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CHAPTER II

CHEMISTRY AND PHYSIOLOGY OF DIGESTION

CHEMICAL CONSTITUENTS OF THE BODY AND OF FOOD

Chemical Elements in the Human Body.

Organic Compounds: Fats, Carbohydrates, Proteins, Albumins; Proxi

mate Principles.

Inorganic Compounds: Water, Acids, Salts.

Scientific Classification of Foods: Nitrogenous, Non-nitrogenous.

Tests to Determine Food Value: Chemical, Physical, Physiological,
Economic.

Of the seventy-four elements known to the chemist but twenty are found in the body. They are: carbon, hydrogen, nitrogen, oxygen, sulphur, phosphorus, fluorin, chlorin, iodin, silicon, sodium, potassium, calcium, ammonium, magnesium, lithium, iron and occasionally manganese, copper and lead. These elements are rarely found in the pure state, being present in the form of chemical compounds which, as they occur in the structure of the various tissues, are very complex in arrangement. These compounds are comparatively unstable, and are readily converted in the body, or changed into other forms, which may be easily shown by chemical analysis.

The elements of both animal and vegetable foods are derived from the earth, air and water. We know that animal life could not subsist upon inorganic substances nor derive nutriment from them alone, notwithstanding the great importance ascribed to the inorganic materials for the vital processes of the human body.

Other combinations of chemical derivatives are also required, which come under the head of plant and animal life, and are known as organic substances. All food is composed of many combinations of these simpler chemical elements, which, for the most part, must be subjected to alterations in the body itself by digestion, before they are ready for assimilation by the tissues. Nutrition of the body, therefore, involves several distinct metabolic processes, viz.:

I. The secretion of digestive fluids and their physiological action. upon food in the alimentary tract.

II. The absorption of food ingredients when digested into the lymphatic and circulatory systems.

III. The assimilation of the absorbed nutrition products by the tissues.

IV. The elimination of the waste material.

Elements and Their Compounds.-The analysis given below will show the relative predominance of the elements of which the human body is composed.

ELEMENTARY COMPOSITION OF THE HUMAN BODY

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All these elements are derived from the food plus the oxygen, either inhaled from the air or liberated within the body as a molecule of oxygen, when food particles are broken down.

Oxygen, hydrogen, and carbon are the three principal elements of the human body (as well as in all our foodstuffs), being the great force producers of the body, and at the same time the tissue builders.

Nitrogen, when present, is capable of acting in a dual capacity; it either accommodates itself to the process of cell and tissue formation, or is broken down from the completed molecule and thrown off when muscular energy is manifested or when cellular disintegration occurs.

Foodstuffs, to play an important part in tissue production, and at the same time stimulate bone formation, must contain sulphur, phosphorus, iron, sodium, potassium, and calcium phosphate. If these elements and compounds are deficient in the food of a growing infant, softening of the bones will follow. Bone cells contain 50 per cent calcium phosphate, and this element is absolutely necessary for normal osseous development. Food without iron salts will cause a change of the hemoglobin content of

the red blood cells and a diminished oxidation, resulting in anemia. The potash salts, especially potassium carbonate and chlorid, if absent or found in insufficient quantities, lead to scurvy-which condition is aggravated by the use of common table salt, sodium chlorid. A diet of salt meat and bread, excluding potatoes, fresh fruit, and vegetables, will produce scurvy.

Sodium chlorid is an important constituent directly concerned in nutrition. Its absence interferes with many of the functions of the body, such as absorption, and materially alters the density and reactions of the different fluids.

PROXIMATE PRINCIPLES.-Chemical elements are rarely found in a pure state, but are present in diverse combinations, or, as they are sometimes termed, "proximate principles," which may be divided into the following groups: (1) mineral, or inorganic compounds; (2) organic compounds, or compounds of carbon. The organic compounds may again be divided very conveniently into two groups: the nitrogenous and nonnitrogenous compounds. The inorganic compounds comprise water and the various acids; for example, the hydrochloric acid of the gastric juice and numerous salts.

Since the same elements in varying proportions are to be found in both foodstuffs and the human body, the various compounds of these elements may be termed "proximate principles." As pointed out by physiologists, "proximate principles" are chemical compounds of which the body is composed and into which it is theoretically, but not analytically, separable without chemical change. They may be classified as follows:

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All of these elements are present, yet all are not of equal importance, nor do they occur in the same proportions. Among the inorganic group, water and the salts are preeminent. Among the organic group, protein, fats and carbohydrates are most numerous.

Inorganic compounds act medicinally and aid in the formation of inorganic constituents of bone, and in osmoses. Oxygen is regularly introduced into the body in large amounts both as an essential constituent of organic foods and as an element by respiration. In the latter form, probably by the assistance of enzymes, it oxidizes all forms of organic components of the body, acting, as a rule, destructively rather than constructively. Most of the other elements (excluding corrosive action) might be administered in the elemental state, and would sooner or later form inorganic or organic combinations while in the alimentary tract. We know that iron in its elemental form can be assimilated as such, though not as readily as from organic compounds. In most instances, all the elements give better physiological results when administered as salts or as organic compounds.

Scientific Food Classification.-Of the chemical classification of foods the one advocated by Baron von Liebig is the simplest and best. He was the first to suggest a real scientific division, viz.: (a) Nitrogenous, (b) Non-nitrogenous.

"Each of these classes contains food materials derived from both the animal and vegetable kingdoms, although the majority of the animal substances belong to the nitrogenous, and the majority of vegetable substances to the non-nitrogenous group.

(a) von Liebig regarded the nitrogenous group as containing plastic elements-i.e., they are essentially tissue builders or flesh formers. Nitrogenous foods are sometimes called azotized foods or albuminoids that is, substances resembling albumin. They consist chiefly of the four elements, carbon, oxygen, hydrogen and nitrogen, to which a small proportion of sulphur and phosphorus is usually joined. Nitrogenous or protein foods are non-crystallizable, but coagulable, consisting principally of fluid or semisolid substances. They are fermentable, and under some conditions will putrefy. This group comprises all forms of animal food, except fats and glycogen. It includes, therefore, albumins and gelatins. Its chief representatives are milk, eggs, crustaceans, fish, flesh and fowl. It also contains such nitrogenous substances as occur in the vegetable kingdom, or vegetable albuminoids.

(b) The second or non-nitrogenous group von Liebig called respiratory or calorifacient foods, because their function in the body is to

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