Food serves two purposes,—it builds and repairs the body tissues, and it generates vital heat and energy, burning food as fuel. Protein and mineral matter serve the first purpose, and hydrocarbons (fats) and carbohydrates (sugars and starches) the second, although, if too much protein be assimilated it will be burnt as fuel, (but it is bad fuel as will be mentioned later), and if too much fat is consumed it will be stored away in the body as reserve supply. Most food contains some protein, fat, carbohydrates, mineral matter, and water, but the proportion varies very considerably in different foods.
Water is the most abundant compound in the body, forming on an average, over sixty per cent. of the body by weight. It cannot be burnt, but is a component part of all the tissues and is therefore an exceedingly, important food. Mineral matter forms approximately five or six per cent. of the body by weight. Phosphate of lime (calcium phosphate), builds bone; and many compounds of potassium, sodium, magnesium and iron are present in the body and are necessary nutrients. Under the term protein are included the principal nitrogenous compounds which make bone, muscle and other material. It forms about 15 per cent. of the body by weight, and, as mentioned above, is burnt as fuel for generating heat and energy. Carbohydrates form but a small proportion of the body-tissue, less than one per cent. Starches, sugars, and the fibre of plants, or cellulose, are included under this term. They serve the same purpose as fat.
All dietitians are agreed that protein is the essential combined in food. Deprivation of it quickly produces a starved physical condition. The actual quantity required cannot be determined with perfect accuracy, although estimates can be made approximately correct. The importance of the other nutrient compounds is but secondary. But the system must have all the nutrient compounds in correct proportions if it is to be maintained in perfect health. These proportions differ slightly according to the individual's physical constitution, temperament and occupation.
Food replenishes waste caused by the continual wear and tear incidental to daily life: the wear and tear of the muscles in all physical exertion, of the brain in thinking, of the internal organs in the digestion of food, in all the intricate processes of metabolism, in the excretion of waste matter, and the secretion of vital fluids, etc. The ideal diet is one which replenishes waste with the smallest amount of suitable material, so that the system is kept in its normal condition of health at a minimum of expense of energy. The value, therefore, of some general knowledge of the chemical constituents of food is obvious. The diet must be properly balanced, that is, the food eaten must provide the nutrients the body requires, and not contain an excess of one element or a deficiency of another. It is impossible to substitute protein for fat, or vice versa, and get the same physiological result, although the human organism is wonderfully tolerant of abuse, and remarkably ingenious in its ability to adapt itself to abnormal conditions.
It has been argued that it is essentially necessary for a well-balanced dietary that the variety of food be large, or if the variety is to be for any reason restricted, it must be chosen with great discretion. Dietetic authorities are not agreed as to whether the variety should be large or small, but there is a concensus of opinion that, be it large or small, it should be selected with a view to supplying the proper nutrients in proper proportions. The arguments, so far as the writer understands them, for and against a large variety of foods, are as follows:—
If the variety be large there is a temptation to over-feed. Appetite does not need to be goaded by tasty dishes; it does not need to be goaded at all. We should eat when hungry and until replenished; but to eat when not hungry in order to gratify a merely sensual appetite, to have dishes so spiced and concocted as to stimulate a jaded appetite by novelty of taste, is harmful to an extent but seldom realised. Hence the advisability, at least in the case of persons who have not attained self-mastery over sensual desire, of having little variety, for then, when the system is replenished, over-feeding is less likely to occur.
In this connection it should be remembered that in some parts of the world the poor, although possessing great strength and excellent health, live upon, and apparently relish, a dietary limited mostly to black bread and garlics, while among ourselves an ordinary person eats as many as fifty different foods in one day.[3]
On the other hand, a too monotonous dietary, especially where people are accustomed to a large variety of mixed foods, fails to give the gustatory pleasure necessary for a healthy secretion of the digestive juices, and so may quite possibly result in indigestion. It is a matter of common observation that we are better able to digest food which we enjoy than that which we dislike, and as we live not upon what we eat, but upon what we digest, the importance of enjoying the food eaten is obvious.
Also as few people know anything about the nutritive value of foods, they stand a better chance, if they eat a large variety, of procuring the required quantity of different nutrients than when restricted to a very limited dietary, because, if the dietary be very limited they might by accident choose as their mainstay some food that was badly balanced in the different nutrients, perhaps wholly lacking in protein. It is lamentable that there is such ignorance on such an all-important subject. However, we have to consider things as they are and not as they ought to be.
Perhaps the best way is to have different food at different meals, without indulging in many varieties at one meal. Thus taste can be satisfied, while the temptation to eat merely for the sake of eating is less likely to arise.