170 NUTRITIONAL PHYSIOLOGY 



fying incomplete absorption and how far it is a true waste- 

 product. If feces are discharged during long fasting the 

 nitrogen contained must be the body's own contribution. 

 On the whole, the fecal nitrogen is nowadays regarded as 

 an excretion unless it is clearly excessive. The nitrogen 

 of the perspiration can usually be ignored. 



When we assume all the nitrogen eliminated to have 

 come from the decomposition of proteins, a certain error \ 

 always exists, but it is not great enough to be considered'/ 

 in the present elementary treatment of the subject. 

 Nitrogen constitutes about 16 per cent, of protein. Ac- 

 cordingly, the excretion of 16 grams is taken to stand for 

 the destruction of 100 grams of protein. This is not far 

 from an average amount when the diet is freely chosen. 

 In the opinion of an increasing number of authorities it 

 is higher than it should be for the best nutritional condi- 

 tion. To arrive at an estimate of the protein metabolized 

 we multiply the quantity of nitrogen in the outgo by 6.25 

 (an operation which is equivalent to dividing by 16 to 

 find 1 per cent, and multiplying by 100 to obtain the total). 

 This was a familiar procedure before the erection of res- 

 piration chambers made possible a complete survey of the 

 metabolism. 



Carbon Elimination. A subject excreting 16 grams of 

 nitrogen may be expected to excrete something like 200 or: 

 250 grams of carbon in the same period. This will be in 

 the respiratory carbon dioxid so largely as to make the. 

 urinary and fecal carbon appear insignificant. Figures: 

 from an actual experiment are: 



In the respiration '. . . . 208 grams. 1 



In the urine 6 " 



In the feces 11 " 



Total 225 " 



This carbon may have been furnished by all three types of 

 body substance the proteins, fats, and carbohydrates 

 in numberless possible combinations. The amount ot 

 protein decomposition has already been fixed at 100 grams. 



