The place of diet in the treatment of affections of the kidney is one of considerable importance inasmuch as so many renal affections run a prolonged and chronic course and many of them are not capable of being very directly influenced by drugs. Further, a large number of renal affections are of toxic origin and dependent upon the action of poisons on the renal elements and on the blood vessels, and some of these toxic agents may be consumed as food, and thus diet is of importance not only in the treatment but also in the prevention of renal disease. Again the composition of the urine can be profoundly affected by diet and even such a fundamental characteristic as the reaction of the urine can be altered by suitable diet. The part played by changes in the composition of the urine in the production of renal disease is no doubt to some extent uncertain, but at any rate the composition of the urine is a factor of considerable importance in some affections as, for example, calculus disease. It may well be that the production of a calculus is not solely dependent on the amount of the particular calculus forming ingredient present in the urine, but still in all cases this must be a factor of some importance. Thus the formation of a uric acid calculus may perhaps take place when the actual percentage amount of uric acid in the urine is not necessarily high, yet the formation of such a calculus in any given patient may be influenced to some extent by the quantity of uric acid present, and perhaps still more so by the form in which it is present, in other words, the relation of the uric acid to the other salts of the urine, as for example, the phosphates and the chlorides. Modifications in the composition of the urine may therefore affect such a process as the formation of a calculus, not only by increasing or diminishing the amount of the calculus forming ingredient present but also by modifying its chemical relationships with other urinary constituents and especially its solubility.
An excessive meat diet may lead to a deposition of uric acid in the urine, not only by increasing the amount of uric acid excreted, owing to the large quantity of purin bases present in the food, but this diet may also bring about a deposition of uric acid as a result of increasing the acidity of the urine, since the meat food will lead to an increase in the acidity of the urine, dependent on an increased excretion of acid phosphate of soda. This illustrates very well the complex effect that may follow a given diet. These alterations in the composition of the urine are produced directly, owing to the presence in the food of certain substances, or owing to certain bodies, e.g. uric acid, being formed as the result of the normal metabolism of the food. Indirect effects, however, are perhaps of greater importance and may also follow the use of certain diets under abnormal conditions. A familiar instance is afforded by the study of gout, there would seem to be but little doubt that gout, at any rate in some individuals, is brought about by a particular diet, and in the gouty an attack of acute gout may be seen to follow certain indiscretions in diet, and it is at any rate possible that gout is associated with and dependent on the production during metabolism of abnormal and toxic substances. These toxins subsequently produce organic disease of various organs and amongst others the kidneys. A better illustration may perhaps be afforded by some instances of oxaluria. A certain degree of oxaluria may be produced in the healthy by the ingestion of certain articles of food, more especially rhubarb, but there are a number of individuals where large quantities of oxalates are passed in the urine associated with disturbance of the gastric functions and the presence of a certain degree of dilatation of the stomach and fermentation of its contents. In these instances the oxalic acid is not present as such in the food, but is formed as the result of decomposition in the stomach, and the oxaluria results from the absorption of these bodies from the gastrointestinal tract. The diet in these patients produces oxaluria, although in the healthy it would fail to do so, the urinary effects being entirely dependent on the abnormal decomposition of the food. Such an oxaluria may be treated by dietetic measures, although these must necessarily be somewhat different from those employed where the oxaluria is merely dependent on the ingestion of oxalates as such.
Thus it may be said that diet may affect the urinary excretion in one of three ways. It may modify the composition of the urine owing to the direct excretion of substances present in the food as such, as for example, the increased acidity of the urine on a flesh diet dependent on the excretion of increased quantities of acid phosphate of soda. Secondly, the composition of the urine may be modified as the result of the excretion of increased or diminished quantities of the normal products of metabolism of the food constituents as, for example, the increased excretion of urea and of uric acid seen in the flesh diet; and thirdly, the urine may be modified in its composition owing to the presence of substances formed as a result of abnormal metabolism of the food constituents owing to the presence of some pathological condition, e.g. the oxaluria in gastric dilatation.
In employing diet in the treatment of renal disease, it is important to bear in mind that the influence of the diet on the renal lesion may be either direct or indirect.
Renal diseases cannot very well be regarded as a whole from the point of view of their treatment by diet, inasmuch as there are such great differences in the effects produced by different renal lesions. Not only is this true with regard to different diseases, but also, to a certain extent, with regard to one and the same disease at different periods of its course. Dropsy is present in some renal diseases, absent in others, and even in the diseases where it is usually present, it may be absent during long periods of time. A diet suitable where dropsy is absent may not be suitable where it is present. The same is true of other phenomena, such as uraemia. Putting aside such conditions as these, renal lesions may be divided into the following groups for the purpose of diet and treatment: First, nephritis in its acute form; secondly, chronic nephritis; thirdly, granular kidney; fourthly, amyloid kidney; fifthly, calculous disease; and lastly, other renal affections which may be grouped either with acute or chronic nephritis. There is no great difficulty with reference to most of these groups, but there is some in regard to chronic nephritis, inasmuch as so many different lesions are included under this term. In one variety dropsy may be present, together with other severe symptoms, and the patient may be profoundly ill; in a second form dropsy may be absent, but albuminuria, with profound disturbance of the general nutrition, and possibly uraemic symptoms may be present; and lastly, in a third variety the patient may present few or no symptoms other than an albuminuria persistent and prolonged. All these different forms are frequently included under the general term of chronic nephritis, but there is at any rate a profound difference between the third group and the other two, inasmuch as in the former the albuminuria is practically the only marked effect present. It is obvious that all these three categories of cases of chronic nephritis cannot be satisfactorily treated on one uniform plan, but it is really in the third group of cases that the greatest difficulty in determining the course of dietetic or other treatment arises.