20 Nutrition

Good health is dependent on satisfactory nutrition, and this is, in turn, dependent on plentiful supplies of the foods necessary for a healthy life. Food is one of the essential needs of the body. Substances that can serve as food for the body are those that it can use as fuel for combustion, or building material for the repair and growth of tissues. Fuel is required to produce the energy for the activities of every living thing and to maintain the heat at which the individual exists. Building materials are necessary to repair the body tissues since they are constantly active and being worn out by their activities. In addition, in infants and children, extra building material is required to build up the new tissues needed for the processes of growth. Fuel supplies and building materials alone are, however, not enough. Certain other substances are necessary to enable the tissues to use the building materials and fuel supplies, and these are known as vitamins.

There are six essential foodstuffs with which the body must be constantly supplied through the foods we eat:

2. carbohydrates

Every article of food contains one or more of these foodstuffs and is only of value as food because it does contain them. The proteins, water and salts are the body-building foods; the carbohydrates and fats are essentially the fuel foods, though the body can and does use protein also as fuel, if more is taken in than is required for body building, or if there is a lack of other fuel, as in starvation. The vitamins and certain salts act as regulators of tissue activity so that, although vitamins are of no use either as fuel or as body builders, the nutrition of the tissues suffers if they are not present in sufficient quantities in the food supply, and diseases appear, which can be prevented and cured by ensuring that the vitamins are present in satisfactory quantities in the diet.

To be of use to the body, these foodstuffs must be digested and absorbed. Food must, therefore, be of such a nature that it can be digested, i.e. broken down by digestive juices into substances that can pass into the blood stream and be carried to the various tissues for their use. Proteins, carbohydrates and fats are complicated compounds found in plant and animal matter, and require digestion.

Water and mineral salts are simple inorganic substances, therefore, they can be absorbed without digestion and they enter into the composition of all animal and plant matter. In fact, all living matter consists largely of water. The inorganic salts, absorbed from the soil or water, are built up by living things into organic salts, which are an essential part of all plants and animals. (See Chapter 1 for more details of the chemical structure of nutrients.)

Proteins

Proteins are the most complicated of the foodstuffs. They consist of carbon, hydrogen, oxygen, nitrogen and sulphur, and usually phosphorus. They are often spoken of as the nitrogenous foodstuffs as they are the only ones that contain the element nitrogen. They are essential for the building up of living protoplasm as this also consists of the elements carbon, hydrogen, oxygen, nitrogen and sulphur. They are found in both animal and plant matter, although the animal proteins are the most valuable to the human body as building material since they are similar to human protein in composition. On the other hand, plant proteins are cheaper: they are more useful as body fuel than as body builders but do provide, at a lower cost, some of the amino acids that the body needs for tissue building. The sources of protein are:

• Animal:

• eggs, containing albumin

• lean meat, containing myosin

• milk, containing caseinogen and lactalbumin

• cheese, containing casein

• Plant:

• wheat and rye, containing gluten

• pulse foods (e.g. peas and beans) containing legumin.

All proteins are made up of simpler substances known as amino acids. There are about 20 of these amino acids but each protein contains only some of these. The amino acids are like letters from which many words can be made, each word being a different combination of letters. The protein of each different type of animal or plant is a different combination of amino acids. Ten essential amino acids are found in human protein; these are amino acids that the body cannot build up for itself. Proteins that contain all ten are called complete proteins, e.g. albumin, myosin and casein.

Proteins that do not contain all the ten essential amino acids are called incomplete proteins, e.g. gelatin, which is contained in all fibrous tissue, and is extracted from bone and calves’ feet in the making of soups and jelly. Animal proteins, such as those of eggs, milk and meat, not only contain all the ten amino acids the body needs but contain all of them in good proportion; these are called first-class proteins and are the best building material for the body tissues. Plant proteins, such as gluten and legumin, contain only slight quantities of one or more of the ten amino acids essential to the body and are therefore called second-class proteins as they are not such good building material. Some first-class animal protein should always be included in the diet.

Carbohydrates

Carbohydrates include sugar and starch. They consist of carbon, hydrogen and oxygen, and always contain hydrogen and oxygen in the same proportions as in water, i.e. twice as much hydrogen as oxygen. They are the chief sources of body fuel, being most easy to digest and absorb and most readily burnt in the tissues, being broken down into carbon dioxide and water. They are obtained chiefly from plant foods:

• Sources of starch:

• cereals, e.g. wheat, rice and barley

• tubers and roots, e.g. potatoes and parsnips

• pulse foods, e.g. peas, beans and lentils

• Sources of sugar:

• sugar cane (sucrose)

• beetroot and all sweet vegetables and fruits, e.g. grape sugar or glucose

• honey

• milk, containing lactose or milk sugar.

Sugars are of three types:

1. simple sugars (monosaccharides), such as glucose or grape sugar (formula C₆H₁₂O₆)

2. complex sugars (disaccharides), such as cane sugar or sucrose, and milk sugar or lactose (formula C₁₂H₂₂O₁₁)

3. polysaccharides, the most complex carbohydrates, e.g. starches such as in potatoes, cereals and root vegetables.

Starch differs from sugar in that it is insoluble in water. Plants store sugar in the form of starch to prevent it from escaping in solution into the water in the soil in which they live. (The formula for starch is (C₆H₁₀O₅)_n, a polysaccharide; n stands for different numbers in the different starches of various plants.) All carbohydrates are reduced to monosaccharides before they can be absorbed from the digestive tract.