The first and leading principle of diet in disease is, to provide for the maintenance of healthy nutrition, under the peculiar alterations of circumstances attendant upon disease. In other words, the diet of disease should be as nearly that of health as the altered conditions of the nutritive functions, and the altered conditions of the patient's life will allow; the grand object being to keep up healthy nutrition of the whole organism.

The true appreciation of this first principle, in its various bearings, will save us from running into either of those extremes which at different times have disfigured medical practice. We shall not be led to starve our patients upon water-gruel, when they are craving for natural food; or to stuff them with beef-steaks and porter when every instinct of their nature rebels against it. In fact, it may be taken as a very safe rule, that it is better cautiously to supply a patient with the kind and quantity of food that his stomach calls for, than to deny it to him without an unquestionably good reason for so doing.

This may seem very much like letting a patient eat and drink what he pleases. But that is not at all what I intend to recommend. All I mean is this, and I wish particularly to impress it, that if we intend to interfere in the subject of diet, we must take care that we thoroughly understand what we are about (see p. 5); and in order to do this it is necessary to keep well up in the following subjects: -

1. The physiology of healthy nutrition.

2. The composition of food, and the essentials of a normal diet.

3. The physiology of disease.

If we keep these matters well before the mind, and at the same time keep our wits about us in watching the case under treatment, it is surprising what an immense deal of good may be done by interfering with the diet; but not otherwise.

We start, then with this as our first principle - never to be lost sight of - that healthy nutrition is to be maintained, if possible, under all circumstances. And we assume that to do this in a healthy adult man of average stature taking moderate exercise," the essentials of a normal diet must be supplied.

It must be borne in mind, that the proportions and quantities of the different elements of this normal diet are arranged to meet the requirements of the different functions of the organism when in a healthy state of activity; and it therefore follows, that if the activity of any of these functions is altered, the requirements will be altered; and hence, the second general principle is this: - To alter the quantities and proportions of the elements of a normal diet to correspond with any alterations in the conditions of life. Thus, when a man is overtaken by sickness, and confined to his room or bed, the adult man taking moderate exercise becomes an adult man taking no exercise; and the ingredients of his diet which were proportioned to his moderate exercise must now be proportioned to his no exercise; and other alterations must be made in like manner, to correspond with other altered circumstances, in addition to any that may be specially required by the nature of his disease.

But I must remind you that, even when a man is confined to his bed, and precluded from taking any kind of exercise, he is still necessarily undergoing a considerable amount of muscular exertion, which must be provided for in his diet. For example, so long as life remains, such all-important muscles as those of respiration, and the heart itself, continue to act, and to require that their healthy nutrition shall be provided for by a supply of plastic materials in the food.

We come next to the long list of alterations of function which may be involved in the term "Sickness." And the third principle is - To alter the forms, quantities, and proportions of the elements of a normal diet, to meet the altered relations in the activity and condition of organs consequent upon disease. It is evident that, in order to carry out our first principle of maintaining healthy nutrition under all circumstances, it may be necessary, under some conditions, to reduce the quantity of every element of diet; and also, under some circumstances, to alter the proportions of the different elements. This we see demonstrated in some of the lower animals by the phenomena of hybernation. When an animal gives itself up to its winter sleep, every vital function is reduced to its lowest degree of activity; and the animal is able to maintain healthy nutrition for a long period without taking any food at all; but as respiration has to be kept up more actively than the rest of the functions, a special store of carbon for this purpose is laid up beforehand in the body.

Now, supposing a man to suffer from any state of disease which should place him in the position, as regards his functions, of an animal during hybernation, it is clear that, while his whole diet must be reduced to a very low scale, the heat-giving elements must be supplied in quantities out of the normal proportions as compared with the rest; because no supply of carbon is stored up in preparation for his illness, as it is in the hybernating animal in preparation for its sleep.

We see conditions, in many respects similar to these in some stages of fevers, in which absorption, nutrition, and every vital function is at its lowest point consistent with life, respiration being the only one sufficiently active to call for any considerable supply of food. But here, of course, we must not lose sight of an element in the case not present in hybernation - viz., the existence of a poison, which by some means, natural or artificial, has to be eliminated or destroyed, and which may be keeping-some functions in activity, the requirements of which must be met. The precision with which we are able to do this in any given case, will depend upon the correctness of our knowledge of the nature of the poison, and of the organs concerned in the restorative process. Here, no doubt, we are often obliged to act in the dark, and to supply many ingredients which may not be needed, in the hope of furnishing among them that which is required, but which our ignorance prevents us from identifying. And we had far better, whenever our knowledge is at fault, act in this safe manner and supply much that may be useless rather than run the risk of withholding that which may be essential to life. But in the majority of cases, our knowledge will be sufficient for the emergency, if we keep in mind the general principles of action.

The fourth principle is this: - To obtain rest for every organ while it is suffering under active disease, by removing from the diet such elements as increase its functions. These are conditions which it is not always easy to fulfil without deviating from our first principle. For example - in the case of diseased kidney - the healthy nutrition of this organ requires a supply of albuminoid materials, while its function is increased by any surplus of these materials in the organism; and when its function is interrupted by disease, a proportion of albuminoids in the diet, necessary to the healthy nutrition of the organism generally, will be tantamount to an excess as regards the function of the kidney, and the accumulation of retained excretory matters will press injuriously upon the affected organ. In such a case other medical aids than diet must be brought to bear; and while the albuminoids in the food are reduced as low as is consistent with healthy nutrition, some auxiliary organs which are not damaged must be stimulated for the time, to save the diseased part from undue pressure upon its functions.

A simpler, but still important principle, may be stated as the fifth, viz.: - In all alterations of diet, to avoid any unnecessary reduction in the number and variety of the forms in which food is allowed to be taken. (See p. 100.) It is especially necessary to bear this in mind when dieting the dyspeptic, who are often still engaged in the active avocations of business and of society while under medical treatment. To treat such cases by cutting off from the daily bill of fare first one article and then another, till the food consists of only two or three permitted forms, is to destroy the appetite and the digestive powers by monotony of diet, and to depress the spirits of the patient by a constant series of petty denials.

This plan of dieting can only be regarded as the resource of ignorance; because an enlightened view of the case will discover some particular defect in the function of digestion or assimilation which will at once indicate the form or element of the food which is to be avoided; and thus it will be only necessary to cut off those articles which specially represent this element, or simply to alter the forms in which they are presented to the stomach. (See p. 4).

The sixth principle is also of great importance, viz.: - When it is necessary to remove from the food any of the essentials of a normal diet, to aim at selecting that which will answer the desired end with least danger to the nutrition of the vital organs. For example, if it is necessary for any special purpose to diminish the heat-giving elements of the diet, it is safer to remove the carbo-hydrates than the hydro-carbons, because the latter not only supply carbon for the evolution of force, but are essential to the nutrition of the nervous system, and of the albuminoid tissues generally. (See Fat, Chapter VII (What You Need To Know About Fat In Nutrition).)

The seventh and last principle which I shall give in this Lecture is of very general application: - When it is desired to increase the normal nutrition of a tissue or organ, we must not only supply it freely with the special materials requisite for its development, growth, and repair, but at the same time call upon it for the performance of its normal functions - over-fed idleness insures morbid nutrition, not healthy life.