Human Foods and Their Nutritive Value, Harry Snyder [free e books to read TXT] 📗

that the digestibility of the meat is influenced by the mechanical character, as toughness of the fiber.

230. Digestibility of Vegetable Foods.—Vegetable foods vary in digestibility with their mechanical condition and the amount of cellulose or fiber. In some the nutrients are so embedded in cellular tissue as to be protected from the solvent action of the digestive fluids, and in such cases the digestibility and availability are low. The starches and sugars are more completely digested than any other of the nutrients of vegetables; in some instances they are from 95 to 98 per cent digestible. Some cellular tissue, but not an excess, is desirable in a ration, as it exerts a favorable mechanical action upon the organs of digestion, encourages peristalsis, and is an absorbent and dilutant of the waste products formed during digestion. For example, in the feeding of swine, it has been found that corn and cob meal often gives better results than corn fed alone. The cob contains but little in the way of nutrients, but it exerts a favorable mechanical action upon digestion. Occasionally too many bulky foods are combined, containing scant amounts of nutrients, so that the body receives insufficient protein. This is liable to be the case in the dietary of the strict vegetarian. Many of the vegetables possess special dietetic value, due to the organic acids and essential oils, as cited in the chapter on fruits and vegetables. The value of such foods cannot always be determined from their content of digestible protein, fat, and carbohydrates. This is particularly evident when they are omitted from the ration, as in the case of a restricted diet consisting mainly of animal foods. Many vegetables have low nutritive value on account of their bulky nature and the large amount of water and cellulose which they contain, which tends to decrease digestibility and lower the amount of available nutrients. Because of their bulk and fermentable nature, resulting in the formation of gases, a diet of coarse vegetables has a tendency to cause distention and enlargement of the intestinal organs. The carbohydrates, which are the chief constituents of vegetables, are digested mainly in the intestines, and require special mechanical preparation in the stomach, hence the nutrients of vegetables are not, as a rule, as easily digested as those of animal foods.

231. Factors influencing Digestion.—There are a number of factors which influence completeness as well as ease of digestion, as: (1) combination of foods; (2) amount of food; (3) method of preparation; (4) mechanical condition of the food; (5) palatability; (6) physiological properties; (7) individuality of the consumer; and (8) psychological influences.

232. Combination of Foods.—In a mixed ration the nutrients are generally more completely digested than when only one food is used. For example, milk is practically all digested when it forms a part of a ration, and it also promotes digestibility of the foods with which it is combined, but when used alone it is less digestible.[27] Bread alone and milk alone are not as completely digested as bread and milk combined. The same in a general way has been observed in the feeding of farm animals,—better results are secured from combining two or more foods than from the use of one alone. The extent to which one food influences the digestibility of another has not been extensively studied.

In a mixed ration, consisting of several articles of food of different mechanical structure, the work of digestion is more evenly distributed among the various organs. A food often requires special preparation on the part of the stomach before it can be digested in the intestines, and if this food is consumed in small amounts and combined with others of different structure, the work of gastric digestion is lessened so that the foods are properly prepared and normal digestion takes place. The effect which one food exerts upon the digestibility of another is largely mechanical.

233. Amount of Food.—Completeness as well as ease of digestion is influenced by the amount of food consumed. In general, excessive amounts are not as completely digested as moderate amounts. In digestion experiments with oatmeal and milk, it was found that when these foods were consumed in large quantities the fat and protein were not as completely absorbed by the body as when less was used, the protein being 7 per cent and the fat 6 per cent more digestible in the medium ration. Experiments with animals show that economical results are not secured from an excess of food.[5] Some individuals consume too much food, and with them a restricted diet would be beneficial, while others err in not consuming enough to meet the requirements of the body. Quite frequently it is those who need more food who practice dieting. When there is trouble with digestion, it is not always the amount or kind of food which is at fault, but other habits may be such as to affect digestion. The active out-of-door laborer can with impunity consume more food, because there is greater demand for nutrients, and the food is more completely oxidized in the body and without the formation of poisonous waste products. The amount of food consumed should be sufficient to meet all the demands of the body and maintain a normal weight.

234. Method of Preparation of Food.—The extent to which methods of cooking and preparation influence completeness of digestion has not been extensively investigated. As is well known, they have great influence upon ease and comfort of digestion. During cooking, as discussed in Chapter II, extensive physical and chemical changes occur, and these in turn affect digestibility. When the cooking has not been sufficient to mechanically disintegrate vegetable tissue, the digestive fluids fail to act favorably upon the food. Cooking is also beneficial because it renders the food sterile and destroys all objectionable microörganisms which, if they remain in food, readily undergo incubation in the digestive tract, interfering with normal digestion. Prolonged heat causes some foods to become less digestible, as milk, which digestion experiments show to be more completely digested when fresh than when sterilized. Pasteurized milk, which is not subjected to so high a temperature as sterilized milk, is more completely digested. See Chapter VII for discussion of sterilizing and pasteurizing milk.[38] The benefits derived from the destruction of the objectionable bacteria in foods are, however, greater than the losses attendant on lessened digestibility due to the action of heat. The method of preparation of a food affects its digestibility mainly through change in mechanical structure, and modification of the forms in which the nutrients are present.[5]

235. Mechanical Condition of Foods.—The mechanical condition of foods as to density and structure of the particles and the extent to which they are disintegrated in their preparation for the table influences digestibility to a great extent. The mechanics of digestion is a subject that has not been extensively investigated, and it is one of great importance, as biological and chemical changes cannot take place if the food is not in proper mechanical condition. In general, the finer the food particles, the more completely the nutrients are acted upon by the digestive fluids and absorbed by the body. Nevertheless, the diet should not consist entirely of finely granulated foods. Some foods are valuable mainly because of the favorable action they exert mechanically upon digestion, rather than for the nutrients they contain.[62] Coarsely granulated breakfast foods, whole wheat flour, and many vegetables contain sufficient cellular tissue to give special value from a mechanical rather than a chemical point of view. The extent to which coarsely and finely granulated foods should enter into the ration is a question largely for the individual to determine. Experiments with pigs show that if large amounts of coarse, granular foods are consumed, the tendency is for the digestive tract to become inflamed and less able to exercise its normal functions. Coarsely granulated foods have a tendency to pass through the digestive tract in less time than those that are finely granulated, due largely to increased peristaltic action, and the result is the food is not retained a sufficient length of time to allow normal absorption to take place. In the feeding of farm animals, it has been found that the mechanical condition of the food has a great influence upon its economic use. Rations that are either too bulky or too concentrated fail to give the best results. In the human ration, the mechanical condition of the food is equally as important as its chemical composition.

236. Mastication is an important part of digestion, and when foods are not thoroughly masticated, additional work is required of the stomach, which is usually an overworked organ because of doing the work of the mouth as well. Although much of the mechanical preparation and mixing of foods is of necessity done in the stomach, some of it may advantageously be done in the mouth. The stomach should not be required to perform the function of the gizzard of a fowl.

237. Palatability of Foods.—Many foods naturally contain essential oils and other substances which impart palatability. These have but little in the way of nutritive value, but they assist in rendering the nutrients with which they are associated more digestible. Palatability of a food favorably influences the secretion of the gastric and other digestive fluids, and in this way the natural flavors of well-prepared foods aid in digestion. In the feeding of farm animals it has been found that when foods are consumed with a relish better returns are secured than when unpalatable foods are fed. To secure palatability the excessive use of condiments is unnecessary. It is possible to a great extent during preparation to develop and conserve the natural flavors. Some foods contain bitter principles which are removed during the cooking, while in others pleasant flavors are developed. Palatability is an important factor in the digestibility of foods.

238. Physiological Properties of Food.—Some food materials, particularly fruits and vegetables, contain compounds which have definite physiological properties, as tannin which is an astringent, special oils which exert a cathartic action, and the alkaloids which serve as irritants to nerve centers. Wheat germ oil is laxative, and it is probable that the physiological properties of graham and whole wheat breads are due in some degree to the oil which they contain.[67] The use of fruits, herbs, and vegetables for medicinal purposes is based upon the presence of compounds possessing well-defined medicinal properties. As a rule food plants do not contain appreciable amounts of such substances, and the use of food for medicinal effect should be by the advice of a physician. The physiological properties of some foods are due to bacterial products. See Chapter XX.

239. Individuality.—Material difference in digestive power is noticeable among individuals. Digestion experiments show that one person may digest 5 per cent more of a nutrient than another. This difference appears to be due to a number of factors, as activity of the organs, as affected by exercise and kind of labor performed; abnormal composition of the digestive fluids; or failure of the different parts of the digestive tract to act in harmony. Individuality is one of the most important factors in digestion. Persons become accustomed to certain foods through long usage, and the digestive tract adapts itself to those foods, rendering sudden and extreme changes in the dietary hazardous. Common food articles may fail to properly digest in the case of some individuals, while with others they are consumed with benefit. What is food to one may prove to be a poison to another, and while general statements can be made in regard to the digestibility of foods, individual differences must be recognized.

240. Psychological Factors.—Previously conceived ideas concerning foods influence digestibility. Foods