Although this short classification includes all the food-stuffs, it is nevertheless not an easy matter to formulate a brief yet comprehensive definition of food. This might pass muster: any substance which when introduced into the living organism will repair the waste of its tissues, and so build them up and furnish it with heat and energy. It might be wise to add that this should be accomplished without causing injury to, or loss of functional activity of, any of its parts. As heat, however important to the economy, is only an indirect product of the combustion of food, perhaps Voit's method of defining a food is preferable: an agreeable and digestible mixture of food-stuffs capable of maintaining the body in an equilibrium, or bringing it into a desired condition, of substance without burdening the organism.

The difficulty experienced in constructing such a definition is in some degree a measure of the extent of our knowledge of the subject, but it is not necessary for my particular purpose to enlarge upon it further than to say that it gives no warrant for the introduction, for example, of elemental nitrogen. I would hardly mention this fact were it not that a few visionary theorists have held it to be capable of extraction from atmospheric air, and so to be utilised for the purposes of nutrition. We may dismiss this theory at once as a pure freak of the imagination, although less exception can be taken to those who claim the admission as a food of elemental oxygen, an ingredient at least absolutely essential to the metabolism of the body.

In considering the theories of diet, it will be found that although there is general agreement as to the necessity for the inclusion of all these alimentary principles in the dietary list of every individual, there is the greatest diversity of opinion as to the proportions in which they ought to be consumed to satisfy the nutritive requirements of the body. The standard which up till quite recently has been almost universally and tacitly accepted was that of Carl Voit, and it is interesting to observe that each of the alimentary principles, but more particularly the protein, has been attacked on the ground that too much of it was permitted in this standard diet of Voit. The tendency has been, therefore, in the direction of a diminution, not so much of the total quantity of food required, as of one or other of the ingredients of the diet, with, of course, a corresponding increase in the remainder.

In addition to this, a number of theories, emanating chiefly from laymen or irregular practitioners, have arisen around what are called food accessories, or with reference to the appropriate manner of preparing, cooking, or eating food, and as there is frequently some glimmering of truth in every emphatic presentation of this kind, it is my purpose to examine some of these, to extract the grain of truth from the chaff of purely fanciful ideas surrounding it.

Before endeavouring to pronounce an opinion as to the whole question, therefore, it will be necessary to endeavour to trace each item of food through all the transformations to which it is subjected by cooking and metabolism until it is finally ejected from the body. As this is a Herculean, and in some respects at present an impossible, task, we shall . consider each food principle in turn, giving a short, concise, and, as far as possible, accurate description of its digestion and metabolism before turning to the theories themselves.

The Nutritive Requirements Of The Body

We ought, however, first to examine the methods of ascertaining the nutritive requirements of the body. It is important to deal with this matter here, as much of the confusion and uncertainty associated with the problem of diet centres round the question of quantity. Practising physicians will easily confirm this statement, if they will reflect on the answers frequently wrested from their patients during any inquiry instituted on the subject of their daily meals. They are unduly sensitive, and inclined to resent such an investigation, sparing no pains to produce the impression that they subsist on absurdly small quantities of food. Obviously, the amount of food will vary according to the age and size of the individual, the amount of work he performs, the external temperature and other less important factors, such as idiosyncrasy. It is futile and misleading to make comparisons between the diet of an eight-stone man and a twelve-stone man, without taking all the above factors into consideration.

It is convenient to classify the methods under two headings, (1) Scientific, (2) Empirical; the former being again capable of subdivision into chemical and dynamical.