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   Many misconceptions about the digestibility of different foods exist, not alone in the minds of laymen, but also in the minds of those who are supposed to know. Studies of this subject have largely centered on the emptying time of the stomach after various foods have been eaten and the amounts of the various foods that are digested as shown in the stools. As both the emptying time of the stomach and the amounts of undigested foods that appear in the stools are affected by a number of factors, it is obvious that a large margin of error is provided for.

   A food is likely to be classed as difficult of digestion merely because it remains in the stomach longer than other foods, whereas, it may not be difficult to digest at all. The process of digestion may merely be different and call for different timing of the digestive secretions. We know, although it is commonly ignored, that the time a food remains in the stomach is determined, not by the ease or difficulty of its digestion, but by the nature of the digestive process required to digest a particular food. A food that digests in the stomach in four hours is not necessarily more difficult to digest and does not demonstrably use up more energy, than one that digests in one hour. The digestive process is simply different. A food may be classed as not very digestible merely because much of it is passed out in the stools in an undigested state, whereas, the undigested food may appear in the stools because the person to whom it is fed is unaccustomed to eating it and has not learned to digest it, or because it was fed in combinations that prevented its digestion, or because of fatigue, or from other causes. A food that is well-digested by one individual at one time under a given set of circumstances may be poorly digested by the same individual at another time under different conditions.

   After making hundreds of digestion experiments Prof. Atwater computed the average coefficients of digestibility of the proteins, fats and carbohydrates in the main groups of foods used by man as part of a mixed diet to be as shown in the following table:

   These figures, which differ in certain particulars from later findings, reveal less differences between the digestibility of various foods than are popularly supposed to exist. Prof. Sherman makes a remark that the table cannot sustain. He says that "it is noteworthy that the coefficients of digestibility are less influenced by the conditions under which the food is eaten and vary less with individuals than is generally supposed." That individual variations are not, on the whole, very great, I believe to be true; but that the conditions under which food is eaten influences the digestion of the food but little, I am sure is a statement that cannot be sustained.

   The figures in Prof. Atwater's table are averages. They do not show individual variations, nor do they show differences under different conditions. They are averages compiled from digestion of food by various individuals under different conditions. It is more than likely, also, that the variations in the conditions under which foods were eaten in his experiments were not great. The digestibility of a food is one thing and is not influenced by the conditions of the individual; the digestive ability of the individual is quite another thing and is markedly influenced by the varying conditions of the individual.

   Atwater did not take into account, in his experiments, the findings of Pavlov that the ability to digest a particular kind of food may be increased or decreased by eating that food or by refraining from eating that food. So far as I can find, no other experimenter has taken this fact into consideration. From Pavlov's findings it is obvious that a vegetarian would not have much flesh-digesting ability. This, no doubt, accounts, in large measure, at least, for the distress experienced by vegetarians when they stray from the fold and indulge in a flesh meal. No doubt, part of their distress is of psychologic origin. On the other hand the man who regularly and habitually consumes nuts as a part of his diet will have greater nut-digesting ability than the man who seldom or almost never eats nuts. It is well-known that many people have trouble with spinach. Repeated tests have shown that if these people are fed spinach daily for a brief period, they acquire the ability to handle it without difficulty. I have seen two people in whom milk did not even coagulate, but was passed out quickly in a fluid state as it was swallowed. In these two individuals, the milk was expelled by the colon within three minutes after it was swallowed. As they were not kept on milk I had no opportunity to learn what adjustments they were capable of. I doubt that they secreted any renin, although the rapidity with which the milk was expelled would indicate that there were also other troubles.

   Body chemistry is, to a large extent, determined by food. Physiology bears out the statement that a particular feeding habit produces particular enzymes or modifications of enzymes, a particular composition of the body fluids and glandular secretions and particular nerve developments. Feeding habits are cumulatively affective and provide the soil for metabolic responses. Perhaps in no other function is the change in enzymic action so obvious and in no other function is it so quickly apparent as in the function of digestion.

   There is a tendency in all living tissues to enter into a condition of a more or less stable nature, when under the influence of forced work or its opposite. Pavlov found this same to be true of the digestive secretions. For example, in feeding animals, it was found that when the diet is altered and the new diet maintained for a length of time, the enzyme-content of the juice becomes, from day to day, more and more adapted to the requirements of the new food.

   A dog fed for weeks on nothing but milk and bread, was placed on an exclusive flesh diet, containing more protein and scarcely any carbohydrate. The ability of the digestive juice to digest protein increased day by day, until it reached its maximum strength, while its amylolytic (starch-splitting) power progressively declined. A dog so fed showed "even after the lapse of three days" "that the proteid ferment tended to increase, while the starch ferment declined." This change has been noted for as long as up to tewnty-three days.

   Reversing the process, a dog fed exclusively upon a flesh diet, was placed on milk and bread and its pancreatic juices observed. The protein-digesting power of the juice decreased progressively, until, after sixty-six days on the bread and milk diet, the whole of the pancreatic juice secreted in twenty-four hours was collected and it was found that "the digestive power for proteid is absolutely nil." Meanwhile the starch-digesting power had increased.

   In some dogs the changes in the character of the pancreatic juice begin to manifest themselves soon after the change of diet, while, in the other dogs, the changes are slower. In this latter type, an abrupt change from one type of diet to another type of diet often produces serious illness--a curative crisis. If a sudden and radical change from the conventional diet to a new one did not result in some systemic reactions we would feel that the new diet was of the same character as the old one and expect no beneficial results therefrom.

   These things are of importance, not alone in showing that the type of juice adapted to the digestion of one type of food is practically valueless in digesting another type of food, but, also, in showing that the longer one type of food is eaten, the more efficient will digestion become. The way to learn to digest a food that is difficult for us to digest, is to continue to eat it. If pepsin or hydrochloric acid is lacking in the diet, not abstinence from, but indulgence in proteins will increase these. There are, of course, limitations. Overstimulation will have the opposite effect.

   Many people have difficulty in handling spinach, but just as "practice makes perfect" in other works, so, persistence in eating spinach will build up the power to digest it. This fact has been demonstrated in hundreds of cases. This rule is applicable to all articles of food.

   We tend to seek foods easy of digestion. This practice undoubtedly weakens our powers of digestion. A true digestive athleticism would seek to build up the powers of digestion by an intelligent employment of foods requiring more and not less work in digestion. Pampering the digestive system undoubtedly makes a molly-coddle of it. I do not mean to sanction abuse of the digestive system--this is another thing.

   Examining the stools to determine how much of a food passes undigested does not determine the digestibility of a food, not at least, until the individual has been fed upon the food under investigation long enough to acquire maximum power to digest it. It is also essential, in determining the digestibility of a food, that it be fed in combinations that do not interfere with enzymic action. Starch that may be freely digested will show up in the stools as undigested starch if eaten with acids or with proteins.

   The vegetarian soon acquires the power to digest and metabolize his new diet. No doubt, too, this power is hereditary. Reinheimer says "that the inheritance of chemical properties which are of great importance for the production of form actually takes place is now as well known as that of morphological properties." From limited observations I am convinced that a child relishes most and digests best the type of foods upon which the mother fed during pregnancy.

   Digestive speed and efficiency vary with individuals and with circumstances. However, in general, foods leave the average stomach about as follows:--fruits, vegetables, bread, eggs, lamb, beef, pork, chicken, nuts, guinea hen. Carbohydrates usually leave the stomach rapidly, proteins remain longer. Foods requiring longer time for gastric digestion are not necessarily harder to digest; it is often merely that the process of digestion is different.

   Foods requiring longer time to digest remain in the stomach longest. Beef requires but slightly longer to digest than lamb. Chicken requires longer than pork despite all of the fat of the latter.

   Red beets pass through the stomach rapidly. So do asparagus, raw tomatoes, lettuce (unless delayed by the usual dressings), and most vegetables low in protein and starch. Vegetables containing much starch are held up for more thorough digestion. Low protein vegetables leave the stomach with little change. Raw cabbage leaves the stomach more quickly than cooked cabbage, a thing most people know. Baked beans are slow to leave the stomach. Spinach is slow, compared with other vegetables.

   Eggs and milk go more slowly than eggs alone. Old eggs require more time in the stomach than fresh, or even cold storage eggs. Boiled eggs remain in the stomach longer than raw ones. Scrambled eggs remain longer still. Raw egg whites leave the stomach rapidly. Egg white does not encourage gastric secretion, unless taken with orange juice, and is poorly digested and badly assimilated.

   Raw milk leaves the stomach slowly. Pasteurized milk more slowly and boiled milk still more slowly. Milk rich in fat leaves the stomach more slowly than milk low in fat content. Buttermilk stimulates gastric secretion.

   Bacon digests slowly and, perhaps due to its fat, lowers stomach acidity. It is difficult for most people to digest. Fat markedly inhibits gastric secretion and the movements of the stomach and slows down digestion.

   Foods are not digested when they have passed out of the stomach. A large part of the work of digestion takes place in the intestine.

   We learned in the previous chapter that such things as coffee, tea, bitters, etc., cause an early emptying of the stomach without, in any way, shortening their digestion time. In other words, foods may be sent out of the stomach before gastric digestion is complete.