528 "f* S- BLTREAU OF FISHERIES 



over arc used for their energy. Their waste products — urea, etc. — 

 are removed by the kidneys and excreted in the urine. 



The proteins of the human tissues are definite chemical compounds. 

 Various proportions of the common amino acids are required for 

 their s}ti thesis. In other words, the amino acids constitute the 

 building stones or units out of which the great protein structure is 

 put together. 



Formerly the nutritive value of proteins was discussed in terms of 

 digestibility, physical properties, and their content of carbon, hydro- 

 gen, oxygen, nitrogen, and sulphur. Osborne and Mendel (1911 to 

 1924) have done much to establish the modern conception that the 

 nutritive value of proteins is largely determined by the amounts of 

 each of the amino acids which they yield. Many, if not all, of these 

 amino acids are essential for the construction and maintenance of 

 tissues. The efficiency of any individual protein in nutrition de- 

 pends upon the minimum of any indispensable amino acid that it 

 yields on digestion, for it is known that several of the amino acids 

 can not be synthesized in the human body. For example, if a pro- 

 tein or mixture of proteins, comparatively deficient in the amino 

 acid tryj^tophane, be supplied in the diet as the sole source of com- 

 bined nitrogen, the synthesis of protein molecules containing this 

 amino acid would be limited by the amount available in the diet. 

 Thus, maintenance and growth would be limited by the minimum of 

 the essential unit. 



As yet we do not know how many of the amino acids are essential 

 in the diet, but nutrition experts have shown that the following 

 amino acids are absolutely essential for growth and maintenance: 

 Tyrosine, tryptophane, lysine, cystine, and either histidine or argin- 

 ine. Further, many believe that valine, leucine, and phenylalanine 

 are also essential units. On the other hand, it has been demon- 

 strated that neither glycine nor proline need be provided ready- 

 made in the food. 



Since the proteins of the protoplasm of our bodies are built up 

 from the amino acids in the digestion products of the proteins of our 

 foods, it is probable that the more nearly the amino acid composition 

 of the proteins of the food resembles that of the body the greater is 

 their nutritive value. 



DIGESTIBILITY 



Several scientists (Milner, 190.5; White and Crozier, 1911; and 

 Holmes, 1918) have shown that the proteins of fish are as easily 

 digested as those of beef and other meats. The digestibility of the 

 proteins of salmon, mackerel, butterfish, and dogfish have been 

 determined both by the use of the enzyme, trypsin, and by experi- 

 ments with human subjects. These proteins were in all cases easily 

 and nearly completely digested. On the average, about 92 per cent 

 of the proteins were digested. 



The proteins of oysters, clams, scallops, and mussels also are 

 readily digested. Whale meat has been found to be as easily and as 

 completely digested as beef. Very few experiments have been carried 

 out to determine the digestibility of crabs, lobsters, shrimp, and 

 other crustaceans, but by analogy we may conclude that the proteins 

 of these shellfish are readily digested. 



