Foods are usually divided into four groups of constituents - fat and oil, starch and sugar, the muscle-making material known as "protein," and the minerals which are especially essential in building the bones and the teeth and in maintaining the richness of the blood. The first three groups are able to provide heat and energy. The fats and oils, however, possess two and a quarter times the value of starch and sugar for this purpose, but neither have the property of building or repairing any portion of the muscular system, which includes all the organs of the body. This is the function of protein alone, although a very small quantity is required by the adult, and that for repair only. Protein is the sole food constituent of flesh, apart from the fat, but it exists in a less stimulating and more healthy form in all vegetables and fruits in sufficient quantities for all our requirements.

A City Man's Lunch

Let us take an example of the average diet of a city man who dines at a popular restaurant on the following food, which represents quite a modest repast compared with what I have frequently seen him enjoy: 4 oz. of roast beef, 6 oz. of potatoes, 3 oz. of bread, 2 oz. of cheese, and 1/2 oz. of butter. This represents an extravagant proportion of muscle-making food which is present to the extent of three times the necessary quantity. Obviously, however, when meat and cheese are eaten together, with potatoes and bread, there is no room for anything else in the luncheon of a moderate man. In this case half the meat and all the cheese should be replaced by green vegetables or some cereal pudding and fruit.

In discussing this subject it is necessary to employ two or three technical terms, with which some readers may not be acquainted. There is no royal road to knowledge, and those who really desire to understand the functions of food will not be deterred by this introduction, which is the key to the whole subject of nutrition.

Food, as suggested above, is divided into four groups of constituents, each group performing particular and necessary functions in the nourishment of the body - in providing for its repair, its warmth, and the exercise of energy, or mental and physical power. These groups are -

1. The Proteids

The nutritious substances in the lean or muscular tissue of flesh, fowl, and fish, the albumen of egg, the casein or curdy matter of milk, the legumen of peas, beans, and lentils, and the gluten of wheat, are all familiar examples.

2. The Carbohydrates

These constituents are chiefly confined to starch and sugar. Gum and the material known as cellulose, of which the cell-walls of plants are chiefly constructed, are also Carbohydrates.

3. The Fats and Oils.

4. The Mineral Salts.

The chief function of the Protein of food is that of building and repairing the muscular system and of assisting in the construction of the bones. Protein - apart from the fat - is the sole nutritious constituent of animal food - milk alone excepted, for milk contains sugar - but it also exists in a less stimulating form in vegetables and fruits.

The function of the Carbohydrates is the provision of heat and energy, while they are also employed in the construction of the fat of the animal body.

The Fats And Oils

The Fats And Oils perform similar functions to the Carbohydrates; two and a quarter parts of the latter are required in the production of heat and energy for every part of the former. Protein, however, as it contains carbon, also possesses the power of producing heat and energy.

The importance of the Mineral Salts - which include lime, phosphorus and iron - will be better realised from the fact that the two former are the chief constituents of the bones.

It has been pointed out that the principal carbohydrates are starch and sugar; of these two all important foods something must be said.

Starch And Sugar

Starch or Sugar forms a large proportion of all the most popular foods of a vegetable character, while sugar is an important constituent of milk. Starch, for example, forms nearly three parts of the weight of white flour. It exists in still larger proportions in rice, cornflour and sago, while in oats, barley, rye, beans, peas and lentils it forms from one-half to two-thirds. Starch is also the prominent constituent of potatoes, and of various other garden vegetables. In a word, it is dominant in most vegetable foods, and therefore forms by far the greater portion of the nutritious matter which we consume. It exists as grains or granules of varying forms, packed in cells, which are beautifully revealed by the microscope. Commercial starch is so commonly associated with the laundry that its great role in food is somewhat obscured. Starch will not dissolve in cold water, but when a food, such as rice, in which it exists in abundance, is boiled, the heat ruptures the walls of the cells in which it is enveloped, and it swells, and becomes more readily amenable to the influence of the saliva and other juices which digest and prepare it for absorption into the blood. When subjected to dry heat starch becomes soluble, and this fact is marked in the case of well-browned dried toast, which is therefore more easily digested than bread. It is not, however, advisable to abandon bread and to eat toast in order to avoid indigestion, as the remedy would be worse than the disease, for the more we depart from nature's rules the more difficult they are to restore.

Sugar is said to be the purest of known foods, but although their composition is closely identical, it exists in various forms. Thus we have the Cane-sugars, the most important of which are those produced from the sugar-cane and the beet, which now forms so large a portion of the table sugar of commerce. Milk-sugar is referred to in the discussion on milk. Grape-sugar is found chiefly in the grape; and Fruit-sugar in a large variety of fruits of other kinds. Although a sample of light brown Demerara cane-sugar with its handsome crystals is distinguishable from Beet-sugar, the two forms are now so similar that few consumers can differentiate. When the question of value arises, beet-sugar is the cheaper, nor should it be condemned on account of its colour.