THE PROTEIN REQUIREMENTS OF MANKIND 103 



They are the fruit of a colossal experience accumulated by count- 

 less millions of men through successive generations . . . and are 

 fitted to yield to observation and study lessons of the highest 

 scientific and practical value." 



Another method that has been put forward as a means of 

 determining the actual needs of the body for protein is that 

 based on Folin's observations on the constancy, under varying 

 conditions, of the output of creatinine and neutral sulphur in 

 the urine. Folin's work and the deductions therefrom have 

 already been discussed at considerable length in the introductory 

 chapter ; it will not, therefore, be necessary to refer to them in 

 detail in the present connection. It is generally acknowledged 

 that protein is only really essential to keep in repair the nitrog- 

 enous tissues of the body, and that its function of supplying heat 

 can be equally well undertaken by fats and carbohydrates. It 

 would therefore appear probable that, if the heat value of a 

 dietary were satisfactory owing to the presence of a sufficiency 

 of these two constituents, the amount of protein necessary would 

 only be that required to cover the loss from tissue wear and tear. 

 With the advent of a reliable method of estimating the katabolism 

 of protein involved in the disintegration of the true tissues, the 

 problem of the nitrogenous requirements of the body from this 

 standpoint would appear to be solved. Such a method has been 

 supposed to be forthcoming in the determination of the creatinine- 

 nitrogen of the urine, which has been regarded as a measure of 

 the endogenous or tissue metabolism. 



If the nitrogen in the form of creatinine were a measure of 

 the katabolism- of the tissue cells, then the heat value of a diet 

 being sufficient, the same amount of nitrogen supplied in the 

 food as lost to the body in the form of creatinine should be 

 sufficient to meet the protein requirements of the body, make 

 good the loss due to true tissue change, and maintain the subject 

 in a condition of nitrogenous equilibrium. 



The average amount of this creatinine-nitrogen is 0-6 gramme 

 daily, remaining, according to Folin, practically constant, whether 

 the diet be protein-rich or protein-free. The rational deduction 

 to be drawn from this argument would be, if the creatinine- 

 nitrogen is any measure of the protein requirements of the body, 

 that under the stated conditions 0-6 gramme of assimilated 

 nitrogen should be sufficient to maintain the body in nitrogenous 

 equilibrium and insure the integrity of the tissue cells. 



