Researches in the field of protein nutrition during the last twenty-five years have had as their main objectives the solution of the following problems: 1. The digestibility of the proteins; 2. the number and nature of the amino-acids contained in proteins of plant and animal origin; 3. the amounts of each of these yielded by different proteins; 4. the extent of the digestive decomposition of proteins in the alimentary tract; 5. the ability or inability of the tissues to synthesize the various digestion products of proteins; 6. the immediate disposition of the products of protein digestion after absorption; 7. the chemical changes which take place during the catabolism or destruction of the amino-acids in the body; 8. the capacity of the body to synthesize protein when the requisite amino-acids are supplied in the food; 9. the determination of the relative efficiency possible in the transformation for purposes of growth of (a) individual proteins and of (b) the mixture of proteins in individual food-stuffs, into body proteins; 10. a comparison of the number and kinds of amino-acids required in the process of repair of tissue waste, an essential result of life activities with those required in the formation of new tissues for growth; 11. the physiological effects of nutrition with protein intake at different planes, or the discovery of the amount of protein from stated sources which serves to maintain the body in optimal nutritive condition.

It is not within the scope of this book to discuss all phases of this subject. An arbitrary selection of topics will be made, and only the last four problems will be considered. This is desirable because the literature relating to research along these lines is especially in need of being summarized and interpreted.