This section is from the book "Practical Dietetics With Special Reference To Diet In Disease", by William Gilman Thompson. Also available from Amazon: Practical Dietetics with Special Reference to Diet in Disease.
Bauer says: "By the digestibility of a food one can obviously understand nothing more or less than the sum of the resistances that it offers to the action of the gastric juice".
The time required for gastric digestion cannot be stated with absolute accuracy. In general, the period for the full digestion of a mixed meal consisting of bread, meat, and vegetables is three and a half hours, but because some kinds of food are thoroughly'digested in the stomach, while others are acted upon but little if at all, the preponderance of one or other article of food in a mixed diet may affect somewhat the whole period of digestion. Tables are sometimes given in text-books of physiology in which the time required for digestion of various kinds of meat are stated in a very definite manner, but all such statements should be received with ample allowance for ordinary variations. If one considers for a moment the different elements concerned in the process of gastric digestion, it is obvious that the normal time required for complete digestion will vary constantly, even in the same individual in a state of health, for it depends upon the kind of food eaten as well as the following conditions: 1. Its state of subdivision, its solubility, complexity, and the process of its cooking. 2. The rapidity with which it is swallowed. 3. The thoroughness with which it is masticated. 4. The activity of the stomach at the time. 5. The interval which has elapsed since the previous meal. 6. The condition of the blood and nerve supply of the gastric glands. 7. The activity of peristaltic movement, which may either retard or hurry the rate of emptying the stomach. 8. The amount of fluid drunk with which the gastric juice is diluted. 9. The strength of the important ingredients of this juice, its volume, and the effect upon it of food itself, neutralising it or not. 10. The rate of absorption. 11. Habit. 12. Idiosyncrasy. 13. The presence of excess of fat or other materials incapable of digestion in the stomach. 14. The diversion of the nervous energy required for digestion to other functions, such as mental or muscular work.
Such statements are often definitely made in articles upon dietetics as that boiled mutton requires three hours for stomach digestion, while roasted mutton requires three hours and eighteen minutes; that raw oysters require two hours and fifty-five minutes and roasted oysters three hours and eleven minutes; that boiled carrots require three hours and sixteen minutes, and the like. These figures may impress the lay mind as being of interest and accuracy, but when the above considerations have been taken into account, it must be seen that they are far from reliable. It is important, however, to state, if possible, the approximate time required for the digestion of certain general classes of food, and the following estimates are probably as nearly correct as possible in view of the above statements.
The average time required for meats cooked by broiling, roasting, or boiling is fully three hours or three and a half hours for their complete digestion. Gigglberger found, as a result of feeding patients with test meals and withdrawing the stomach contents through a tube, that meat requires from two and a half to five and a half hours for digestion, according to its quality, method of cooking, etc. Stewed meats need less time than roasts. Pork and very fat meat may require four or five or more hours for digestion, and veal needs at least four hours. Fresh lamb may be digested in two and a half to three hours. Chicken, capon, and turkey may be digested in from two to two and a half hours, but the meat of many of the forms of wild birds, such as ducks, may require four hours. Some of the viscera of animals which are eaten as food, such as brains, tripe, liver, and kidneys, are digested in less time - approximately two hours. Fish and shellfish require from two and a half to three hours for their digestion. Raw eggs are usually digested in less time than cooked ones, and they may be ready for absorption in two hours, whereas hard-boiled eggs require at least three and a half hours. Milk is usually digested somewhat sooner when boiled than if raw.
Milk whey is absorbed from the stomach, but the curds often pass on into the duodenum.
The majority of the heavier vegetables, such as peas, beans, corn, beets, turnips, etc., remain in the stomach between three and three and a half hours, but potatoes, if baked and mealy, may remain but two and a half hours. Raw vegetables, such as coleslaw or raw cabbage and lettuce, etc., may remain two and a half hours or more in the stomach, and the same is true of most raw fruits, such as berries, apples, pears, peaches, etc. The more digestible cereals - rice, sago, tapioca, etc. - should pass on from the stomach within two hours. Fats and oils taken alone may remain in the stomach only a few moments. Sugars may, to some extent at least, be absorbed from the stomach wall in the course of the first hour of digestion.
Since gastric juice is fairly constant in composition, its effect in health upon the duration of digestion of the same kind of food may not vary in the same individual; but persons differ from one another very much in the rapidity of their digestive processes. In some persons, even in health, stomach digestion may be uniformly an hour or more slower than it is in others. Variations from the usual period of digestion are almost always upon the side of its retardation. The quality, composition, and quantity of the food all affect the rate of gastric digestion.
Coarse food, badly cooked starchy food, excess of fats, tough-fibred meats, unripe fruits or vegetables, all retard digestion, and may prove very irritant.
Among articles of diet, substances are sometimes eaten which are purely refuse material, such as the skin of potatoes and of coarse fruits, grape seeds, shells, soft-shell crabs, etc.
They may pass on unaltered into the intestine and cause diarrhoea, or they may sometimes linger for several days and excite reflex irritation. I have known undigested capsules of cod-liver oil to be retained in the stomach for three or four days before being vomited. (See Cod-liver Oil, p. 205).
The temperature of food and drink affecting gastric digestion is discussed under the heading Temperature and Digestion (p. 338).