We have reached the conclusion that one method of estimating the needs of the body is to ascertain how much heat the body liberates. If we assume that for the average, active individual the heat liberated in 24 hours corresponds to 3000 calories, then it becomes perfectly evident that in order to retain our health, we must consume a quantity of food which, when burnt in the body, will give such 3000 calories. From tables that can be found in any book on dietetics (see Appendix) we can find how many calories a pound any of the common foodstuffs will yield when consumed. It becomes then merely a matter of selecting enough food to give the necessary 3000 calories.

The Calorie Is Not The Only Factor Involved

Unfortunately for the simplicity of the science of dietetics, the question of adequate nutrition is a far more complicated one. Calories alone do not completely solve the problem. For take an example borrowed from our previous chapter. We there stated that one pound of tomatoes represents a fuel value of 100 calories. If your requirements are 3000 calories a day, suppose I were to suggest to you that you satisfy these requirements by eating 30 pounds of tomatoes? You would laugh at me. You would say you could not do it; it would make you sick. And so it would, despite all assertions that your fuel requirements would be satisfied. You do not object to tomatoes, provided that meat and bread and butter and milk take the place of most of the 30 pounds of tomatoes. Quantity alone is not at all sufficient; quality and variety are equally important. And this brings us to the next step in our subject.

Carbohydrates, Fats And Proteins

When the chemist examines the foodstuffs common to man, he finds that he can classify them under three broad divisions: carbohydrates, fats and proteins. There are plenty of differences among the individual foods in each group, but they show enough common characteristics for them to belong to one family and to be differentiated from foods belonging to either of the other families. The fate of different carbohydrates given to the body is much the same. This is true of the fats and proteins. It is much like classifying the people of the world into the white, yellow and black races. The 80 or 90 million white people in America show plenty of differences among themselves; but their color and other anthropological features divide them very sharply from the yellow and black people.

We can best form an idea of these three classes of foodstuffs by naming foods rich in one of the three. Starch and sugar are excellent examples of carbohydrates. Starch is found in abundance in flour and all cereals, in rice and potatoes. Besides the sugar cane and sugar beet, sugar (of several varieties) is found in milk and fruits. Butter and the "fat" of meat, and much of cheese and all oils represent the class of fats. Much of the egg and meat and cheese and some of the milk contain proteins. Very few of the common foods contain one hundred per cent of any one of the three classes of foodstuffs; usually each foodstuff consists of various mixtures of two or three of them, as the following table, representing the composition of some common foods, will make clear. (Approximate figures are given. The difference between 100 and the sum of the first three figures in a horizontal column represents - roughly - the % of water. Mineral salts, though present, and though enormously important, are not included because they will be discussed separately in the next chapter. See particularly page 37 and following.)

This table is instructive. You will notice - what has already been intimated - how all of the common foods are mixtures of two or three of the foodstuffs. Cane sugar and butter are notable exceptions. You will notice, furthermore, how relatively rich in carbohydrates are bread and potatoes, and how relatively poor in carbohydrates but rich in protein and fat are cheese and eggs and meat and fish. You will notice, if you glance once again at the table, that in the case of milk, though the percentages of fats, carbohydrates and protein are rather small, there is a fairly equal distribution of these three.

Now why should milk, the sole food of infants, contain substantial quantities of all three foodstuffs? Why from time immemorial have men selected their food in such a way as to include substantial amounts of the three? There must be some good reason for it. There must be some reason why milk does not contain one hundred percent of carbohydrate alone, and why the adult's food does not consist of one hundred percent fat or protein alone.