FOOD is required for two purposes - to build up the body and repair tissue waste, and to supply potential energy, which can be converted into heat and work. Foods are classified as follows: -
Proteins . . e.g.
Albuminoid, e.g. gelatine..
Carbohydrates . e.g.
Sugars and starch ....
Fats . . .e.g.
Sodium, potassium, calcium, mag-nesium, iron, phosphorus, chlorine, sulphur ....
The proteins contain carbon, hydrogen, nitrogen, oxygen, and a small amount of sulphur. Carbohydrates and fats contain carbon, hydrogen, and oxygen only.
Almost all articles of food are complex substances containing both nitrogenous and non-nitrogenous ingredients, and the element which predominates in the food gives it its distinctive class. The various foods perform the functions of supplying building material, muscular energy, and heat in different degree.
The building material for forming and repairing tissue is commonly supposed to be supplied only by the the carbohydrates and fats not ranking as tissue-builders.
Mineral salts .
In recent years, however, it has been shown that a carbohydrate radicle enters into the composition of the protein molecule, and this suggests that carbohydrates may play a much more important part in the growth and repair of tissue than has formerly been supposed. The second great function of food - source of heat and energy - is shared by ail the organic foodstuffs:
Proteins . . ,
Sources of heat and energy.
These possess potential energy. The force which holds the food elements together in combination is called potential energy. In the body the proteins, fats, and carbohydrates undergo oxidation, the oxygen necessary for their combustion being taken in with the inspired air. The oxidation processes are attended by the liberation of energy, which may take the form of heat or work. Fats require the greatest amount of oxidation, and are thus the most important fuel food. It is owing to fat's great value as a heat producer that it forms such a prominent article of food in the dietaries of the inhabitants of the Arctic regions.
The animal body has been aptly compared to a steam engine. The fuel, the source of energy, is represented by the food. The inlet for the draught of air and the outlet for the waste gases, the products of combustion, are combined in one organ, the lungs, which we use to take in Oxygen, and give out C02 and H20; and just as the coal used in engines has some incombustible constituents which remain as ash and have to be raked out, so there are parts of our food which the body cannot make use of, and which leaves the body as excrementitious matter or faeces, having passed through the digestive tract without at any time having formed part of the tissues.
There are still two constituents of foodstuffs which have to be got rid of after the elimination of CO., and H20 by the lungs, viz., the nitrogen and sulphur present in proteins. This function is served by the kidneys. The nitrogen is combined in the body with carbon and oxygen and forms urea, and in this form is excreted by the kidneys, together with salts and water as urine. The sulphur is oxidised to a sulphate, and in this form appears in the urine. The income and output of the body may be arranged in the form of an equation - Food + Oxygen taken up = Faeces + C02 + H20 + Urea; and in the same way we may make an equation of the income and output of energy -
Energy set free by the combustion of food = Work done by the body, and heat given off.
A knowledge of the nutritive value of food may be gained in the following ways : -
1. By a study of its chemical composition.
2. By ascertaining its heat value.
3. By reference to its physiological properties - the ease with which it is digested and absorbed.
Chemical analysis of a food tells us the proportion of protein, fat, carbohydrates, salts, and water present in its composition. The results of chemical analysis, therefore, give us some information as to the value of food, both as a tissue-builder and as a source of energy or heat production.
The heat value of different foods may be determined experimentally by the use of an instrument known as the Bomb calorimeter, the result being expressed in calories. The standard of heat production is the calorie, which means the amount of heat required to raise the temperature of 1 kilogram of water 1° centigrade. The amount of heat evolved by the combustion in a calorimeter of I gramme of the different foodstuffs is as follows: -
1 gramme of Carbohydrate produces 4 calories. 1 „ Protein produces 4 calories.
1 „ Fat produces 8.9 calories.
These figures represent the amount of energy set free by I gramme of the foodstuff combining with oxygen to form the end-products of its combustion. This constitutes the physical heat value of the food - that is, the amount of heat produced by complete combustion of the food in a special calorimeter chamber. The physiological heat value of the food is the amount of heat produced by the complete combustion of the food in the living tissues. Carbohydrates and fats are completely oxidised in the tissues; their physiological and physical heat values are therefore the same. Proteins, on the other hand, are not completely burnt up, urea, the end-product of nitrogenous metabolism in the tissue, being an incompletely oxidised product. The physiological heat value of protein is, therefore, less than the physical heat value by the extent to which urea is capable of further oxidation outside the body.