NUTRITION. 365 



are now experimenting to learn whether crude mother liquors — another 

 by-product obtained from the milk-sugar manufacture — can be used 

 in place of our "protein-free milk" so as to raise animals to maturity 

 without any other source of protein or inorganic salts. These observa- 

 tions pave the way for extending to other and larger species of animals 

 the study of the problems which we are investigating with the rat. 



Precisely as the use of zein has made it possible to ascertain certain 

 quantitative features regarding the needs of tryptophane and lysine, 

 feeding experiments with gliadin from wheat have enabled us to obtain 

 additional information with respect to the physiological requirement 

 for lysine, in which this protein is deficient. The extent of growth can 

 be almost completely controlled at will from day to day by the quantity 

 of a single factor, lysine, which is furnished in the diet. 



These experiments with "deficient" proteins have led to the exten- 

 sion of our investigations to gelatin, from which tryptophane, tyrosine, 

 and cystine are missing and in which phenjdalanine and glutaminic 

 acid are apparently present in very small proportions. The experi- 

 ments in this direction may enable us to learn something regarding the 

 part played by these other amino-acids in nutrition. An animal can 

 be maintained for long periods without loss of body-weight on a diet 

 devoid of lysine; such maintenance is impossible when tryptophane is 

 missing in the food. It would seem, therefore, as if the need of lysine 

 (if there be any) in the wear-and-tear or maintenance functions can 

 be supplied from the reserves in the tissues more readily than the 

 requirement for tryptophane can be thus satisfied. If, as now seems 

 probable, new protein can not be synthesized unless the food furnishes 

 lysine, the actual destruction of tissue protein in maintenance must 

 be far smaller than it is currently believed to be. One of several 

 experiments may be cited in support of these statements. A young rat 

 weighing 50 grams was maintained on a zein-tryptophane diet at con- 

 stant weight for six months, during which time it remained active and 

 apparently in fair physical condition. Although body- weight was 

 maintained, the animal grew extremely thin and showed every indica- 

 tion that its musculature was being drawn on to supply the deficiencies 

 of the diet. The amount of lysine which could thus be furnished for 

 the synthesis of new protein required for the maintenance of the physio- 

 logical functions was manifestly small. Thus if we assume that 25 

 per cent of the animal is protein, yielding all of the amino-acids required 

 for the synthesis of new protein, the total protein available during the 

 six months was only 12.5 grams. Since only a small part of this 12.5 

 grams was actually used, and since presumably no new protein could 

 be made from the zein-tryptophane food, it is nearly certain that the 

 wear-and-tear quota of protein is extremely small. Our experiments 

 have further indicated that food protein serves some further purpose 

 than to furnish the amino-acids for the synthesis of new protein to 



