless often occasioned by this same phenomenon.
Voit shows that there are many other agencies which may exert similar influence upon intestinal movement. His assistant, Dr. Hoffmann, has observed that when cellulose is added to human food, for instance to flesh-meat, such meat then gives a largely-increased proportion of excrementitious matter. Purgatives, or sudden refrigeration of the abdominal region, may have the same effect. Bread containing all the constituents of wheat causes, according to Meyer's experiments, prompt evacuation, by reason of the indigestible cellulose it contains. It yields a widely disproportionate quantity of excreta, as has been demonstrated by Panuni in the case of dogs fed on bread, when, though evacuation increased, the amount of urea was diminished. White wheaten bread gives least excreta. The proportion of water contained in this matter affords the means of judging how long it had remained in the intestine, and how far the process of extracting the nutritive properties had gone. The substance evacuated in small quantity after a meal of flesh-meat contains hardly any remains of meat and 50 per cent, of solid matter, while the excreta from bread-food, evacuated in greater quantity, with much of the bread not transformed, has only 23 per cent, solid matter.
It might be supposed that the yeast in the bread is the cause of this; but Meyer has shown that a ration of starch-pudding and unleavened bread gives residuum in equal proportion with leavened bread. He has also shown that bread, into the composition of which enter nutritive salts, according to the Horsford process, gives a residuum equal to that of common bread. The result, therefore, is not due to the absence of these salts from bread. Dr. Bischoff has shown, in the course of some experiments he made at the instance of Liebig, that the addition of extract of meat, with or without salt, to the bread given to a dog, does not affect the intestinal absorption, nor does it lessen the amount of excrementitious matter. The objection might perhaps be urged that, as the dog is purely carnivorous, experiments made on him will not warrant a universal conclusion. Therefore, Dr. Hoffmann made the same experiment with a man. The man was fed on potatoes, pulse, and bread, and there were then 116 parts of dry excreta; when extract of meat was added, there were still 109.
According to Haubner, the addition of a little peas to potatoes notably diminishes or even entirely dissipates the starch, which else is found in great quantity in the excreta of sheep. This result he attributes to the influence of the albumen in the peas. Dr. Bischoff gave a dog 800 parts of bread and 100 of meat. The amount of residuum was not lessened, nor the assimilation of the bread increased. Dr. Meyer found a dog, on 1,000 parts of bread, to give TO of excreta; on 1,000 of bread and 100 of meat, 66 parts; and on 1,000 of bread and 300 of meat, 75 of solid residue. There is, then, no means of promoting the digestion of bread or potatoes, or other vegetable food, in the intestine whether of man or of dog, nor of preventing the loss of the starch.
From the persevering observations of Dr. Hoffmann, it follows that this imperfect digestion and this voluminous excretion are unavoidable a fact which, on a vegetable diet, necessitates larger consumption, even though each of the elements were in itself capable of absorption. If a man consumes in a day 1,000 parts potatoes, 207 lentils, 40 bread and beer, he takes in 14.7 of nitrogen. Of the latter he gives out 7 by the kidneys, and 6.9 in 116 of dry excreta. The latter contain 24 per cent, of the dry food, and 47 per cent, of the nitrogen. But, when he takes in animal food, the same amount of azote and of starch in its respiratory equivalent of fat, i. e., 390 parts meat and 126 fat, he eliminates daily only 28.3 parts solid matter, with 2.6 of nitrogen; while on the contrary, the liquid excretion holds 14.2 if nitrogen. Consequently, though the amount of albumen in the two cases was the same, still twice as much of it was absorbed by the intestine from the ration of meat as from that of vegetables. The 800 parts of meat yielded only 27 of dry residuum.
This difference in absorption makes the essential difference between vegetable and animal food. Consequently, we are not justified in saying that 2.4 of starch is the equivalent of one part fat. Further, 1 part fat and 2.4 albumen, both absorbed by the same organism, have not the same action on the transformation of albumen or the admission of oxygen. Great caution must therefore be observed, in treating of such equivalencies, even where the elements of nutrition are analogous, but come from diverse sources. Their mutually different behavior in digestion has not been always taken into account.
Dr. Hoffmann has studied this subject for the human economy, as regards various alimentary substances or nutritive elements. Bread, potatoes, rice, maize, etc., taken in any quantity whatsoever, can scarcely support the life of man or of carnivorous animals, communicating to them no bodily strength. Too large a proportion of their nutritive elements is eliminated in the excretion. Still, with the addition of a small quantity of albumen, whether animal or vegetable, they may suffice. They are poor in albuminates, but rich in starch. Even herbivorous animals often take in an excess of food, so as to get the requisite amount of albumen.
The same occurs with man, and therefore he will waste non-azotized material. The quality of the food he takes may be told from the excreta. It is Liebig who said that you might make out the boundary-lines of those countries where the coarse brown bread of Westphalia is used, from certain indications found along the hedge-rows. An Irish laborer, according to Buckle, consumes daily 9½ lbs. of potatoes, a weight too great for all the intestines to carry. These potatoes would contain of water 3,200 parts, dry albumen 70, and of non-nitrogenous substances 725. The latter quantity is far in excess of what is necessary to nourish a strong man; but there is a deficiency of albumen, to say nothing of the amount lost in the excreta. As a consequence, the body is capable of but little work, notwithstanding the great quantity of potatoes taken in, and it is but ill provided with the means of resisting disease, owing to the excess of water in the organs. The same is to be said of rice, which is poor in nitrogen. According to Salvatore Thomassi, the farmers of the rice-fields in Italy, who enjoy liberal fare, reach an advanced age, while the day-laborers who live on rice succumb prematurely to diseases caused by exhaustion. In Western India, where rice is the chief food of the natives, they always add some element of food which is richer in azote. Those Italian laborers who come into Germany to work upon the railways always eat cheese with their staple food, maize. Other populations derive all their nutriment from 800 parts of bread and 100 of meat, besides potatoes and herrings. All these facts are in full accord with the experiment made on the dog, which lost weight on 800 parts of bread, or on the same with extract of meat, and finally died in convulsions.
On this subject we have the valuable researches of William Stark, dating from 1789. He tried experiments upon himself as to the relative value of different kinds of food. For 42 consecutive days he lived on 556 to 849 parts of bread and 900 to 1,800 water per day. Meanwhile he lost 17 lbs. weight. Then he took 736 to 962 of bread, 113 to 226 sugar and 900 to 1,300 water, and in 28 days lost 3 lbs. But he gained with 849 bread, 1,800 milk and 1,300 water. Hence it will be seen that prison-fare of bread-and-water is justly to be regarded as a punishment. In fact, the sentence condemning a man to live four weeks on such fare is in Danish law equivalent to sentence of death, and in Denmark no case has ever occurred of a culprit surviving his punishment.
Of course, Prof. Voit is far from condemning the use of vegetable substances for food; but he insists upon it that they must be combined with proper nutritive elements in assimilable form, in order to keep the body vigorous. These vegetable substances are deficient in albumen, and nothing can supply it better than flesh-meat. It is also advisable to substitute, for a part of the mass of starchy material, animal or vegetable fat. It is not denied but that one may sustain life on purely vegetable fare. The only conclusion the author insists upon in this first portion of his essay is, that the alimentation of man is always best secured, as regards azotic and fatty food, when the latter is got from animal matter, and oftentimes the elements in question cannot be derived from any other source. Chemistry alone will not account for this disparity. The principles of which we speak do not differ from one another chemically in their origin. It is only physiological experimentation, with the living being as the reactive agent, that can show these differing properties, as we have seen. This is a point of great importance. The question of nutritive salts, which we are next to consider, is no less important.—Revue Scientifique.