A Practical Physiology Part 11

101. Cla.s.sification of Foods. Foods may be conveniently divided into four great cla.s.ses, to which the name food-stuffs or alimentary principles has been given. They correspond to the chief "proximate principles" of which the body consists. To one or the other of these cla.s.ses all available foods belong[16]. The cla.s.sification of food-stuffs usually given is as follows:

I. Proteids, or Nitrogenous Foods.

II. Starches and Sugars, or Carbohydrates.

III. Fats and Oils.

IV. Inorganic or Mineral Foods,--Water, Salt.

102. Proteids; or Nitrogenous Foods. The proteids, frequently spoken of as the nitrogenous foods, are rich in one or more of the following organic substances: alb.u.men, casein, fibrin, gelatine, myosin, gluten, and legumin.

The type of this cla.s.s of foods is alb.u.men, well known as the white of an egg. The serum of the blood is very rich in alb.u.men, as is lean meat. The curd of milk consists mainly of casein. Fibrin exists largely in blood and flesh foods. Gelatine is obtained from the animal parts of bones and connective tissue by prolonged boiling. One of the chief const.i.tuents of muscular fiber is myosin. Gluten exists largely in the cereals wheat, barley, oats, and rye. The proteid principle of peas and beans is legumin, a substance resembling casein.

As the name implies, the proteids, or nitrogenous foods, contain nitrogen; carbohydrates and fats, on the contrary, do not contain nitrogen. The princ.i.p.al proteid food-stuffs are milk, eggs, flesh foods of all kinds, fish, and the cereals among vegetable foods. Peas and beans are rich in proteids. The essential use of the proteids to the tissues is to supply the material from which the new proteid tissue is made or the old proteid tissue is repaired. They are also valuable as sources of energy to the body. Now, as the proteid part of its molecule is the most important const.i.tuent of living matter, it is evident that proteid food is an absolute necessity. If our diet contained no proteids, the tissues of the body would gradually waste away, and death from starvation would result. All the food-stuffs are necessary in one way or another to the preservation of perfect health, but proteids, together with a certain proportion of water and inorganic salts, are absolutely necessary for the bare maintenance of animal life--that is, for the formation and preservation of living protoplasm.

103. Starches and Sugars. The starches, sugars, and gums, also known as carbohydrates, enter largely into the composition of foods of vegetable origin. They contain no nitrogen, but the three elements, carbon, hydrogen, and oxygen, the last two in the same proportion as in water. The starches are widely distributed throughout the vegetable kingdom. They are abundant in potatoes and the cereals, and in arrowroot, rice, sago, and tapioca. Starch probably stands first in importance among the various vegetable foods.

The sugars are also widely distributed substances, and include the cane, grape, malt, maple, and milk sugars. Here also belong the gums and cellulose found in fruit, cereals, and all vegetables which form the basis of the plant cells and fibers. Honey, mola.s.ses, and manna are included in this cla.s.s.

The physiological value of the starches and sugars lies in the fact that they are oxidized in the body, and a certain amount of energy is thereby liberated. The energy of muscular work and of the heat of the body comes largely from the oxidation, or destruction, of this cla.s.s of foods. Now, inasmuch as we are continually giving off energy from the body, chiefly in the form of muscular work and heat, it is evident that material for the production of this energy must be taken in the food. The carbohydrates const.i.tute the bulk of our ordinary food.

104. Fats and Oils. These include not only the ordinary fats of meat, but many animal and vegetable oils. They are alike in chemical composition, consisting of carbon and hydrogen, with a little oxygen and no nitrogen. The princ.i.p.al kinds of fat used as food are the fat of meat, b.u.t.ter, suet, and lard; but in many parts of the world various vegetable oils are largely used, as the olive, palm, cotton seed, cocoanut, and almond.

The use of the fats in the body is essentially the same as that of the starches and sugars. Weight for weight they are more valuable than the carbohydrates as sources of energy, but the latter are more easily digested, and more easily oxidized in the body. An important use of fatty foods is for the maintenance of the bodily heat. The inhabitants of Arctic regions are thus enabled, by large use of the fat and oil from the animals they devour, to endure safely the severe cold. Then there is reason to believe that fat helps the digestion of other foods, for it is found that the body is better nourished when the fats are used as food. When more fat is consumed than is required to keep up the bodily heat and to yield working power, the excess is stored up in various parts of the body, making a sort of reserve fuel, which may be drawn upon at any future time.

105. Saline or Mineral Foods. All food contains, besides the substances having potential energy, as described, certain saline matters. Water and salts are not usually considered foods, but the results of scientific research, as well as the experience of life, show that these substances are absolutely necessary to the body. The princ.i.p.al mineral foods are salt, lime, iron, magnesia, phosphorus, potash, and water.

Except common salt and water, these substances are usually taken only in combination with other foods.

These saline matters are essential to health, and when not present in due proportion nutrition is disturbed. If a dog be fed on food freed from all salines, but otherwise containing proper nutrients, he soon suffers from weakness, after a time amounting to paralysis, and often dies in convulsions.

About 200 grains of common salt are required daily by an adult, but a large proportion of this is in our food. Phosphate of lime is obtained from milk and meats, and carbonate of lime from the hard water we drink.

Both are required for the bones and teeth. The salts of potash, which a.s.sist in purifying the blood, are obtained from vegetables and fruits. An iron salt is found in most foods, and sulphur in the yolk of eggs.

106. Water. Water is of use chiefly as a solvent, and while not strictly a food, is necessary to life. It enters into the construction of every tissue and is constantly being removed from the body by every channel of waste[17].

As a solvent water aids digestion, and as it forms about 80 per cent of the blood, it serves as a carrier of nutrient material to all the tissues of the body.

Important Articles of Diet.

107. Milk. The value of milk as a food cannot be overestimated.

It affords nourishment in a very simple, convenient, and perfect form. It is the sole food provided for the young of all animals which nourish their young. It is an ideal food containing, in excellent proportions, all the four elements necessary for growth and health in earlier youth.

[Table: Composition of Food Materials. Careful a.n.a.lyses have been made of the different articles of food, mostly of the raw, or uncooked foods. As might be expected, the a.n.a.lyses on record differ more or less in the percentages a.s.signed to the various const.i.tuents, but the following table will give a fair idea of the fundamental nutritive value of the more common foods:

In 100 parts Water Proteid Fat Carbohydrate Ash Digestible Cellulose Meat 76.7 20.8 1.5 0.3 -- 1.3 Eggs 73.7 12.6 12.1 -- -- 1.1 Cheese 36-60 25-33 7-30 3-7 -- 3.4 Cow's Milk 87.7 3.4 3.2 4.8 -- 0.7 Wheat Flour 13.3 10.2 0.9 74.8 0.3 0.5 Wheat Bread 35.6 7.1 0.2 55.5 0.3 1.1 Rye Flour 13.7 11.5 2.1 69.7 1.6 1.4 Rye bread 42.3 6.1 0.4 49.2 0.5 1.5 Rice 13.1 7.0 0.9 77.4 0.6 1.0 Corn 13.1 9.9 4.6 68.4 2.5 1.5 Macaroni 10.1 9.0 0.3 79.0 0.3 0.5 Peas and Beans 12-15 23-26 1-2 49-54 4.7 2-3 Potatoes 75.5 2.0 0.2 20.6 0.7 1.0 Carrots 87.1 1.0 0.2 9.3 1.4 0.9 Cabbage 90 2.3 0.5 4-6 1-2 1.3 Fruit 84 0.5 -- 10 4 0.5 ]

Cheese is the nitrogenous part of milk, which has been coagulated by the use of rennet. The curd is then carefully dried, salted, and pressed.

Cheese is sometimes difficult of digestion, as on account of its solid form it is not easily acted upon by the digestive fluids.

108. Meats. The flesh of animals is one of our main sources of food.

Containing a large amount of proteid, it is admirably adapted for building up and repairing the tissues of the body. The proportion of water is also high, varying from 50 to 75 per cent. The most common meats used in this country are beef, mutton, veal, pork, poultry, and game.

Beef contains less fat and is more nutritious than either mutton or pork.

Mutton has a fine flavor and is easily digested. Veal and lamb, though more tender, are less easily digested. Pork contains much fat, and its fiber is hard, so that it is the most difficult to digest of all the meats. Poultry and game have usually a small proportion of fat, but are rich in phosphates and are valued for their flavor.

109. Eggs. Consisting of about two-thirds water and the rest alb.u.men and fat, eggs are often spoken of as typical natural food. The white of an egg is chiefly alb.u.men, with traces of fat and salt; the yolk is largely fat and salts. The yellow color is due partly to sulphur. It is this which blackens a silver spoon. Eggs furnish a convenient and concentrated food, and if properly cooked are readily digested.

110. Fish. Fish forms an important and a most nutritious article of diet, as it contains almost as much nourishment as butcher's meat. The fish-eating races and cla.s.ses are remarkably strong and healthy. Fish is less stimulating than meat, and is thus valuable as a food for invalids and dyspeptics. To be at its best, fish should be eaten in its season. As a rule sh.e.l.l-fish, except oysters, are not very digestible. Some persons are unable to eat certain kinds of fish, especially sh.e.l.l-fish, without eruptions on the skin and other symptoms of mild poisoning.

111. Vegetable Foods. This is a large and important group of foods, and embraces a remarkable number of different kinds of diet. Vegetable foods include the cereals, garden vegetables, the fruits, and other less important articles. These foods supply a certain quant.i.ty of alb.u.men and fat, but their chief use is to furnish starches, sugars, acids, and salts.

The vegetable foods indirectly supply the body with a large amount of water, which they absorb in cooking.

112. Proteid Vegetable Foods. The most important proteid vegetable foods are those derived from the grains of cereals and certain leguminous seeds, as peas and beans. The grains when ground make the various flours or meals. They contain a large quant.i.ty of starch, a proteid substance peculiar to them called gluten, and mineral salts, especially phosphate of lime. Peas and beans contain a smaller proportion of starch, but more proteid matter, called legumin, or vegetable casein.

Of the cereal foods, wheat is that most generally useful. Wheat, and corn and oatmeal form most important articles of diet. Wheat flour has starch, sugar, and gluten--nearly everything to support life except fat.

Oatmeal is rich in proteids. In some countries, as Scotland, it forms an important article of diet, in the form of porridge or oatmeal cakes.

Corn meal is not only rich in nitrogen, but the proportion of fat is also large; hence it is a most important and nutritious article of food. Rice, on the other hand, contains less proteids than any other cereal grain, and is the least nutritious. Where used as a staple article of food, as in India, it is commonly mixed with milk, cheese, or other nutritious substances. Peas and beans, distinguished from all other vegetables by their large amount of proteids--excel in this respect even beef, mutton, and fish. They take the place of meats with those who believe in a vegetable diet.

113. Non-proteid Vegetable Foods. The common potato is the best type of non-proteid vegetable food. When properly cooked it is easily digested and makes an excellent food. It contains about 75 per cent of water, about 20 per cent of carbohydrates, chiefly starch, 2 per cent of proteids, and a little fat and saline matters. But being deficient in flesh-forming materials, it is unfit for an exclusive food, but is best used with milk, meat, and other foods richer in proteid substances. Sweet potatoes, of late years extensively used as food, are rich in starch and sugar. Arrowroot, sago, tapioca, and similar foods are nutritious, and easily digested, and with milk furnish excellent articles of diet, especially for invalids and children.

Explanation of the Graphic Chart. The graphic chart, on the next page, presents in a succinct and easily understood form the composition of food materials as they are bought in the market, including the edible and non-edible portions. It has been condensed from Dr. W. O. At.w.a.ter's valuable monograph on "Foods and Diet." This work is known as the Yearbook of the U.S. Department of Agriculture for 1894.

KEY: 1, percentage of nutrients; 2, fuel value of 1 pound in calories. The unit of heat, called a _calorie_, or gramme-degree, is the amount of heat which is necessary to raise one gramme (15.43 grains) of water one degree centigrade (1.8 Fahr.). A, round beef; B, sirloin beef; C, rib beef; D, leg of mutton; E, spare rib of pork; F, salt pork; G, smoked ham; H, fresh codfish; I, oysters; J, milk; K, b.u.t.ter; L, cheese; M, eggs; N, wheat bread; O, corn meal; P, oatmeal; Q, dried beans; R, rice; S, potatoes; T, sugar.

This table, among other things, shows that the flesh of fish contains more water than that of warm-blooded animals. It may also be seen that animal foods contain the most water; and vegetable foods, except potatoes, the most nutrients. Proteids and fats exist only in small proportions in most vegetables, except beans and oatmeal. Vegetable foods are rich in carbohydrates while meats contain none. The fatter the meat the less the amount of water. Thus very lean meat may be almost four-fifths water, and fat pork almost one-tenth water.

[Ill.u.s.tration: Fig. 45.--Graphic Chart of the Composition of Food Materials. Composition of Food Materials. Nutritive ingredients, refuse, and fuel value. ]

114. Non-proteid Animal Foods. b.u.t.ter is one of the most digestible of animal fats, agreeable and delicate in flavor, and is on this account much used as a wholesome food. Various subst.i.tutes have recently come into use. These are all made from animal fat, chiefly that of beef, and are known as b.u.t.terine, oleomargarine, and by other trade names. These preparations, if properly made, are wholesome, and may be useful subst.i.tutes for b.u.t.ter, from which they differ but little in composition.

115. Garden Vegetables. Various green, fresh, and succulent vegetables form an essential part of our diet. They are of importance not so much on account of their nutritious elements, which are usually small, as for the salts they supply, especially the salts of potash. It is a well-known fact that the continued use of a diet from which fresh vegetables are excluded leads to a disease known as scurvy. They are also used for the agreeable flavor possessed by many, and the pleasant variety and relish they give to the food. The undigested residue left by all green vegetables affords a useful stimulus to intestinal contraction, and tends to promote the regular action of the bowels.

116. Fruits. A great variety of fruits, both fresh and dry, is used as food, or as luxuries. They are of little nutritive value, containing, as they do, much water and only a small amount of proteid, but are of use chiefly for the sugar, vegetable acids, and salts they contain.

In moderate quant.i.ty, fruits are a useful addition to our regular diet.

They are cooling and refreshing, of agreeable flavor, and tend to prevent constipation. Their flavor and juiciness serve to stimulate a weak appet.i.te and to give variety to an otherwise heavy diet. If eaten in excess, especially in an unripe or an overripe state, fruits may occasion a disturbance of the stomach and bowels, often of a severe form.

117. Condiments. The refinements of cookery as well as the craving of the appet.i.te, demand many articles which cannot be cla.s.sed strictly as foods. They are called condiments, and as such may be used in moderation. They give flavor and relish to food, excite appet.i.te and promote digestion. Condiments increase the pleasure of eating, and by their stimulating properties promote secretions of the digestive fluids and excite the muscular contractions of the alimentary ca.n.a.l.

The well-known condiments are salt, vinegar, pepper, ginger, nutmeg, cloves, and various substances containing ethereal oils and aromatics.

Their excessive use is calculated to excite irritation and disorder of the digestive organs.

118. Salt The most important and extensively used of the condiments is common salt. It exists in all ordinary articles of diet, but in quant.i.ties not sufficient to meet the wants of the bodily tissues. Hence it is added to many articles of food. It improves their flavor, promotes certain digestive secretions, and meets the nutritive demands of the body.

The use of salt seems based upon an instinctive demand of the system for something necessary for the full performance of its functions. Food without salt, however nutritious in other respects, is taken with reluctance and digested with difficulty.

Salt has always played an important and picturesque part in the history of dietetics. Reference to its worth and necessity abounds in sacred and profane history. In ancient times, salt was the first thing placed on the table and the last removed. The place at the long table, above or below the salt, indicated rank. It was everywhere the emblem of hospitality. In parts of Africa it is so scarce that it is worth its weight in gold, and is actually used as money. Torture was inflicted upon prisoners of state in olden times by limiting the food to water and bread, without salt. So intense may this craving for salt become, that men have often risked their liberty and even their lives to obtain it.