282 Biological Chemistry. 



periments bearing upon this subject have been carried 

 out by the determination of the nitrogen balance with 

 varied diets over prolonged periods of time. The results 

 obtained have been, however, the subject of much con- 

 troversy. 



It is possible, also, to determine the utilization value 

 of any given protein. The more completely a protein has 

 been digested, the greater the proportion of the nitrogen 

 excreted in the urine, and the smaller the amount ap- 

 pearing in the f seces. Again, proteins may be well digested, 

 and yet they may be by themselves inefficient in supplying 

 the bodily needs of an organism even when administered 

 in relatively large quantities. Reference has been already 

 made to the fact that when gelatin is the only protein 

 ingested, the body will not remain in nitrogenous equili- 

 brium. Zein, when administered to adult rats, is also 

 inefficient, as the animals will constantly excrete more 

 nitrogen than they have received in the foods, and lose in 

 body weight. These proteins are deficient in aromatic 

 groups, yielding no tryptophan on hydrolysis. It is 

 supposed that certain definite groups must be contained in 

 the proteins in order that they may efficiently supply the 

 nitrogenous needs of the organism. The results obtained 

 by the chemical investigation of the proteins, combined 

 with those got from metabolism experiments on animals, 

 have in recent years added largely to the knowledge of 

 the physiology of nutrition. 



Another interesting example of a diet which lacks 

 some constituent essential for the normal well-being of an 

 animal, although the deficiency is not due apparently to 

 the quality or quantity of the protein, is that of polished 

 rice. When pigeons are fed exclusively on such a diet 

 they develop poly neuritis, which is stated to be cured by 

 the injection of certain constituents of the rice-polishings. 



