172 NUTRITIONAL PHYSIOLOGY 



ing chiefly on the relation of the income and outgo of 

 water. 



Could there be conditions under which all the non-pro- 

 tein carbon could confidently be assigned to carbohydrate 

 sources? Not in the fasting state nor commonly on a 

 mixed diet. The case might be approximated by giving 

 ample rations with minimal fat and maximal carbohydrate 

 for days together. This is nearly equivalent to the nutri- 

 tion of the herbivora. If our supposed human subject 

 yielded the amounts of carbon and nitrogen already 

 quoted while adequately fed upon protein and carbo- 

 hydrate, we should not be much in error in assuming 

 that the carbon from non-protein had been evolved from 

 starches and sugars metabolized. Carbon forms about 40 

 per cent, of carbohydrate, and, if we reckon according to 

 the same principle as before, we find that 172 grams of 

 carbon could have come from 430 grams of carbohydrate, 

 more or less. Under more ordinary conditions of feeding 

 and on the first day of a fast both carbohydrate and 

 fat would share with protein the sustaining of the body's 

 activities. 



The Respiratory Quotient. The modern respiration 

 chamber is a small room with impervious walls and care- 

 fully controlled ventilation. The carbon dioxid of the 

 air drawn off is either determined directly or estimated 

 from measured samples bearing a known relation to the 

 total volume. In chambers of the best type the oxygen 

 consumption is also ascertained. If we know both the 

 carbon dioxid production and the oxygen absorption we 

 can, of course, compute the ratio between the two. The 

 value of this ratio, based on the volumes and not the 

 weights of the two gases, is known as the respiratory quo- 

 tient. It is figured by dividing the volume of carbon dioxid 

 by the volume of oxygen. So determined it has most of the 

 time the character of a proper fraction, or, rendered as a 

 decimal, it is less than one. This is a way of saying that 

 the carbon dioxid discharged is generally less than the 

 oxygen which has disappeared in the exchange. 



