A careful consideration of the facts contained in the last chapter will quickly reveal in a concrete form many important principles of dietetics which we shall now proceed to elucidate. Needless to say, it will be impossible to treat these principles in the detailed manner of a text-book, but I hope to be able to give a great deal of valuable practical information which will stimulate my readers to pursue the study of the subject further, with the aid of the bibliography at the end of the book.

Definition Of Food

I have no doubt most of my readers want to know first just what constitutes a food and in what manner it is supposed to nourish the body. The most comprehensive description of a food with which I am acquainted is this: Any substance which when introduced into the body is capable of repairing the waste of its tissues and so building it up and furnishing it with heat and energy.

In thinking over the various substances which would fulfil these functions, we naturally recur to those already mentioned as actually entering into the composition of the body, viz. proteins, fats, carbohydrates, mineral matters, and water. A most searching examination into all the varied diets of all the manifold races of mankind reveals the fact that, despite their enormous differences, each one can be resolved into these five substances, which are hence called the alimentary principles.

Irresponsible professors of dietetics amongst the laity do not hesitate to advance preposterous claims for the inclusion of other principles, such, for instance, as the possibility of inspiring nitrogen from the air and so adding it to the resources of the body, but there is not an atom of evidence in support of any theory which suggests any other than the above means of obtaining nourishment for the body. It is not necessary for more than one of these principles to be present to constitute the substance a food, e.g. sugar is a pure carbohydrate and butter is almost a pure fat, whilst white of egg is almost a pure protein. Usually, however, there is more than one present, and we have seen that bread contains all the alimentary principles in certain proportions.