FOOD made by plants and taken in by animals is utilized in three ways: (a) it may be oxidized to produce energy that is expended in work and physical exercise; (b) it may be incorporated into cells and tissue to produce growth; or (c) to renew and replace parts of tissue that are constantly being broken down by the chemical changes that occur during life.

Energy value of food

A Joule" is a unit of energy, and the number of kilojoules that 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 greatly according to age, sex, occupation and activity but a general indication of the daily requirements of normal people can be obtained by considering estimates given for adults engaged in different occupations, e.g. a labourer doing heavy Work requires about 15,000 kJ per day, a clerk needs between 7500 and 10,500 kJ, and a child of six years about 5000 kJ Clearly, heavy manual work, rapid growth and vigorous activity will require a greater supply of energy. The following Table indicates roughly how the energy requirements during the day depend on activity:

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 6000 k 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.


Carbohydrates are substances containing the elements carbon, hydrogen and oxygen. Examples are glucose, CHO;cane sugar, CH,H22011 starch (C,H1,Os), and cellulose. The formula for starch means that the molecule is a large one, made up of repeated C H,O units. The "n may be be equivalent to 300 or more. Foods relatively rich in carbohydrates are milk, fruit, jam, honey and cakes, all of which contain sugar, and bread, yam and cassava, all of which Contain starch.

Carbohydrates are principally of value as energy giving foods. Each gram can provide 17 kJ of energy. In mammals, excess carbohydrate is stored as glycogen in the liver and the *1000 joules =1 kilojoule (kJ). muscles, or converted to fat and stored beneath the skin.


  Proteins content the element carbon, oxygen, nitrogen, usually sulphur and, possibly, others according to their source. Examples of foods containing Proteins are lean meat, egg white, ground nuts, and milk and it's products such as cheese.

Substances, mainly amino acids, formed from the digestion of proteins are used in making new protoplasm and celle proteins are essential for growth and replacement.  and the diet must include protein of one sort or another. Animal protein usually contains a higher concentration of the necessary amino acids than plant protein. Proteins provide energy when Oxidized, giving 17 kJ/g. They cannot be stored in the body but if too many are taken in they are converted to glycogen which is stored in the liver.

Kwashiorkor. This is a Ghanaian term meaning, the sickness the old baby gets when the new baby comes, and it is a proteins deficiency disease which occurs all over the world, though it is known by thirty eight different names. The condition shows itself as the mother's milk falls off in quality and quantity after the first six months of breastfeeding and the child's diet is supplemented by starchy, protein deficient yam and cassava. It occurrence is often most severe when breastfeeding stops altogether in order to make way for the next baby. The child becomes irritable and listless; the skin cracks and scales; the liver is damaged and the child often dies before the age of five. In fact, about 30 per cent of African children affected by the illness die. Among the Masai, who eat meat and drink milk, the disease is unknown.

It was known before the Second World War that the disease was due to a protein deficiency arising from a change to a vegetable diet after the more nutritious milk. Feeding with sufficient protein both cures and prevents the disease. The problem, however, is to find a proteins rich diet that can be provided and afforded in localities where animal protein, including milk, is very scarce. Soya beans are one source, and dieticians have evolved a mixture of corn, cotton seed, yeast and leaf meal which is both palatable and effective in curing the disease.

In Nigeria, a mixture of ground-nut flour, dried yeast, milk casein, sugar, five minerals and seven vitamins is marketed under the name of "Amama".It is inexpensive and, if added to the food at the rate of 30 g per day, prevents kwashiorkor in children up to six or seven years old.


Like carbohydrates, fats contain only carbon, hydrogen an oxygen but in different proportions. They are present in foods such as milk, butter and cheese, animal fat, egg-yolk, marga and ground-nuts. Although fats are less easily digested and absorbed than carbohydrates, they have more than double the energy value, providing 39 kJ/g, and are an important source of energy.


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.

If a diet is deficient in one or more vitamins this results in a breakdown of normal bodily activities and produces some 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.

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

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 proteins of 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, cocoyam or ground-nuts, all of which contain iron, or by taking tablets containing suitable compounds of iron.


Water makes up a large proportion of all the tissues in the body and is an essential constituent of normal protoplasm. It plays an important part in the digestion and transport of food material and all the chemical reactions of the body take place in solution.


Roughage consists largely of the cellulose in plant-cell walls; it cannot be digested by man. It adds bulk to the food and enables the muscles of the alimentary canal to grip it and keep it moving by peristalsis, particularly in the large intestine. Absence of roughage is likely to lead to constipation and its attendant disorders.


weeks or months of 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 principle 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.

Balanced diet

          A balanced diet must contain:

(a) a sufficient quantity of energy, 

(b) proteins, fats and carbohydrates in the correct proportions, 

(c) vitamins, 

(d) mineral salts, 

(e) water, 

(f) roughage.

Tropical diets are often unbalanced for the following reasons:

(i) They contain too much yam, cassava, banana and maize. These foods are easy to grow but deficient in protein. In pregnant mothers, babies and young children, a protein shortage can lead to serious malnutrition as in kwashi-orkor. Rice, millets, guinea corn and beans are a far better source of vegetable protein than cassava and yams. Maize contains some protein but this does not provide all the amino acids needed by the body and moreover, one of the B, vitamins, nicotinic acid, cannot be adequately obtained from maize.

(ii) They contain too little vitamin A. Special care needs to be taken to include green vegetables, or red palm oil in the diet in addition to tomatoes and other sources of carotene. Alternatively, the vitamin A in the diet may be supplemented with fish-liver oil.

(iii) In areas where the diet is mainly rice or maize there is likely to be a shortage of vitamin B. Different methods of milling and cooking rice could reduce this deficiency while a maize diet needs to be supplemented with other cereals to avoid pellagra.

(iv) Insufficient energy, or in other words, not enough food. About half the people in the world, principally in Asia, South America and Indonesia, have an average daily in take of less than 9000 kJ. Even allowing for children whose needs may be less than this, these figures suggest that many people have only enough food to stay alive and not enough to do a good day's work. There appears to be no general food shortage in tropical Africa; the problems arise from eating the wrong kind of food rather than not eating enough.


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