There are two methods of adapting diet for the sick: First, by giving one half, one third, or one fourth of the ordinary quantity of food for health; second, by altering the quality of the different classes of foods, and reducing the total amount simultaneously or not, as required. The former method is simpler and less troublesome, and for some few cases it may prove sufficient. It is to be regretted that it is a routine method adopted in many hospitals, for it is unscientific, besides being often wasteful and radically wrong. The second method presupposes a knowledge of the relations of diet to disease, and requires careful supervision. In speaking of this subject Parkes wrote: "It is certain that the physician can in this way wield a great power, which, if less striking than that obtained more rapidly by drugs, is yet of immense moment".

The same aliments which in health produce definite results in furnishing energy and repairing tissue loss may fail completely in disease. Such is the case in diabetes when starches become not only a useless but a harmful food, and in severe albuminuria, when meats only aggravate the albuminuric waste.

In the majority of severe diseases, and especially of acute and febrile diseases, the proteids and fats, aside from those of milk, become less assimilable than the carbohydrates. Beneke attributes this to the fact that carbohydrates are respiratory foods - i. e., their consumption results in the production of carbon-dioxide gas and water, and the elimination of carbon dioxide proceeds in disease very much as in health. If carbohydrates are not therefore furnished, inanition ensues, for the tissues begin to consume their own substance.

There are, on the other hand, some few diseases, such as tuberculosis, chlorosis, and secondary anaemia, in which proteids and fats may be distinctly required, and others again may temporarily demand a purely nitrogenous diet.

Food improper in quality or deficient in quantity is sure to be not only a direct source of disease, but indirectly, by lowering the vitality of the body, it leaves it an easy prey to epidemic and contagious diseases of every sort. This was strikingly shown during the ravages of the plague in the Middle Ages, and it has been apparent in the history of all recent famines. (See Starvation, p. 301, and Famine, p. 308).

The important relation of food to disease in regard to its quantity and composition, as well as the frequency and method of its administration, are becoming more and more thoroughly appreciated. This is owing in great part to improved methods of diagnosis and to modern means of clinically discriminating between different kinds of gastric and intestinal indigestion. It is also due to an increasing knowledge of the chemistry of food, of food preparation by cooking, and of "artificial digestion".

There is still much difference of opinion in regard to the best dietaries for certain diseases, such, for example, as gout and obesity, but the general principles of dietetics are to-day well established and more widely understood and practised than ever before, and starvation in fevers, like excessive venesection and similar depleting measures, is no longer carried to the former injurious extreme. There remains much to be learned in regard to dietetics; and the great variations in digestive power which are encountered in individual cases of disease and in personal idiosyncrasy, together with the increasing varieties of foods and food preparations, make the relation of diet to disease a constantly broadening subject, the importance of which cannot be overrated. It is to be regretted that the study of dietetics is not more thoroughly taught as a necessary part of medical education, and that thorough systematic instruction in this practical branch of science is omitted from the curriculum of most medical schools.

For many physicians it seems far easier to write a brief prescription for a "nerve tonic " or cathartic pill than to take the time and pains to state in writing for their patients a definite dietary which in very many cases will prove an invaluable adjunct to medicinal treatment, if not in itself a curative measure. Sometimes indeed they appear possessed with the idea that the minutiae of dietetics are beneath consideration, whereas in reality, like the details of good nursing, they are most important, and a careful attention to them and a close interrogation into the ordinary habits of the patient in regard to his daily regimen is sure of appreciation and of beneficial results.

In all acute febrile diseases and in all so-called "wasting " diseases in which there is faulty assimilation of the food, the digestive functions are impaired, while the rapid tissue waste increases the need for nutriment to replace it. In such cases enfeebled digestive powers must be taken into account, and while endeavouring to supply the increased demand for food, every effort should be made to relieve the alimentary canal of unnecessary labour by supplying those preparations which will yield the necessary amount of nutrition and force with a minimum expenditure of energy. Moreover, the conditions of elimination of waste materials from the tissues must be studied in relation to the composition of different foods in order to prevent the overworking of the excretory organs.

It may be asserted that there is almost no disease of long duration and severity, and certainly no disease accompanied by grave constitutional disturbances, the course of which cannot in a measure be controlled or benefited by thorough study of the nature and uses of foods. There are many acute aliments in which undoubted benefit is derived from greatly diminishing or altogether withholding temporarily the consumption of food, while there are others in which forced feeding - that is, increasing the quantity of food to the utmost capacity of the digestive organs - is a necessity.

The general relations of food to the organism, as well as the nature of foods and the processes involved in their digestion, absorption, and assimilation, have received full consideration in the preceding pages. In the following chapters the principles briefly outlined above will be considered in their application to individual diseases. At the risk of some repetition, it has been thought best to make the dietetic treatment of each disease as comprehensive as possible under the different headings considered, and to emphasize its value by a brief synopsis of the more important symptoms which arise, and a discussion of the indications to be met by appropriate dieting.