Instinct and experience combined guided us in our choice of food long before there was anything like a science of nutrition. The studies of our generation have confirmed what instinct and experience had taught us. But these studies have done more than that. They have in the first place supplied us with answers to the questions, why have we selected the substances we call food as material fit for body consumption? and why are some foods preferred to others? But aside from supplying us with a raison d'etre, studies in the science of nutrition are helping to avert the food famine which in these days constantly hovers over us. In days past a comparative abundance of food supply enabled the individual to be prodigal with his produce. He ate much of many things. He ate more than he actually needed, and so assured himself a minimum quantity of certain essentials.

What these essentials were the man of the past did not know. Eating much, and eating a variety of things, he usually obtained the necessary elements in his food; but he also ate much that was not necessary. Or sometimes, despite abundance, he indulged in a one-sided diet, which led to many diseases of which he and his offspring were victims. But to-day, with our limited food supply, we can no longer afford to be prodigal. Unless the essential factors in diet are known, and unless such knowledge is put into practice, we shall suffer from deficiency diseases even more than our forefathers. Fortunately, the science of nutrition has reached that stage where we can point out, with no little certainty, many if not all the essential factors in food. Such knowledge, combined with progress in the manufacture of artificial fertilizers, and in the manufacture by the chemist of artificial foods, will in time decrease the percentage of unfit among us. No longer will it be necessary to record that 20 per cent of our school children suffer from malnutrition, and that 25 per cent of the country's manhood are physically unfit.* And remember that these figures refer to the richest and most prosperous country in the world.

Calories

We have discussed the fuel needs of the body in terms of calories, the calorie being a unit used to measure heat; and we have seen that the adult needs from 2500 to 4500 calories per day, depending upon the type of work he does. Women need somewhat less, and the need of children is in proportion to their age.

Proteins, Fats And Carbohydrates

But we cannot make the sweeping statement that any food which when taken in the body will burn to yield the necessary calories, is a satisfactory food; for a satisfactory diet means more than merely satisfying fuel needs. We need protein for the construction and repair of cellular tissue; and of the three classes of foodstuffs, protein alone answers this purpose. Not even fats and carbohydrates in quantities yielding 10,000 and 20,000 calories will serve as a substitute for the protein. Without protein life becomes impossible.

* I, of course, do not overlook the fact that mothers are not always ignorant, nor are they always careless; and that a fair proportion of malnutrition cases is due to actual want in the family. The food expert can do little without the help of the social reformer.

The fuel needs are supplied principally by the carbohydrates and fats.

Most of the foods we eat are mixtures of two or more of the three common classes of foodstuffs. Milk is one of the very few that contains all three. In a general way, meat is rich in protein; so is fish; whereas bread and the vegetable foods are rich in carbohydrate. Butter and animal fats, and animal and vegetable oils, are almost wholly fat.

Experience and scientific experiments have taught us the wisdom of carefully distributing our food among the three classes of foodstuffs. The standard diet for an adult suggested by Voit some 40 years ago still serves as the basis for our calculation to-day. The Voit diet consisted of protein 118 grams, fat 56 grams, and carbohydrate 500 grams. Or, in round numbers, protein 100, fat 50, and carbohydrate 500 (remembering 30 grams to be equal, approximately, to one ounce). Of late, the tendency has been to increase the fat somewhat.

The available statistics for Great Britain showing the quantity of food consumed during 1909-1913 (in an average population of 45.2 million) are:

(The numbers refer to grams. "Per man per day" represents the average workman doing an average day's work; 100 of the total population - men, women and children - are reckoned as the equivalent of 77 men.)

For the composition of some of the common foods, see the Appendix.

"Ash" Or Mineral Matter

In addition to the three classes of foodstuffs, the presence of "ash" or mineral matter in the diet is absolutely imperative. A diet of protein, fat and carbohydrate, in amount such as outlined above, and supplying even more than enough of fuel needs, will quickly cause death, unless mineral matter is also added to the diet. Fortunately for us, almost all of the foods we eat contain some mineral matter (see the table in the Appendix giving the composition of foods). The elements present in mineral matter are essential in the construction of protoplasm and bone, and in regulating the concentration of fluids in the body. Lime is needed for bone and teeth construction and repair; salt, for the formation of the juice in the stomach, iodine for the thyroid, etc.

Water, Oxygen, Etc

Both water and oxygen are essential foods, for without either, life is impossible. The water is necessary for cell construction and for the fluids of the body. The oxygen makes combustion possible; so that the foods, or their products, coming in contact with this element, are oxidized, and so liberate the heat necessary to propel the human engine.

The stimulants, condiments, etc., are of lesser importance. Their chief function is to "add spice to life"; they make life more agreeable.

Amino-Acids

Modern studies in nutrition have shown that not all proteins have the same value; which means that merely to talk of 118 grams of protein without specifying the kind of protein is of little consequence. The proteins have been shown to be complex substances composed of simpler units which the chemist calls amino-acids. All proteins when decomposed in a certain way yield these amino-acids (see Appendix). There are some 17 or 18 - recently another one has been discovered - of these amino-acids, and all proteins are composed of the majority of these, but in varying proportions. Since what the body needs is not protein as such, but rather the amino-acids which go to make up the protein, modern nutrition stresses the need for satisfying amino-acid, rather than protein minimum.

And here again it has been shown that not all amino-acids are of equal physiological value. Some, such as tryptophane, are absolutely indispensable in a diet. Others, such as glycocoll, are not. Hence the importance of selecting those proteins which contain the necessary amino-acids.

Vitamines

But the most marked advance in modern nutrition is due to the proof that besides proteins, fats, carbohydrates and mineral salts, there are other as yet ill-defined substances which, though needed in but minute proportions, are yet essential to life. These substances are known as vitamines. At least three well-defined vitamines have been detected, which for purposes of identification we shall call fat-soluble A, water-soluble B and water-soluble C. The presence of all three of these vitamines is essential to well-being. As a matter of fact, very few foods contain all three. Milk is one of the rare exceptions, but even here the quantity of vitamine C that it contains is dangerously small. It is only by eating a variety of foods that we assure ourselves a liberal allowance of all three types of vitamine.

Fat-Soluble A

This is present in abundance in milk and in butter and in egg yolk, and, to a lesser extent, in beef fat and in many vegetable foods (lettuce, spinach, cabbage, carrots, potatoes, etc.). Lard and vegetable oils, such as olive oil, are devoid of it. Cereals in general (wheat, rye, barley, etc.) contain little. In a general way the statement may be made that this vitamine is present in green leaves and in the embryos of many seeds.

Water-Soluble B

This is more abundant than either of the other two. In fact, nearly all natural foods contain some of it. Yeast is particularly rich in this vitamine. So are milk and orange juice. The cereals contain it but only the outer layers; so that in patent flour vitamine B is absent, but in whole wheat flour it is present.*

Water-Soluble C

Most fresh fruit and fresh vegetables contain this vitamine. The emphasis is advisedly put on fresh material. The orange and the tomato are particularly good examples.

Effect Of Heat

All three vitamines are more or less susceptible to heat, so that any process involving this operation - cooking or canning - is apt to destroy, or greatly lessen, the efficacy of the vitamine. Of the three, water-soluble C seems most susceptive and water-soluble B least. One of the problems of the immediate future is to devise methods of drying, preserving and canning food, without at the same time lessening the vitamine value of such foods.