Physicians prescribe, cooks prepare and people eat all manner of combinations of food, without the slightest regard for the physiological limitations of man's digestive system. It is the general view, lay and professional, that the human stomach should be able to digest about any number and variety of food substances that may be put into it at one time.

Digestion is governed by physiological chemistry but the so-called food scientist continues to disregard this fact. He writes out his menus without the slightest thought of the decomposition that his jumbled mixtures are certain to cause in the digestive tracts of his patients. He never thinks of the fact that he is actually poisoning those who pay him for advice and instruction.

Certain physiological limitations of the digestive glands and of the digestive enzymes and juices should be considered in planning a meal for either the well or the sick. With a knowledge of these limitations at hand, we are in a position to plan a dietary which will adjust itself to the physiological limitations of the digestive glands and their secretions.

Not what we eat but what we digest and assimilate adds to our health, strength and usefulness. An unhampered or unimpeded digestion may be guaranteed only to the extent to which we guard our stomachs against food combinations and mental and physical conditions which disturb and impair digestion. A stomach that is reeking with decomposition will not supply to the body the "calories" and "vitamins" originally contained in the food eaten.

The specific action of the digestive enzymes, the careful timing of their secretion and the adaptation of the strength and character of the digestive secretions to the character of the food upon which they are to act was seen in our study of the processes of digestion. The more these facts are studied, the more it appears to be utterly impossible to digest the conglomeration that makes up the usual meal of the average man or woman.

In this chapter it is intended to shed more light upon the principle that the digestion of different foods requires special adaptations in the digestive juices. This is true in man, in animals, in plants. Suppose we begin with the carnivorous plants.

Carnivorous plants are of three general types--namely, (1) plants with an adhesive apparatus with which to catch their prey, such as Drosophyllum Lusitanicum, (2) plants which show movements in the capture of their prey, as Pinguiculavulgaris (Butter-wort) and Drosera Rotundifolia (Sun-dew), and (3) plants with traps for the capture of insects, as Nepenthatceoe (Pitcher Plants), Cephalotus Follicularis, Lathroea Squamaria (Toothwort), Dionacea Muscipula (Venus' Fly-trap), Utricularice Neglecta (Bladderwort), etc.

The leaves of the first class, of which Drosphyllum, which grows in Portugal and Morocco, is representative, are covered with a viscid and very acid substance, secreted by the glands in the leaves and flower stalks. The drops of secretion are readily removed from the leaves but are replaced with great rapidity, the glands being able to secrete large quantities of acid juice.

An insect alighting on the leaf becomes clogged by the viscid secretion adhering to its legs and wings. Crawling on, as it is unable to fly, it is soon bathed in the acid fluid, and sinks down dead. It is not uncommon to see a plant covered with the refuse of dead insects, which have been exhausted of their nutriment, while, at the same time, there are numerous recently alighted insects struggling to get away.

Experimenting with these plants, Darwin found that their secretion is absorbed in about an hour and a half if a proteid is placed upon them; no absorption can be detected when small bits of glass or charcoal are used in the same way. Unlike many of the carnivorous plants, Drosophyllum does not secrete very much more fluid after stimulation by albuminous substances, but the fluid becomes more acid and contains more ferment.

Butterwort, in addition to secreting a viscid fluid, with which to catch its prey, also curves up its leaves on the prey in digesting it. Soon after an insect is captured, the glands in contact with it (none of the glands except those in actual contact with the insect secret the digestive fluid), pour out a large quantity of fluid, which is more viscous and strongly acid in reaction, the previous secretion having been neutral. Although fragments of grass, placed on a leaf of this plant, occasion a slight degree of incurvation, which begins as soon as does incurvation following the capture of an insect, it does not last long and is not accompanied by any secretion. The secretion of the glands of this plant is neutral in the fasting state, but shortly after stimulation with a nitrogenous substance becomes acid and contains an enzyme with an action similar to that of pepsin. A close parallelism exists between the digestive power of Drosera and that of the mammalian stomach, both of which are able to digest vegetable as well as animal proteins. The secretions of Drosera digest those same substances--albumen, milk proteids, fibrin, etc.,--that are digested by hydrochloric acid and fail to digest those substances — cellulose, epithelial cells, mucin, starch, oils, etc., that gastric juice fails on. The digestive process is stopped in both by the addition of an alkali--such as soda--and recommences on the further addition of acid. In both, the enzyme requires the presence of an acid to activate it.

Darwin's experiments with these plants showed that while digestible nitrogenous substances excite the secretion of an active juice, indigestible substances, even if nitrogenous (with the exception of ammonium salts) only occasion an increased secretion of an acid but inactive fluid.

Venus' Fly Trap, which closes up on its prey, pours out an active juice with which to digest it. Unlike the glands of Drosera, the glands of this plant do not secrete before stimulation, nor do they act unless the stimulant is nitrogenous and soluble. The secretion is acid and contains a peptic ferment. It is also strongly antiseptic.

I need not consider more of the many types of carnivorous plants that exist and have been studied. In general we may say that during the "fasting" state, the digestive secretions of all carnivorous plants are either neutral, or are but mildly acid in reaction; but, that when digestible nitrogenous substances are placed upon them, the secretion becomes strongly acid and contains an enzyme, previously absent, which acts upon proteins as does pepsin. If non-nitrogenous substances are placed upon the leaves there may or may not be an increased secretion. If the secretion is increased, it may be acid, but will contain no enzyme.

In the case of the nitrogenous substances, the juice contains active digestive properties, which have the same action on protein substances as does pepsin; while with indigestible bodies, even if nitrogenous, and in the case of all non-nitrogenous substances, these active digestive agents are absent. We note here a precise adaptation that is seen in all forms of life.

There is not only a difference in the character of the juice poured out upon different substances; there is also a marked difference in the amount of fluid poured out and in the length of time it is secreted. There are also marked differences in the physical behaviour in these plants towards the various substances. When protein is placed upon the leaves of some plants, the incurvation of the leaf is often great enough to completely envelop the bodies and lasts until digestion and absorption are completed. But if bits of glass are dropped on their leaves, a certain degree of incurvation occurs, but it is not accompanied with any secretion and does not persist.

Dr. N. Phillip Norman, Instructor in Gastro-enterology, N. Y. Polyclinic Medical School and Hospital, New York City, says: "In studying the action of different enzymes, one is struck by Emil Fisher's statement that there must be a special key to each lock. The ferment being the lock and its substrate the key, and if the key does not fit exactly to the lock, no reaction is possible. In view of this fact is it not logical to believe the admixture of different types of carbohydrates and fats and proteins in the same meal to be distinctly injurious to the digestive cells? If since it is true that similar, but not identical, locks are produced by the same type of cells, it is logical to believe that this admixture taxes the physiologic functions of these cells to their limit."

The digestive juices are complex and elaborately contrived fluids. The work of the digestive canal is beautifully performed and most carefully adapted to the work in hand. For each set of raw materials, a suitable combination of digestive secretions, with special properties, are required and are, therefore, produced. The digestive glands are able to vary their work considerably, not alone with respect to the quantity of juice secreted, but also with respect to the properties of the juice.

There is more water or less water, a higher degree of alkalinity, or acidity, a different degree of concentration of the enzyme or a total absence of enzyme, as required by the different classes of foods and as required at various stages of the process of digestion of one kind of food.

All of these separate conditions of juice activity are not without their importance, although they are wholly disregarded by "orthodox" dietitians and by the physicians and practitioners of all schools.

Physiologists usually gloss over these facts, so that the student of physiology is not impressed with their practical importance.

It is clear that the character of the juice corresponds with the requirements of the food to be acted upon. Carbohydrate foods receive a juice rich in carbohydrate-splitting enzymes, protein foods receive a juice rich in protein-splitting enzymes, etc. This alteration of the juice extends both to its strength and to the absolute quantities of the ferment, just as we saw it do in the case of the carnivorous plants. So different in character are the specific secretions poured out on each different kind of food, Prof. Pavlov speaks of "milk-juice," "bread-juice," "flesh-juice," etc.

Based on our knowledge of the chemistry of digestion, briefly presented in this and the chapter on digestion, I present the following rules. These rules have been carefully tested in practice by the writer and many others and have stood the test of experience. They should be followed by well and sick alike.