The science of nutrition has now reached a stage of development where it seems highly desirable that its history should be written, notwithstanding the fact that at present it is attracting the attention of a greater number of investigators than at any time in the past. There are distinct signs that discoveries of practical value in this branch of physiological chemistry will not be made so rapidly in the future as they have been during the past seven years for the reason that the relatively simple technic which has characterized the experimental work in this field gives no promise of bringing to light further spectacular results.

The experimental methods hitherto described can, when the necessary labor and money are expended upon them, perfect our understanding of the quantitative relations of the amino acid content of the individual proteins and mixtures of proteins which occur in natural foods of animal and vegetable origin. They can, furthermore, serve as an aid to the isolation in a state of purity of the several vitamins, although they are but poor and unwieldy substitutes for chemical tests, which, unfortunately, are still wanting. The tree of nutritional knowledge appears, however, to have grown to proportions which reveal the general outlines which it will always present, and further researches by the methods which have hitherto been so productive, can, it seems, only clothe it in attractive foliage and aid it in maturing the rich setting of fruit which has not yet ripened and fallen for the service of man, although a few windfalls which have been tasted reveal the keen enjoyment with which the human race will one day reap the full harvest.

In the preface to the third edition of The Science of Nutrition, Professor Graham Lusk said in 1917: "In another decade the development of scientific knowledge will probably permit the formulation of the subject from the standpoint of physical chemistry. It cannot now be so treated." It seems to the author that a few years must elapse, during which the structural changes in the protoplasm of the different tissues which result from specific faults in the diet are accurately described, before the methods of physical chemistry and the chemistry of colloidal substances can be most effectively applied to the problems of physiology. Recent researches on the anatomic changes seen in beri-beri, rickets, scurvy and ophthalmia of dietary origin, represent the beginning of the task of preparing the field for the physical chemist. It is perhaps not too optimistic to hope that the next six years may see the fulfillment of Professor Lusk's prophecy.

It seems certain that a new era in nutrition investigations is at hand. Hitherto this branch of science has been the monopoly of the physiological chemist. He must now share his heritage with the histologist and the pathologist, and the three must work together. The chemist cannot progress far toward interpreting the nature of the changes which take place when the diet is faulty in any specific sense. He can interpret now, the exact nature of the dietary error which has produced in his experimental animal a pathological state, but with existing methods of analysis he cannot discover what part of the structures of the protoplasm have been injured. He can see loss of function but cannot explain its mechanism. He cannot tell what changes have taken place within the cells of special tissues as the result of his deliberate disturbance of their nutrition.

The science of pathology now suddenly has revealed to it an opportunity in a broad field of great practical importance, which it has never before enjoyed, viz.: an opportunity to investigate morbid anatomy and changes of function with a background of chemical explanation as to etiology.

In the vast literature which has grown up within a few years, relating to nutrition investigations, there is much that is confusing to one who would undertake to familiarize himself with it. Faulty technic, unwarranted deductions, premature conclusions, and conflicting data were inevitable in investigations involving so many variables and unknown factors. It seemed to the author that there is great need of a careful and critical interpretation of the existing data relating to nutrition, in order that the science may appear in its proper perspective. It was with this objective that the present volume was written. In order that the subject might be clarified it was necessary that elements of weakness in a considerable amount of experimental data should be pointed out.

The ultimate aim of scientific investigations is to give man control over the forces of Nature. The science of nutrition gives promise of making possible the realization of the optimal condition of physical well-being, with all that this implies for the mental achievement, freedom from physical defects including tooth decay, and immunity to many of the ills which result from invasion of the tissues through breeches in the barriers of bodily defense, especially through decayed teeth and a debilitated alimentary tract.

The author has, during recent years, devoted much study to the dietary habits of peoples in different parts of the world with a view to correlating the results of animal experimentation with those of human experience in the use of different types of diets. The results of this study are presented in this volume. It is believed that these two lines of investigation, discussed as they are in their relation to each other, point unmistakably to a dietary regimen which will when adopted bring about marked improvement of the human race.

It is a privilege to acknowledge the services rendered by Dr. V. E. Levine, who carefully read the manuscript during its preparation and made many helpful suggestions for its improvement, and by Dr. P. G. Shipley, who also read the manuscript and assisted in the preparation of the chapter on rickets. Special appreciation is due Miss Nina Simmonds for her constant interest and criticism, and for assistance in the preparation of the bibliography and the index. Her familiarity with the extensive literature of nutrition and the critical sense with which she has examined it, contributed in no small degree to give the book such value as it may possess.

E. V. McCollum

Laboratory of Chemical Hygiene School of Hygiene and Public Health, Johns Hopkins University, Baltimore, Md.