PROTEIN REQUIREMENTS 87 



equilibrium is reached. Thus Falta (122} superimposed different pro- 

 teins on to a standard diet, whose effect on the metabolism was known, 

 and found in his experiments a variation from about three to six 

 days in the rate at which the nitrogen was excreted. This observa- 

 tion was made on man, but in the carnivora the differences in the rate 

 of excretion did not hold good, or at any rate were much less marked. 

 This step-like excretion is not due to slowness of absorption, as the 

 excretion is spread over days, but must be due to a step-like breakdown 

 of protein in the body. Falta has suggested that in the absorption of 

 protein, part is taken up in some high molecular form, and part in a 

 small molecular form. According to the immediate needs of the 

 organism part is retained and part burnt, the low molecular form 

 being much more labile than the high. A certain amount of evidence 

 in support of this step-like breakdown is available, and will be con- 

 sidered presently. In the light of Falta's results, then, it is not only 

 the amount of protein present which influences the amount of nitrogen 

 excreted, but also the nature of the protein given plays quite as an 

 important part. As Ehrstrom has remarked, the capacity which the 

 body possesses of catabolizing protein is constant, not variable. The 

 variable factor lies in the differing chemical constitution of the pro- 

 ducts of digestion and their varying resistance to such catabolic action. 



Examination of the Nitrogen : Sulphur Ratio. 



Evidence of the rate of protein catabolism has also been put for- 

 ward from quite another point of view, namely from the investigation 

 of the sulphur output. Sulphur, just like nitrogen, is found in practi- 

 cally all proteins, but in more varying amount. It had been suggested 

 that from the study of the ratio of the sulphur : nitrogen output evidence 

 might be obtained of differences in the rate and form of the breakdown 

 of proteins. This ratio has accordingly been fairly fully investigated 

 by several workers. Von Wendt (419 A) carried out some extremely 

 interesting and careful work which demonstrated clearly that the 

 amount of protein catabolized can be calculated quite as accurately by 

 estimation of the sulphur as of the nitrogen output. Using as a factor 

 9' 3 the ratio of nitrogen to sulphur excretion in a nitrogen starvation 

 experiment and multiplying the daily output of sulphur by it, figures 

 were obtained which were practically identical with those obtained by 

 the Kjeldahl nitrogen method. The drawback of course to the utiliza- 



