Food made by plants and taken in by animals is utilized in three ways:

Most animals, particularly humans, need a diet providing:
(a) a sufficient quantity of energy,
(b) the correct proportion of carbohydrates, proteins and fats,
(c) mineral salts,
(d) Vitamins,
(e) water,
(f) fibre (roughage)

Energy value of food

A joule is a unit of energy and the number of kilojoules* which can be obtained from a sample of food is a measure of its possible value as a source of energy in the body. Only about 15 per cent of the energy in the food can be obtained as mechanical energy, but the additional heat energy set free is important in maintaining body temperature.

The number of kilojoules needed by human beings varies very greatly according to age, sex, occupation and activity, but a general indication of the daily requirement of normal people can be obtained by considering estimates given for adults engaged in different occupations, e.g. a lumberjack doing eight hours' work per day requires from 23,000 to 25,000 kj, a tailor needs between 10,000 and 11,000 Id and a child of six years about 8000 kJ.

If the daily intake of food does not provide sufficient energy, a human being will lose weight as the existing food stores and tissues of the body are oxidized to release energy, and the capacity for work will fall off. An intake of less than 6300 kJ per day, if maintained for a long period, would probably produce wasting and death, though the conditions in which this is likely to occur would probably give rise to malnutrition through lack of certain vitamins in the first instance.

The energy value of food is calculated by burning a known weight of it completely to carbon dioxide and water.  The burning takes place in a special apparatus, a bomb calorimeter designed to ensure that all the heat given out during the combustion is transmitted to a known weight of water whose temperature rise is measured.

Since 4.2 joules of heat raise the temperature of I g water by 1⁰ C, the number of joules given out by the burning food can be calculated. Whether the body can obtain the same quantity of energy depends on the efficiency of digestion, absorption and the chemical processes undergone by the food. 

Carbohydrates
Carbohydrates are substances containing the elements bon, hydrogen and oxygen. Examples are glucose, (C₆H₁₀O₆); cane sugar, (C₁₂H₂₂O₁₁); starch, (C₆H₁₀O₅)_n; and cellulose. The formula for starch means that the molecule is a large one, made up of repeated (C₆H₁₀O₅) units. The "n" may be equivalent to 300 or more. Foods relatively rich in carbohydrates are milk, fruit, jam and honey, all of which contain sugar, and bread, potatoes and yams, all of which contain starch.

Carbohydrates are principally of value as energy-giving foods, each gramme of which can provide 16 kJ of energy. In mammals, excess carbohydrate is stored as glycogen in liver and the muscles, or converted to fat and stored in fat cells beneath the skin.

Proteins
Proteins contain the elements carbon, hydrogen, oxy nitrogen, usually sulphur and, possibly, others according their source. Examples of foods containing protein are lean meat, eggs, beans, groundnuts, fish and milk and its pr such as cheese.

Proteins are broken down by digestion to substances amino acids which are absorbed into the blood stream eventually reach the cells of the body. In the cells the acids are re-assembled to form the structural proteins of cytoplasm and its constituent enzymes. There are only twenty different kinds of amino acids but there are hundreds different proteins.

Plants can build all the amino acids they need from carbohydrates, nitrates end sulphates but animals cannot. They must therefore obtain their amino acids from proteins already made by plants or present in other animals and the diet must therefore include a minimum quantity of protein of one sort or another.

A diet with sufficient energy content of fats and carbohydrates and rich in vitamins and salts will lead to illness and death because of its lack of proteins. Proteins are particularly important during periods of pregnancy and growth when new cytoplasm, cells and tissues are being made.

Although animals cannot make amino acids they can, in some cases, convert one amino acid into another. There are, however, ten or more amino acids which animals cannot produce in this way and these essential amino acids must he obtained directly from proteins in the diet.

Animal proteins generally contain more essential amino acids than do plant proteins hut since milk and eggs contain the highest proportion of all, a vegetarian who includes these in his diet should not lack essential amino acids.

Proteins must be included in the diet because it is from proteins alone that new cells and tissues can be built for growth and replacement. If proteins are eaten in excess, there will be more amino acids in the body than are needed to produce or replace cells.

The excess amino acids are converted in the liver to carbohydrates which are then oxidized for energy, or converted to glycogen and stored. The energy value of protein is 17 kJ/g.

Kwashiorkor.

For a long time it was thought that kwashiorkor was the result of a shortage of protein in the diet. It is when babies are weaned from protein-rich breast milk to protein- deficient yam and cassava that they tend to develop kwashiorkor. They become listless and miserable; their skin cracks and becomes scaly; their abdomens swell and their hair takes on a reddish colour. Because kwashiorkor occurs mainly in children whose diets are deficient in quantity as well as in protein, the disease has also been described as protein-calorie, or protein- energy malnutrition (PEM). In many cases it has been possible to cure the disease by providing more food and increasing the protein content of the diet.

However, there are many cases where some children develop Kwashiorkor and others do not, even though they all have the same low-protein, low-energy diet. Also, kwashiorkor occurs mainly in hot, humid regions rather than in hot dry regions despite the fact that diets in the two regions differ very little.

Recent studies point to the possibility that the disease might be caused by poisons produced by a mould which grows in certain types of food. These poisons, are called aflatoxins, and hey have been found in ground-nuts, chick peas and dried okra and affected children have higher levels of aflatoxins in their blood than normal. Aflatoxins are known to cause liver damage. It seems likely that some children are able to break down the aflatoxins in their bodies better than others, which would explain why they do not develop kwashiorkor even though they eat the same food. If this theory is correct, one way combat the disease would be to improve the methods of storing and marketing the suspect food, to prevent the growth of mould.

fats

Like carbohydrates, fats contain only carbon, hydrogen and oxygen hut in different proportions. They are present in milk, utter and cheese, fat, egg-yolk, groundnuts and margarine.

Although fats are less easily digested and absorbed than carbohydrates, they have more than double the energy value, providing 38 kJ/g. Fat can be stored in the body.

Mineral salts

A wide variety of salts is essential for the chemical activities in the body and for the construction of certain tissues. The red pigment in the blood contains iron; bones and teeth contain calcium, magnesium and phosphorus; sodium and potassium are essential in nearly all cells, in the blood fluid and in nerves; iodine is necessary for the proper functioning of the thyroid gland. In addition to the sulphur and nitrogen in the body, traces of copper, cobalt and manganese are required.

Salts of these elements are present in small quantities in a normal diet and the body can absorb and concentrate them.

Anaemia is a shortage of red cells or haemoglobin in the blood. It may result from the failure of the red bone-marrow to make enough cells or sufficient pigment, or from an excessive rate of destruction of red cells, or simply a shortage of iron in the diet.

In the latter case, iron-deficiency anaemia, the condition can be cured by eating meat, liver, or green vegetables such as the leaves of spinach or groundnuts, which contain iron, or by taking tablets containing compounds of iron.

Vitamins
Vitamins are complex chemical compounds which, although they have no energy value, are essential in small quantities for the normal chemical activities of the body.

It has long been known that certain diseases could he prevented or cured by making alterations in the diet. In about 1750, James Lind, a naval doctor, cured scurvy in seamen by providing them with citrus fruits.

Christiaan Eijkman, a Dutch doctor working in Java, in 1896, was searching for the "germ" which he thought transmitted the disease beriberi, but found that the disease could be caused by feeding chickens with polished (dc-husked) rice and cured by feeding them unpolished rice.

Although Eijkman concluded from this result that the disease was caused by a poisonous substance in the polished rice, which was normally neutralized by something present in the husk of the unpolished grain, it was realized in the next ten years by Eijkman and other scientists, notably the English biochemist Gowland Hopkins, that the husk contained an accessory food factor whose absence led to the disease. Many similar accessory food factors have been identified and although their composition varies widely they are usually listed under the general heading of vitamins.

Fifteen or more vitamins have been isolated and most of them seem to act as catalysts in essential chemical changes in the body, each one influencing a number of vital processes. Vitamins A, D, E and K are the fat-soluble vitamins, occurring mainly in animal fats and oils and absorbed along with the products of fat digestion. Vitamins B and C are the water- soluble vitamins.

If a diet is deficient in one or more vitamins, this results in a breakdown of normal bodily activities and produces symptoms of disease. Such diseases can usually be effectively remedied by including the necessary vitamins in the diet.
Plants can build up their vitamins from simple substances, but animals must obtain them "ready-made" directly or indirectly from plants.

Some of the important vitamins are set out in the Table overleaf together with their properties. It must be emphasized that nearly all normal, mixed diets will include adequate amounts of vitamins, and deficiency diseases arc most likely to occur where, as in rice-eating countries, the bulk of the diet consists of only one or two kinds of food.

Vitamins and their Characteristics

Calciferol helps the absorption of calcium from the intestine and the deposition of calcium salts in the bones.

Natural fats in the skin are converted to a form of calciferol by sunlight.

There are several other substances classed as vitamins, e.g. riboflavia (B2), tocopherol (F), phylloquinone (K), but these are either (a) unlikely to be missing from the diet, or (b) not known to be important in the human diet. Vitamin K is synthesized by bacteria in the colon (p. 89). If it is absorbed, this might explain why dietary deficiency is unimportant.

Water
Water makes up a large proportion of all the tissues in the body and is an essential constituent of normal protoplasm.

In the intestine, water is needed to digest and dissolve the food, and water in the blood plasma carries the dissolved food all round the body. Nitrogenous waste products are removed from the body in a water solution, called urine. Water is present in all kinds of food but a water balance in the body (seep. 108) is maintained by drinking.

Dietary fibre (roughage)

Fibre consists largely of plant-cell walls which digested by man but are digested by bacteria in the c fibre is thought to be important in maintaining digestive system in a variety of ways.  

Cereals, vegetables and fruits contain a high proportion of fibre.

 

NAME AND SOURCE OF VITAMIN	DISEASES AND SYMPTOMS CAUSED BY LACK OF VITAMIN	NOTES
Retinol (vitamin A; fat-soluble) Liver, cheese, butter, mar- garine, milk, eggs.	Reduced resistance to disease, par-	The yellow pigment, carotene, present in green
Carotene (vitamin A precursor; water-soluble) Spinach, red palm oil, carrots.	particularly those which enter through the	leaves, carrots, red peppers and palm oil, is
	epithelium. Poor night vision. Cornea of	turned into retinol by the body.
	eyes becomes dry and opaque leading to	Modified retinol forms part of the light-
	keratomalacia and blindness,	sensitive pigment in the retina (page 131). Retinol is stored in the liver.

 

Vitamin B complex	The B vitamins are present in most		Many of the	B vitamins act as catalysts in the
Ten or more water-soluble vita-	unprocessed food.	Deficiency diseases	oxidation of	carbohydrates during respiration.
mins usually occurring to-	usually arise only in populations living		Absence of	these catalysts upsets the body
gether. Four are described here,	on restricted diets,		chemistry an	d leads to illness.

 

Thiamine (vitamin B1) Whole grains of cereals, beans, groundnuts, green vegetables, meat, yeast and "Marmite". Nicotinic acid or Niacin (vitamin B3) Beans, lean meat, liver, yeast and "Marmite", Nicotinic acid can be made in the body from the essential amino acid trypto- phan which is present in most cereals. Cobalamin (vitamin B) From animal products only, e.g. meat, milk, eggs, cheese, fish. Folic acid Liver, spinach, fish, beans, peas.	Wasting and partial paralysis, or water- logging of the tissues and heart failure. These are symptoms of the two forms of beriberi. Skin eczema on exposure to sunlight, diarrhoea, wasting and mental dc- generation; all symptoms of pellagra. Vitamin deficiency anaemia (pernicious anaemia). Vitamin deficiency anaemia Not enough red blood cells are made	Rice husks contain both thiamine and tryptophan, so highly milled rice is deficient in both. Populations living largely onmilled rice are very prone to beriberi. Maize lacks tryptophan, so a diet which consists mainly of maize can lead to pellagra. This vitamin contains cobalt and is made by the bacteria in the intestines of herbivorous animals. , Likely to affect pregnant women on poor diets, and people suffering from malaria and hookworm.
Ascorbic acid (vitamin C; water- soluble) Oranges, lemons, guava, paw- paw, mango, tomatoes, fresh green vegetables,	Vitamin C is needed for the formation of collagen fibres in connective tissues, e.g. in the skin. If ascorbic acid is deficient, wounds do not heal properly; bleeding occurs under the skin, particularly at the joints; the gums become swollen and bleed easily. These are all symptoms of scurvy.	Possibly acts as a catalyst in cell respiration. Scurvy is only likely to occur when fresh food is not available. Milk contains little ascorbic acid so babies need additional sources. Cannot he stored in the body; daily intake needed.

 

Calciferol (vitamin	D; fat-	Calcium is not deposited properly in the
soluble) ‘		bones, causing rickets in young children
Butter, milk, cheese,	egg-yolk,	because the bones remain soft and are
liver, fish1iver oil,		deformed by the child's weight. Deficiency in adults causes osteamalacia; fractures are likely.

Milk
The sole article of diet during the first few weeks or months of a mammal's life, Milk is an almost ideal food since it contains proteins, fats, carbohydrates, mineral salts, particularly those of calcium and magnesium, and vitamins.

For adults, however, it is less satisfactory because of its high water content and lack of iron. Large volumes would have to be consumed if it were the principal article of diet for an adult, and serious blood deficiencies would result from the lack of iron. In the body of the embryo mammal, iron is stored while the embryo develops inside its mother, and this supply must suffice until the young mammal begins to eat solid food.

The Digestion, Absorption Metabolism of Food

To be of any value to the body, the food taken in through the mouth must enter the blood stream and be distributed to all the living regions.
Digestion is the process by which insoluble food, consisting of large molecules, is broken down into soluble compounds having smaller molecules. These smaller molecules, in solution, pass through the walls of the intestine and enter the blood stream. Digestion and absorption take place in the alimentary canal (Fig. 18.1 and Plate 15), digestion being brought about by means of active chemical compounds called. enzymes.

The alimentary canal is a muscular tube, with an internal glandular lining, running from mouth to anus. Some regions have particular functions and, accordingly, different structures.

Juices are secreted in the alimentary canal from glands in its lining or are poured into it through ducts from glandular organs outside it. As the food passes through the alimentary canal it is broken down in stages until the digestible material is dissolved and absorbed. The indigestible residue is expelled through the anus.

Enzymes
Enzymes are chemical compounds, protein in nature, made in the cells of living organisms. They act as catalysts substances which accelerate the rate of most chemical changes in the organism without altering the end-products. They occur in great numbers and varieties in all protoplasm and without them the chemical reactions would be too slow to maintain life.

The vast majority of enzymes are intracellular, that is, they carry out their functions in the protoplasm of the cell in which they are made. Some enzymes, however, are secreted out of the cells in which they are made, to be used elsewhere. These are called ext racellular enzymes.

Bacteria (p. 73) and fungi (p. 75) secrete such extracellular enzymes into the medium in which they are growing. The higher organisms secrete extracellular enzymes into the alimentary tract to act on food taken into it.

These digestive enzymes accelerate the rate at which insoluble compounds are broken down into soluble ones. Enzymes which act on starch are called amylases, those acting on proteins are proteinases, and lipases act on fat.

The digestive system consists of organs that break down food and absorb nutrients such as glucose. Organs of the digestive system are shown in Figure 23.14. Most of the organs make up the gastrointestinal tract. The rest of the organs are called accessory organs.

Every enzyme has the following characteristics:
(a) it is destroyed by heating, sincc it is a protein,
(b) it acts best within a narrow temperature range,
(c) it acts most rapidly in a particular degree of acidity or alkalinity (pH),
( it acts on only one kind of substance,
(e) it always forms the same end-product or products, since an enzyme affects only the rate of reaction.

Movement of food through the alimentary canal

Ingestion is the act of taking food into the alimentary canal through the mouth. Swallowing (see Fig. 18.3). In swallowing, the following actions take place:

(a) the tongue presses upwards and back against the roof of the mouth, forcing the pellet of rood, called a bolus, to the back of the mouth, orpharynx;

(b) the soft palate closes the opening between the nasal cavity and the pharynx;

(c the laryngeal cartilage round the top of the trachea, or windpipe, is pulled upwards by muscles so that the opening of the larynx lies beneath the back of the tongue, and the opening of the trachea is constricted by the contraction of a ring of muscle; and

(d) the epiglottis, a flap of cartilage, directs food over the laryngeal orifice. In this way food is able to pass over the trachea without entering it. The beginning of this action is voluntary, but once the bolus of food reaches the pharynx swallowing becomes an automatic or reflex action. The food is forced into and down the oesophagus, or gullet, by peristalsis.