Читать книгу Dietetics for Nurses - Fairfax T. Proudfit - Страница 15

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Up to a few years ago it was believed a complete diet should contain an adequate amount of protein of a proper type, a sufficient amount of calcium, phosphorus and iron, and enough carbohydrates and fats to furnish the body with sufficient fuel to cover its energy expenditures. This belief was proved to be incorrect a number of years ago by Dr. Hopkins of England. In making certain feeding experiments with rats, Dr. Hopkins showed that some substance or substances present in milk, other than those already mentioned, was essential for the growth of the animal; that animals deprived of this material grew for a time, but gradually ceased to do so. Later on, Osborne, Mendel, McCollum and Davis discovered a like substance in butter fat; and still later Dr. McCollum found the same growth stimulating material, or one very like it, existing in the leaves of plants. These scientists found, upon investigation, that there were probably two substances in milk—one soluble in the fat, the other in the protein-free and fat-free whey—both of which were essential for normal growth. In 1911 Dr. Funk discovered in rice polishings a substance which he believed to be a cure and preventive of Beri-beri; to this substance, which is now believed to be identical with the second substance found in milk, he gave the name “vitamine.” Dr. Funk’s name “Vitamine” is now accepted to cover a number of substances essential to growth, and for the prevention and cure of certain diseases. To the first two has been added a third member of the vitamine family, which has proved to be a cure and preventive of scurvy. These substances are called—on account of the substances in which they are soluble—“Fat soluble A,” “Water soluble B,” and “Water soluble C.” The table on page 496 shows the sources from which these factors may be obtained. The four plus system is used by Dr. Eddy to describe the abundance with which they occur.[11]

Function of “Fat Soluble A.”—All investigators agree that the “A” vitamine is an essential factor in the growth of young tissue, and the repair of mature tissues. McCollum claims that this vitamine is likewise a factor in the prevention of the eye disease known as xerophthalmia, and other scientists also hold this opinion. Eddy states that a diet lacking in the “A” vitamine will, in the majority of cases, result in stunted growth and the development of the eye disease, and that the appearance of the latter may be taken as a sure indication of the absence or deficiency of this vitamine.

The following diagram shows the effect of adding fat soluble “A” to the diet which was adequate in other respects. This chart represents the growth curve of young rats.[12]

Figure showing the effect upon growth of adding “fat soluble A”

to a diet adequate in all other respects.

Courtesy of Dr. E. V. McCollum.

Mellanby of England believes the “A” vitamine to be a factor in the prevention of rickets. Scientists of America have recently investigated this disease, and Dr. Hess (New York) has found cod liver oil to be a remedy for it. Cod liver oil is known to be rich in “Fat soluble A,” but whether the cure of rickets is due to the presence of this vitamine in the oil, or to a possible fourth vitamine, is still undetermined.

Effect of Heat on the “A” Vitamine.—Heat, as applied in the ordinary methods of cooking, is not believed to exert a great deal of destruction upon the “A” type of vitamine; but hydrogenation, the process used in the hardening of certain fats in the manufacture of lard substitutes, is said to destroy it completely.

“Water Soluble B.”—The second vitamine discovered in milk and believed to be identical with the Funk vitamine is more widely distributed than the “A” vitamine. For this reason it is not so likely to be deficient in the diet as is found to be the case with the “A.” A glance at the table shows that the best sources outside of yeast are the seeds of plants and the milk and eggs of animals. In beans and peas the “B” vitamine is distributed throughout the entire seed, but in the cereal grains it is found chiefly in the embryo. As a result, bread made from fine white flour or meal is much more apt to be deficient in vitamine of the “B” type than that which is made from the whole grain; the same is true of rice and other cereals. Spinach, potatoes, carrots and turnips show an appreciable amount of the vitamine, but beets are known to be extremely poor in it. Nuts too are considered a valuable source.

Function of the “B” Vitamine.—Like the “A” vitamine, water soluble “B” is believed to be essential to growth. Funk established its value as a preventive and cure of Beri-beri, the disease common in the Orient among people living largely upon a diet of polished rice and fish. Besides being a growth-stimulating substance and an antineuritic, the “B” vitamine is highly valued for its stimulating effect upon the appetite. To this property is probably due at least part of the credit for which certain substances work for the promotion of growth in animals. This can be utilized to good advantage for children showing a disposition to refuse food, by supplementing formulas made from milk,[13] with the expressed juice of vegetables and fruits known to be rich in the “B” vitamine.

Effect of Heat on the “B” Vitamine.—This vitamine also shows a resistance to heat; that is, as applied in the methods generally used in cooking, pasteurization temperatures do not materially affect the vitamine property of the formula as far as the “A” and “B” factors are concerned.

The Effect of Alkali (Soda) upon the “B” Vitamine.—It has been an ordinary practice to add soda to the water in which certain vegetables are cooked, for the ostensible purpose of softening the vegetables and hastening their cooking. The practice has been condemned by many scientists who are making experiments along these lines, on account of its destructive power upon the “B” vitamine. Chick and Hume in England claim that when the amount of food given contains originally just sufficient vitamines to cover the growth factor the use of soda in the cooking water does serious harm to these vitamines. This is a point well worth remembering. It is often difficult to persuade certain individuals to eat vegetables in appreciable quantities; if the vitamines were reduced though the method of preparing the food, these individuals would not obtain a sufficient quantity of the vitamines.

“Water Soluble C.”—The third member of the vitamine family is known for its antiscorbutic property; that is, it is the best known cure and preventive of scurvy. It likewise exerts a certain amount of influence upon the growth of the animal and must be present in the diet, in order that the health and well-being of the individual may be safeguarded. The “C” vitamine, like the “B” vitamine, is soluble in water, and is present to an appreciable extent in the fresh juices of the fruits and vegetables. Some are richer in this respect than others (orange and tomato juice), while the cereals, grains, seed of plants, sugars, oils, and meats are singularly deficient. Milk (whole) does not contain a great amount of the “C” vitamine, and this amount is still further reduced under certain methods of preparation. Milk powders, made either from the whole or the skimmed milk, are found to contain only very small amounts of this essential substance. Condensed milk and cream are supposed to be free of “C,” and the same is true of eggs.

Effect of Heat on “C” Vitamine.—All authorities agree that the “C” vitamine is much more sensitive to heat than the other two; and for this reason much of the value obtained from this vitamine in uncooked material may be lost when the food containing it is subjected to long-continued heat. Hess claims that the temperature used for pasteurizing milk for some time, is more destructive to this vitamine than boiling water temperature continued for a few minutes only.[14] There is need for care in formulating the diet for children to see that they are given fresh fruit every day; or when that is not possible, to see that they are at least given tomato juice. This substance is rich in the antiscorbutic vitamine, and according to experiments made by Sherman, LeMer and Campbell, loses fifty per cent. of its antiscorbutic power when boiled one hour. Dr. Delf at the Lister Institute experimented with raw and cooked cabbage, and found that when this material was cooked for one hour at temperatures ranging from 80° to 90° C the loss in antiscorbutic power amounted to 90% in the cooked leaves over the raw material. Dr. Delf also concluded from her experiments that it was advisable to add neither acid nor alkali in the cooking of vegetables if these substances were to give their maximum value of vitamines.

From the foregoing description of these vitamine factors, it is readily seen why so many dietaries are deficient in these essential substances. The limited sources from which to obtain the “A” vitamine; the sensitiveness of the “B” vitamine to the action of alkalies; the sensitiveness of the “C” vitamine to heat, alkali and acid, moreover the limitation of its presence chiefly to the fresh fruits and plant juices—all point to the need of special care in the selection of the food materials and of the manner in which these materials are prepared for consumption.