Chronic interstitial nephritis may be of primary origin, or it may occur as a result of arterio-fibrosis and other conditions. It is usually a very slow process, and is often provoked by chronic alcoholism.

Urine of a low specific gravity is passed in greater amount than normal, and thirst is increased. The arterial walls are thickened and their tension is high.

That an exclusive meat diet is not the cause of chronic Bright's disease is proved by the fact that Eskimos and other races, who subsist altogether upon it, are not especially liable to the disease, although in the long sunless winter they live under very bad hygienic conditions. (See the relation of proteid food to albuminuria, p. 501.) It is nevertheless a fact that the disease is very prevalent in this country among men past forty years of age who for years have been overworked with business cares, and who have neglected to take sufficient exercise and care in eating. In the early stages of the disease much may be accomplished by dietetic treatment. Osier says that "care in food and drink is probably the most important element in the treatment of these early cases".

Whatever views may be entertained in regard to the causation of albuminuria, it is agreed by all that cooked meats, eggs, and highly seasoned food in general are injurious in all cases of existing albuminuria. One should be cautioned against regarding the amount of albumin present in the urine from time to time in the course of chronic albuminuria as an exact indication of the extent of the renal disease. The fluctuations observed are often due to diet merely, and the albumin may occasionally be considerably reduced in quantity, or actually absent temporarily from the urine, although the lesion of the kidneys is making uninterrupted progress. The volume of albumin in the urine must not, therefore, be estimated alone as an indication for dietetic treatment; it is merely one of many symptoms incident to the course of chronic Bright's disease winch are benefited by a strict regimen.

Extensive disease of the excretory surface of the kidneys always interferes with the normal elimination of nitrogenous waste matter which accumulates in the system until it finally produces toxic or uraemic symptoms of a most serious character. If the alimentary canal is overloaded with food above the actual requirements of nutrition, or beyond the capabilities of the system for oxidising food products, toxic symptoms are sure to follow. For these reasons the diet in chronic Bright's disease must be definitely regulated, and a mean must be maintained between overburdening the digestive apparatus and withholding food which is needed to support the bodily strength. This applies especially to the waste of nitrogenous food, which passes off through the kidneys, whereas the waste of farinaceous food is eliminated largely from the lungs and skin in the form of water and carbonic acid.

Milk Diet

In many cases of chronic Bright's disease the effect of feeding the patient for six weeks, or even two or three months, upon an exclusive milk diet is remarkably gratifying. The quantity of urine, urea, and extractives increases, while the albumin diminishes, and oedema and general anasarca disappear. The patient's strength and general condition improve, the pulse is strengthened, and if dyspnoea pre-existed it subsides.

The quantity of milk to be prescribed for an exclusive milk diet in Bright's disease must depend on the age and size of the patient, as well as upon his ability to take exercise and use up force in muscular energy. If the patient is invalided so as to be confined to his room or the house, from five to seven pints of milk daily are quite sufficient. Seven pints of milk contain 216 grammes of albumin and casein, 172 grammes of fat, and 161 grammes of lactose; whereas, according to Pettenkofer and Voit, a healthy adult requires 137 grammes of dry albumin, 117 grammes of fat, and 352 grammes of carbohydrates (see p. 290). The milk diet is therefore deficient in carbohydrates, but the latter are estimated in Voit's table for a person taking active exercise, and requiring the development of more muscular energy and heat than the invalid. If the patient loses weight on a milk diet, although it otherwise agrees with him, it may be well to add farinaceous food in the shape of rice or bread. Many patients will live contentedly with no other food than a bowl of bread and milk four or five times a day.

In some cases it is inadvisable to commence the milk diet immediately, and a good result can be best attained by cutting off one article of solid food after another and replacing the loss by an additional tumbler of milk. Where it is expected to employ a milk diet for many months, as in an advanced case of Bright's disease, it is usually found that the milk is better borne if taken fresh at a natural temperature and without flavouring of any kind. Although the treatment may seem severe at first, if the patient understands the gravity of the situation he is usually willing to accept it, and after a few days seldom complains of the monotony of this most restricted form of feeding. In the worst cases it is desirable to give the milk at brief intervals, in quantities of six ounces, once an hour during the daytime, with an extra tumblerful at night, and on awakening in the morning. Ordinarily, however, it is tiresome to have to drink milk so often, and the quantity may be so regulated as to give the requisite amount once every three hours.

The quantity of milk necessary to support life for any length of time and maintain good nutrition, especially if the patient is exercising at all, is considerable, and he must take from fourteen to eighteen, or even twenty-two six-ounce tumbler-fuls of milk in the twenty-four hours. It is usually impossible to commence at this rate without producing gastric disturbance from souring of the milk in the stomach, and possibly diarrhoea. The latter symptom is an almost certain indication that the milk is being imperfectly digested, and a temporary reduction in its quantity is advised.