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CHAPTER XXVI

FOOD

FOODS are the substances which are required for the nutrition of the body. It has been calculated that a man of average weight loses about 1000 grammes of matter daily; this passes out in the expired air, the urine, sweat, fæces, and other excretions; the substances that pass out are comparatively simple bodies, like water, carbonic acid, and urea, formed in the chemical decompositions always going on in living

matter.

Food is necessary to replace this waste if the body-weight is to remain constant; but the substances taken in in the form of food undergo many changes before they ultimately become a constituent part of the body. The changes are partly of a physical nature, such as mastication in the mouth, a thorough mixing by the peristalsis of the stomach, and solution in the watery secretions of different parts of the alimentary tract; these same secretions also fulfil a much more important function, namely, that of causing chemical changes in the food, converting insoluble into soluble, indiffusible into diffusible substances. These two sets of changes, physical and chemical, constitute what is called digestion. Absorption follows digestion; that is, the products of digestion pass through the walls of the alimentary canal into the blood or lymph circulating there. The blood- or lymph-stream carries the absorbed products to the tissues, which take them and make them part of themselves: this is assimilation. In some cases, however, assimilation may not occur immediately, but an organ like the liver may intercept such an absorbed material as sugar, and store it (in the form of glycogen), giving it out by degrees as it is wanted.

Before we can study the chemical processes concerned in digestion, absorption, assimilation, and nutrition, it is necessary that we should be acquainted with the raw materials, the foods, on which the digestive juices act.

THE PROXIMATE PRINCIPLES OF FOOD

If we examine the food-stuffs, such as milk, eggs, meat, and vegetables, we find that they are mixtures of various inorganic and organic materials, which are named proximate principles; and, more

over, the chief proximate principles of food are in the main the sameas the chief proximate principles of the body they are destined to build up. They may be classified as follows:

INORGANIC

Nitrogenous.

ORGANIC

Water.

Salts, e.g. chlorides, phosphates, carbon

ates of sodium, potassium, calcium, &c. Proteids, e g. albumin, myosin, casein, &c. Albuminoids, e.g. gelatin, chondrin, nuclein, &c.

Simpler nitrogenous bodies, lecithin,. creatine, &c.

Iron-containing compounds.

Fats, e.g. cream, fats of adipose tissue.
Carbohydrates, e.g. sugar, starch, &c.

Non-nitrogenous Simpler organic bodies, e.g. alcohol,

vegetable acids and salts, &c.

Liebig inaccurately divided the organic foods into assimilable or plastic (proteids) and combustible or respiratory (fats and carbohydrates); we, however, now know that all varieties of food are both assimilable and respiratory.

Water. It is recognised as a matter of every-day knowledge that a good water supply is essential to the health and well-being of the community. The water used for drinking must be clear, odourless, and colourless. It must not be contaminated with sewage, nor with the pathogenic bacteria apt to occur in sewage. For the of purposes safety it should be filtered through an efficient filter, or, better still, boiled before it is consumed. Distilled water is insipid; rain water has the same disadvantage; the softness of water, i.e. its freedom from salts, deprives it of its pleasant taste. The salts of spring or river water vary immensely according to the rock through which the river or spring passes.2 When the salt (magnesium sulphate, iron salts, &c.), or gas (carbonic acid, sulphuretted hydrogen, &c.) contained in any special spring, is accentuated either in quantity or quality, the water is often found to be useful as a therapeutic agent. Pure water alone, without any specially dissolved salt, is also a most useful addition to the physician's stock of remedies. The subjects of water analysis, of

1 There is some doubt whether inorganic compounds of iron are absorbed (see p. 800); iron-containing foods are therefore classified with the organic proximate principles.

? A good drinking water should not contain more than twenty degrees of hardness, i.e. twenty parts of lime in 100,000 of water.

water as a therapeutic agent, and of mineral waters are obviously too large to be treated of in a work on physiology.

Water is taken into the body, not only as water pure and simple, but all other forms of food, and especially beverages, are composed of water mixed with something else. Even the solid foods contain large proportions of water; meat, for instance, contains about 75, bread 37, milk 86, eggs 74, potatoes 75 per cent. of water.

Inorganic salts.-The importance of these agents in nutrition has been already dwelt on; they are daily excreted in certain amounts and must be daily replaced by the same or approximately the same amount; and the enumeration of the salts of the body (p. 60) is also an enumeration of the salts of the food. Sodium salts, especially the chloride, are essential. About twenty grammes of sodium chloride is in the mean taken per diem, partly in the articles of diet themselves, but mostly in a separate form as a condiment. This salt doubtless supplies the chlorine for the acid of the gastric juice. Potassium salts are found more abundantly in muscle, nerve, and other solid structures of the body, and are especially contained in meaty foods and in potatoes. Calcium salts, particularly the phosphate and carbonate, are more especially necessary for bone and tooth, but are also universally distributed in the tissues, though in smaller proportions; these salts are chiefly derived from milk, eggs, cereals, and other vegetables. Iron is found not only in hæmoglobin, but also in the organs, such as liver and spleen, and in most of the fluids of the body, in milk, in eggs, and in many vegetable foods. Probably the iron absorbed from the alimentary canal is furnished wholly by organic compounds of iron formed either during plant life or during the life of other animals, being there again ultimately derived from plants. Bunge terms these organic compounds of iron hæmatogens. The following table compiled by Beaunis 2 gives the percentage composition of the ash of various foods:

Food

1

Potash Lime Magnesia Soda NaCl Iron oxide PO SO

Silica

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1 Bunge's Physiol. Chem. transl. by Wooldridge, 1890, p. 100. Hæmatogen in most

cases appears to be a compound of iron and nuclein.

Physiol. humaine, i. 621..

Proteids. These form the most abundant source of nitrogen to the body, and they are found in larger quantity in animal than in vegetable foods. The chief animal proteids used in food are the myosin of flesh, the casein and albumin of milk, the proteids of egg, and of blood. The chief vegetable proteids are glutin, vegetable myosin, and other vegetable globulins. Gelatin has also a certain nutritive value, but an animal fed on gelatin, to the total exclusion of proteids, wastes.

Carbohydrates, on the other hand, are derived chiefly from vegetable foods; the most important are starch, cane sugar, and grape sugar. The carbohydrates found in animal food are lactose in milk, glycogen in liver and muscle, and inosite in muscle, and other organs. Cellulose, gums, and mucilages are of little or no use in nutrition.

Fats. The fat of adipose tissue consists of olein, stearin, and palmitin. The fat of milk contains certain lower glycerides in addition. The vegetable oils consist chiefly of olein and palmitin.

Vegetable acids and salts of those acids. -Oxalic, tartaric, citric, and malic are the most important of this group. In the body they are converted into carbonates.

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We do not actually use as foods the various organic proximate principles in the pure condition; it is necessary that in a suitable diet these should be mixed in certain proportions, and in nature we find them already mixed for us. In milk and in eggs, which form the exclusive food-stuff of young animals, all varieties of proximate principles are present and mixed in suitable proportions; hence these are spoken of as perfect foods. Eggs, though a perfect food for the developing bird, do not form a perfect food for mammals, as they contain too little carbohydrate. In most vegetable foods carbohydrates are present in excess, while in most animal foods the proteids are predominant; hence in a suitable diet these should be mixed in proper proportions. The food-stuffs we shall consider fully are milk, eggs, meat, bread, various flours, and seeds of plants used as food, and in conclusion certain accessories of food, such as alcoholic beverages and other stimulants and condiments. The table on the next page gives at a glance the percentage composition of the principal food-stuffs.'

Milk

Milk is a secretion which is characteristic of mammals. The acini of the mammary glands are during periods of non-lactation lined by flattened epithelium. During lactation, which begins after the birth of

1 From McKendrick's Physiology, ii. 9.

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