374 CARNEGIE INSTITUTION OF WASHINGTON. 



by observing the weight of the animals for which maintenance is either barely 

 possible or no longer satisfactory in the numerous individual experiments on 

 different fixed daily doses of yeast. 



Our numerous data indicate, however, that per unit of body-weight the 

 larger animals may have a somewhat smaller requirement than the small 

 growing ones. This, perhaps, is explained in part by the relatively larger 

 content of adipose tissue in the adult individuals. In any event, the major 

 thesis is clearly substantiated. 



The problem has further been considered from a somewhat different angle 

 by observing the effect of measured daily doses of vitamine B, administered iD 

 the form of yeast, upon animals which had previously grown on our customary 

 mixed diet instead of the specific unvarying ration discussed in the previous 

 experiments, as it might be assumed that if vitamine B storage in the body 

 depends on the character of the diet, animals raised upon different sorts of 

 food might subsequently respond differently to the same daily vitamine 

 dosage. No appreciable difference, however, was observed. 



Our experience with dry yeast as a source of vitamine B has been corrob- 

 orated by similar experiments with the concentrate from yeast extract first 

 prepared by us several years ago. Thus, with a daily dose of 40 milligrams of 

 this product, growth usually stopped when the rats reached a size not far from 

 200 grams in body-weight; but weight was subsequently gained when more 

 of the concentrate was fed, and growth then continued until a new mainte- 

 nance level was reached, whereupon larger doses of the yeast-concentrate 

 again promoted growth. Likewise, when naturally occurring foods such as 

 green vegetables were used as the sole source of vitamine B, larger quantities 

 were required for normal growth as the animals increased in size. 



The possibility of supplying the vitamines in small daily doses apart from 

 the energy-bearing portion of the ration has made it possible to consider some 

 of the problems of metabolism from new viewpoints. It has already become 

 apparent that some of the current traditions regarding the requisite 

 relative proportions of protein and the non-nitrogenous nutrients in the diet 

 must be, in part at least, abandoned in the light of our findings. 



The unexpected growth of animals to considerable size on diets almost free 

 from carbohydrate or fat, or from both, encouraged us to undertake further 

 investigations on the effects of such unusual food mixtures. In our more 

 recent experiments some animals have grown at a normal rate to more than 

 260 grams on diets of which more than 90 per cent consisted of protein. 

 Evidently, if any considerable quantity of carbohydrate is requisite for normal 

 metabolism it must have been derived from the amino-acids of the ingested 

 proteins. It has been reported that different species of animals on high- 

 protein diets suffer from albuminuria, and the implication is that kidney 

 defects arise. It is hardly conceivable in the case of our rats fed on high- 

 protein diets that they should have continued to grow to almost normal adult 

 size at a satisfactory rate if the renal organs had been seriously damaged. 

 The histology of animals which have grown up on such unique diets remains 

 to be investigated; likewise the possible changes in tissue composition, fat 

 production, and other chemical factors incident to gain in weight. 



Thus far none of the animals on the high-protein diets has ultimately reached 

 the average adult size attained by rats fed on our standard food mixture con- 



