120 THE PROTEIN ELEMENT IN NUTRITION 



to the real nitrogenous metabolism ; but for a short period of 

 twenty-four hours, as in the observations on these 108 individuals, 

 the urinary nitrogen could not be accepted as a true measure of 

 the level of nitrogenous metabolism. It is well known to all 

 who have worked on the elimination of nitrogen in the urine that, 

 even with a uniform, non- varying dietary, there may be con- 

 siderable variations in the daily excretion of urinary nitrogen ; 

 the diets of these students were freely selected, and doubtless 

 varied from day to day. 



The only true measure of the amount of nitrogen undergoing 

 metabolism is the difference between the nitrogen intake and 

 the loss of nitrogen to the body in the faeces. (The question of 

 the nitrogen derived from the intestinal canal that has really 

 been metabolized has already been discussed.) 



The average quantity of 12-87 grammes given by Chittenden 

 as the amount of metabolized nitrogen is therefore too low, as 

 it makes no allowance for nitrogen that has undergone meta- 

 bolism, but which has either been retained or has left the body 

 in some way other than in the urine. 



2. How are we to discover what the real average level of the 

 nitrogenous metabolism of these 108 subjects is ? In the latter 

 part of the same paper Chittenden records the results of observa- 

 tions on six laboratory assistants over a period of 130 days. We 

 have discussed this experiment already, and shown that under 

 fairly similar conditions these six men, with an average intake 

 of 12-02 grammes of nitrogen, metabolized 10-66 grammes, 

 whilst only 9-16 grammes on the average appeared in the urine. 

 That is, 1 -5 grammes of nitrogen daily for each subject had actu- 

 ally undergone metabolism, but did not appear in either the 

 urine or faeces. If we accept this average, which is based on 

 Chittenden' s figures, as the amount to be added to the nitrogen 

 of the urine in order to arrive at the real nitrogenous metabolism, 

 we get 12-87 + 1-50 = 14-37 grammes of nitrogen per man daily. 

 This works out at practically 90 grammes of metabolized protein 

 as contrasted with 100 grammes provided in the ordinary stan- 

 dard, or 90 per cent, of its protein value. 



3. Voit's dietary is fixed for an average man of 70 kilos doing 

 an average amount of work, and provides 0-228 gramme of 

 nitrogen per kilo of body weight. Now, Chittenden records, 

 details, in the paper referred to, of 33J per cent, of the subjects 

 under observation ; the average body weight of these thirty-six 

 persons was only 61-7 kilos, so that it would be expected they 



