The best known modern theory of diet is the purin-free, or, as its author, Dr. Alexander Haig, with a certain lack of scientific precision entitles it, the uric-acid-free diet theory. It is quite fair at the very outset of our inquiry to seek for an accurate definition of the term "purin-free diet," for it is pretty apparent that amongst the foods permitted by its advocates many are included, e.g., milk and its derivatives, which are by no means devoid of poring. It should be noted that the term "purin bases" applies to hypo-xanthin, xanthin, adenin and guanin, the four precursors of uric acid, while the term "purin bodies " includes these four substances with the addition of uric acid.

It must be clearly understood, therefore, that an absolutely purin-free diet is practically unattainable. Probably this is a very fortunate circumstance, for it is indubitable that the animal cell requires both nucleic acid and purin bases for the purpose of building up nucleo-proteins, which are absolutely essential for enabling the body to carry on its business, and can, indeed, be separated from almost every one of its organs. It is possible, however, that the body may be able to manufacture purin bases for itself, as we know this is effected in incubating eggs. Despite this fact, we are quite justified in saying that the purin bases and other components of nucleic acid to be found in so-called purin-free articles of diet are used in the construction of the body cells and especially of the nuclei.

It is therefore impossible to separate with precision the urinary uric acid which is derived from the food from that which comes entirely from cell metabolism, because manifestly some of the endogenous uric acid may owe its origin at no remote date to the purin bases of milk products. For all practical purposes, however, it may be asserted that a purin-hee diet is one which is capable of maintaining the output of uric acid at the lowest level possible for the individual, and this is, presumably, almost entirely of endogenous origin.

The advantages to be derived from the adoption of such a diet are sometimes so notable that there is a tendency on the part of an enthusiast to look upon it as a panacea for "all the ills that flesh is heir to." In recent times there has been a surprising accumulation of favourable opinions in support of it, and amongst a certain section of the profession it has been taken for granted that the objections, which have been exposed time and time again, no longer hold any weight, and that the uric-acid theory so-called has been proved to a demonstration. From the theoretical point of view, the arguments in its favour are so plausible and so difficult to refute, that one is apt lightly to advocate its adoption, without considering the consequences. For some years now I have been much attracted by its possibilities in the treatment of disease, and in selected cases, where the ordinary therapeutic methods have failed, I have had no hesitation in encouraging patients to give it a thorough trial.

It is astonishing what a fascination such a theory has for the young and enthusiastic physician in the first flush of his disappointment with both old, well-worn remedies and new much-vaunted specifics. On the face of it, the whole thing seems so absurdly simple that it does not need any elaborate demonstration to prove its truth. The body of your ailing patient is a more or less intricate filter, into which, for purposes of nutrition, have been poured certain solutions of nutriment containing certain poisons which are not completely excreted, and hence accumulate therein, producing well-defined acute or chronic diseases, according to the temperament of the patient. To eliminate the disease, all that seems necessary is to expunge the poison from the food, and, where this cannot be done, to select foods which contain no poison, and you go forward with the simple, and at first not easily shaken, faith that disease in such circumstances is really an impossibility.

The practice of the purin-free diet may have existed long before Dr. Haig's time, but most certainly the theory - and, coeval with this, its inclusion as a branch of dietotherapy of decided value - originated with Dr. Haig. The description of its evolution in his fertile brain is carefully detailed in his now well-known treatise on the subject - a treatise which, from the empirical point of view, may be looked upon as one of the first efforts to put the practice of dietetics on a scientific basis. It reads like a romance, and would bear complete transcription; but I must content myself with giving the briefest resume of the facts, in order that we may understand the theory and demonstrate some of its fallacies.