The processes of digestion of food are so remarkable that they have been made the subject of extensive study by many of the ablest students of physiology and biochemistry. The chemistry of the fats and carbohydrates is somewhat simpler than that of the proteins, and their structure and transformations came to be earlier understood. It was not until the close of the nineteenth century that the nature and extent of protein digestion became fully appreciated. Soon after 1900 the researches of Kossel and of Fischer revealed the great variation in the composition of proteins from different sources (2). This discovery introduced into nutrition studies the idea of quality of protein in addition to quantity. A statement of the quantity of protein had hitherto seemed satisfactory to students of nutrition. Most proteins were found to be resolvable into seventeen or eighteen simple digestion products called amino-acids, and it was observed that the proportions in which these were present in the protein molecule varied greatly in the proteins from different sources. All or nearly all of these digestion products appear to be indispensable components of an adequate diet. The extensive and valuable researches of Osborne have shown that all natural foods contain several proteins (3). Although there are individual proteins, such as gelatin and zein (from maize), which are entirely lacking in one or more of the essential digestion products, every natural food appears to contain more or less of each of them. The protein mixture, which is found in any natural food, such as the grains, tubers, meats, etc., may be regarded as biologically complete, but their biological values differ greatly, depending on the yield of the several amino-acids obtained from them.

This appreciation of the differences in biological values of proteins from different sources represented an awakening from a state of tranquillity among physiologists, who had come to believe, about the year nineteen hundred, that chemistry had revealed all the more important facts relating to foods and nutrition. As we look back now upon the recorded observations which had been made from time to time for centuries, especially by those whose viewpoint was essentially that of the pathologist, it is difficult to see how all the able students of nutrition during the nineteenth century should have so completely ignored their findings which pointed to a close relation between the diet and disease.