Bacteric fermentations in the small intestine are unimportant under normal conditions, with the exception of the fermentation of cellulose. Cellulose has the same empirical formula as starch. It is completely insoluble, and is not affected by any of the digestive juices. The greater part of the cellulose consumed by herbivora is, however, broken up by bacteria into acetic and butyric acids, carbonic acid, and marsh-gas. In Man also a small quantity is similarly destroyed.
In the large intestine the bacteric fermentations are not unlike those which occur in the stomach, with, in addition, the destruction of proteins, or of products of proteid digestion. The greater the quantity of undigested food which reaches the large intestine, the greater is the development of bacteria. When the stomach is dilated, the ascending colon, and especially its cæcum, is usually dilated also. Bacteric fermentation in the large intestine, with resulting flatulence, is evidence of imperfect digestion, due either to an excess of food or to weakness of the alimentary organs, or, as is more commonly the case, to the combination of these two factors. The relation of fermentation to alimentation can be shown by counting the microbes in a specimen of the contents of the large intestine. In a particular case it fell from 65,000 per milligramme upon a mixed diet to 2,000 per milligramme upon a diet of milk.
In the world at large bacteria perform many offices of the utmost usefulness to other living things. They fix nitrogen in the soil, sweeten polluted rivers, reduce animal and vegetable matter to a condition in which it is available as plant-food. Their presence within the alimentary canal is inevitable; but it is somewhat doubtful whether, with the exception of the fermentation of cellulose, they do the economy any service with which it could not dispense. As parasites of the alimentary canal, some kinds are less desirable than others. Recently a method of limiting their variety has been introduced and advocated with much enthusiasm, as favourable to the hygiene of the digestive tract. In countries in which the cows are driven, in summer, to mountain pastures, the peasants of the plains live during their absence largely upon milk brought down at intervals, and allowed to turn sour. Sour milk, in Bulgaria, develops a bacterium of extraordinary vigour. It can live in a medium containing as much as 10 per cent. of lactic acid, a concentration fatal to other forms of Bacterium acidi lactici. It is easily cultivated, and when ingested continues to multiply in the alimentary canal. So peculiarly lusty is this bacterium that it makes life impossible for other germs. As it dies out after two or three months, it seems unlikely that a man who swallows the Bulgarian milk-germ runs a risk of inviting a repetition of the tragedy which followed the acclimatization of the mongoose in Jamaica. Its supremacy has been attributed to its capacity of developing a concentration of lactic acid too high for the well-being of other bacteria; but it is improbable that it has the opportunity of doing this in the alimentary canal of a person living on a mixed diet. The extinction of other bacteria (if they are extinguished) is more likely to be due to an antagonism of a more subtle kind, at present inexplicable, but not without parallel. The purifying influence of the water of the Ganges has for ages been an article of faith. Pilgrims from fever-stricken districts bathe in it, foul it, drink it, with the corpses of their fellows floating down the stream. Recently it has been shown that this belief is not without foundation. The water of the Ganges at Benares contains bacteria which are as tigers among lesser vermin. The germs of cholera and typhoid fever disappear from cultures into which these overbearing microbes are introduced.
Conditions Requisite for Normal Digestion.—When M. Chevreul, Professor of Chemistry at the Jardins des Plantes of Paris, attained his hundredth year, an interviewer very naturally inquired of him, “Have you always had a good digestion?” To this the still vigorous Professor answered: “I really cannot say, for I have never noticed.” So long as it is well used, the stomach is an unobtrusive organ. It is tyrannical when it deems itself the victim of inconsiderate treatment. A study of its physiology serves to show that it will work contentedly only upon certain clearly defined terms, of which the following are perhaps the most important: The stomach exacts due warning that its services are wanted. The nerves of smell and taste must announce the approach of food and guarantee its quality. “What may I eat?” asked a large-framed, strenuous, eager, overworked barrister of a great physician. “Eat, sir? You may eat whatever you like. But be quite sure that you do like it.” Wise advice. The human race would not have developed its strong preferences for certain kinds of food if all foods were equally suitable to satisfy its needs. Taste is not a matter of fashion. It is the expression of the experience of mankind. Fanciful as civilization has made us, and easily as appetite is perverted, if we are sure that we really like, and want, a food, we may trust that our liking will guide us as safely as it guides a buffalo or a deer. “Eat what you like.” Eating with liking carries with it the idea of obtaining the maximum of satisfaction from the exercise of this necessary function. Most things which are reckoned unwholesome are full in flavour or rich in consistency. They satisfy the palate when spread out very thin. It is poor economy to help oneself to caviare with a table-spoon. In the second place, the stomach must be assured that the teeth are doing their proper share of work. Among the many half-truths which every year are exalted to the level of a revelation or a rule of conduct is the doctrine of the “chewers”—persons who take no meals, but industriously and almost continuously masticate nuts and biscuits. Thirdly, the meal must not be so large that the stomach cannot deal with it “at a sitting.” In from two to three hours the last of the food should have passed through the pylorus, allowing the stomach to rest before it is called into activity again. As proteins are practically the only foods which are digested in the stomach, the work required of this organ depends upon the quantity of proteins present amongst the constituents of a meal. Meat is the food richest in proteins, although bread, vegetables, milk, cheese also yield them. Some people can digest three meat meals every day; but others, probably the majority, find that it is unwise to take any considerable quantity of meat more than once in twenty-four hours. It is only when the cells of the gastric glands have accumulated a store of pepsinogen-granules that proteid digestion is vigorously carried on. Fourthly, the food must be in a form in which it does not irritate the stomach, provoking an outflow of acid out of proportion to the pepsin which accompanies it. Experience alone can teach the foods which are to be avoided on this account. But speaking generally, it may be said that the stomach resents the presence of substances which cannot be amalgamated into chyme. Its task is the reduction of the mixture of foods which compose a meal to the consistence of a smooth cream. Hot buttered toast or pie-crust are made of wholesome constituents enough, but, fat being melted into the starch, the fragments are impermeable to the gastric juice. They act mechanically as irritants of the mucous membrane. Again, it may be said that “pure” foods are apt to provoke acidity. Nothing could be more wholesome than eggs or pounded meat or custard pudding; but taken by themselves these articles of diet over-stimulate the mucous membrane. They need to be diluted with starch-foods, or even with cellulose.
And this calls attention to the dietetic value of vegetables. Vegetables, which consist chiefly of innutritious cellulose, distribute the digestible constituents of a meal and increase its bulk, greatly favouring its progress through the alimentary canal. Especially in herbivora is it important that the bulk and looseness of the food should be well maintained. Rabbits thrive on sugar, starch, and albumin, mixed with such an absolutely indigestible substance as horn-shavings. If the inert substance be omitted, they die of intestinal inflammation, although fed on the same mixture of pure foods. Other rules which govern digestion might be mentioned; and it is needless to point out that, when the mechanism is deranged, steps adapted to the particular malady must be taken to bring it back to a normal condition. There is, however, one precaution upon which, in a certain number of cases, it is impossible to lay too much stress. The digestion of proteins is seldom carried out satisfactorily when much sugar, and especially much cane-sugar, has been eaten at the same meal. Excessive lactic fermentation prevents the proper peptonization of meat. The chemistry of digestion is not sufficiently well understood to enable the physiologist to say what is amiss; but probably by-products of peptic digestion are produced. To many people this is of little consequence; but to those who exhibit a gouty tendency it is, unfortunately, a most serious matter. Civilized races are particularly subject to the uric acid diathesis. In the course of nitrogenous metabolism uric acid is formed in place of fully oxidized and easily soluble neutral urea. Although the chemical sequence has not been discovered as yet, there is no question but that imperfect gastric digestion means the formation of uric acid, with all its lugubrious results: malaise, neck-ache, emotional depression. Birds and reptiles form uric acid as the end-product of nitrogenous metabolism, not urea. So also do city-fathers, butchers, and others whose diet consists too largely of meat. Many nervous, ill-nourished men and women tend to do the same, however abstemious their meals. It is useless to tell such persons to reduce the amount of proteins in their diet. Their attempts at increasing the starch, sugar, and fat at the expense of nitrogenous foods lead to dyspepsia, which makes matters worse. They often find, however, that if they are careful to restrict to the narrowest limits the amount of carbohydrates (especially sugar) which they take in conjunction with meat, fish, eggs, or other proteid foods, the formation of uric acid ceases. Sugar, bread, fruit, and other carbohydrates, may be taken in abundance, and with great advantage, at breakfast and lunch, without proteid food, if dinner consists of broth, fish, meat, cheese, vegetables, with a minimum of bread.
The History of the Foods after Absorption.—All foods, with the exception of inorganic salts and salts of various vegetable acids, fall into three classes: (1) Proteins—substances of complex chemical constitution, containing nitrogen; (2) carbohydrates—so called because hydrogen and oxygen, in the proportions in which they enter into the formation of water, are united with carbon; (3) fats. Proteins of various kinds are consumed as food. The peptones produced from them by digestion also vary. Yet very little is known as to the differences in physiological value which distinguish the various kinds of protein when absorbed into the fluids of the body ([cf. p. 134]). All carbohydrates after digestion and absorption appear as dextrose. The various fats preserve their individuality until they are taken up by the tissues. When fixed in the tissues, they assume, except under somewhat abnormal conditions, the composition characteristic of the fat of the animal which has eaten them. If a dog which has been severely starved is fed upon mutton-fat, it puts on in the first instance fat which resembles that of a sheep rather than the normal fat of a dog. As soon, however, as it is well nourished (which would never occur unless some protein and carbohydrate were added to the mutton-fat), its fat assumes the usual form.
For practical purposes we are obliged to speak of the three classes of food—proteid, carbohydrate, and fatty—as if there were but one member in each class. And we have abundant evidence that such a simple classification is fully justified. The body has so large a power of altering chemically the nature of the food which it absorbs that it makes little difference in the further history of the food whether the protein supplied to it be an albumin or a globulin; the fat, stearin, palmitin, or olein; the carbohydrate, starch or sugar.
In earlier days it was customary to regard the body as the receiver of a variety of foods which it could break down into simpler substances by oxidation, but could not reconstruct. Plants were regarded as the manufacturers of organic compounds, animals as the destroyers of the complex substances made by plants. The union of molecules, synthesis, was looked upon as the function of the vegetable kingdom. Animals built into their tissues the products elaborated by plants; some of these products they shook to pieces for the purpose of setting their energy free; others slowly disintegrated as the result of tissue “wear and tear.” Gradually it was realized that many chemical changes occur in the body which cannot be viewed as merely exhibitions of its analytical capacity. The tissues were recognized as laboratories in which reactions occur which consist in something more than the splitting of complex into simpler molecules. The instances earliest understood were connected with the history of carbohydrates and fats. In the disease diabetes an enormous quantity of sugar is excreted, amounting in extreme cases to between 1 and 2 pounds per diem. When carbohydrates are present in the food, the amount of sugar excreted in diabetes is greater than it is when they are withheld; on an almost exclusively proteid diet the amount of sugar excreted far exceeds the amount of carbohydrates in the food. Another illustration of the power of making sugar possessed by the animal economy is afforded by a dog fed upon lean meat, and nothing else. Sugar is found in its blood, and a store of carbohydrate (glycogen) in its liver. The formation of fat is an instance of constructive metabolism. There is abundant evidence that the quantity of fat produced may greatly exceed the quantity contained in the food. Animals are fattened for the market on a diet which contains less fat than that which accumulates in their bodies. When nursing her young, an animal may secrete in her milk much more fat than she obtains as such in food. It was a great mistake to suppose that the body is dependent upon its tradesmen for fat and sugar. It can make either of these substances out of a mixed diet in which it is relatively deficient. It must, however, be a mixed diet. An animal cannot live exclusively on fat or exclusively on carbohydrate. It is impossible, therefore, for us to determine whether, if given the one alone, it can turn it into the other. Chemists were very unwilling to credit the body with the power of performing even the simpler of these transformations—the conversion of carbohydrate into fat. Proteins are essential constituents of a fattening diet. Their immensely complex molecule has always afforded a tempting field for arithmetical ingenuity. It is easy to remove from it the atoms needed for the composition of fat, and yet to leave such groups of atoms as might reasonably be supposed to constitute its “nitrogenous moiety.” The hypothesis that the metabolic capacity of the body is limited to analytical processes justified the supposition that, when more fat is laid on than the food contains, the balance comes from proteid substances, which split into nitrogenous and fatty moieties. It has been shown, however, that an animal during fattening may put on more fat than is contained as such in the food, or obtainable from its diet, even though all the atoms of carbon and hydrogen in its proteid food were devoted to its formation. The balance must come from carbohydrates. Perhaps a still more striking illustration of constructive capacity is the power of making glycerin. If a dog receive fatty acids in its diet, it accumulates normal fats. The glycerin which, united with fatty acids, constitutes the fat, was not contained in its food. Starch and sugar are sources of fat. As yet there is no evidence that fat can be converted into sugar.
The chemistry of the nitrogen-containing compounds appears to present more difficult problems. Plants build up proteins. Is the animal’s relation to these substances limited to their disintegration? Do proteins inevitably descend from step to step until they reach urea? There are reasons for thinking that, even when dealing with nitrogenous substances, the metabolic power of the body is not exclusively analytical. The liver can make urea from ammonia-salts, such as lactate, or even carbonate, of ammonia—substances more stable, and therefore in the chemical sense simpler, than urea. This is an indication, though a faint one, that the body has a constructive capacity, a power of producing more complex from simpler substances, even in the case of nitrogenous compounds. Beef-tea, mutton broth, meat-extracts have long been regarded as foods of value when the power of assimilation is low. Chemists point out that the nitrogenous substances which these decoctions contain are so near the bottom of the ladder that the energy set free by their further oxidation to urea is scarcely worth consideration. They admit that their ready availability renders them useful as restoratives, but they deny them the status of foods, on the assumption that their further progress must be downward. As was stated when the conversion of peptones into leucin and tyrosin was described, evidence is beginning to accumulate which shows that within certain limits, at present impossible to define, the system can reconstruct its proteins from amides and other simple products of their degradation.
The animal economy receives, and after due digestive preparation absorbs, three classes of food—nitrogenous, fatty, and carbohydrate. If either of the two latter kinds be deficient in the diet, the body can to a certain extent produce it from the other two. What is the special value of each kind of food? What use is made of it? Before attempting to answer these questions, we must endeavour to trace the further history of the foods after they have traversed the wall of the alimentary canal.