THE ESTIMATION OF METABOLISM 173 



Every molecule of carbon dioxid holds combined the 

 equivalent of a molecule of oxygen. It follows that if all 

 the oxygen were devoted to the formation of carbon dioxid 

 the two volumes would be equal and the ratio between 

 them would be unity. The failure p| apart L of_the oxygen 

 to reappear as carbon dioxid indicates that it has been 

 combined in some other way. It has actually gone to form 

 the second great respiratory product, the water of the 

 metabolism. The interest which physiologists feel in the 

 respiratory quotient springs from the fact that it varies 

 with the prevailing employment of one kind of material 

 or another in the general oxidation which is going on. 

 The decimal value of the ratio is elevated in proportion 

 to the prominence of carbohydrate in the process. It is 

 lowest when fat is bearing a principal part. Since, at the 

 beginning of a fast, carbohydrate is called upon to meet 

 the requirement, while its place is taken by fat a few hours 

 later, the respiratory quotient will show a decline which 

 marks with exactness the shifting' of the current! 



It would not be easy to show how the respiratory quotient 

 can be made the basis of equations which determine 

 how much fat and how much carbohydrate are broken 

 down to give a certain output of carbon dioxid. Suffice it 

 to say that the possibility exists and is highly fruitful of 

 results in the quantitative studies of nutrition laboratories. 

 The meaning of the respiratory quotient is sometimes al- 

 tered by temporary conditions. Perhaps the most in- 

 teresting of these is the peculiar increase in carbon dioxid 

 outgo exhibited by an animal which is rapidly fattening on 

 a diet rich in carbohydrates. Such an animal may show 

 for days together a respiratory quotient in excess of unity, 

 that is, it produces carbon dioxid not accounted for by the 

 observed oxygen intake. This extra carbon dioxid is ex- 

 plained satisfactorily as having come from the carbo- 

 hydrate undergoing transformation to fat. (See also 

 Chapter XV, page 143.) 



Equilibrium. Our nearly uniform weight, maintained 

 for periods of years, suggests that income and outgo are 



