THE ESTIMATION OF METABOLISM 175 



The roughly maintained equilibrium, which is, after all, 

 a striking example of the adjustments of the organism, is 

 to be traced to the singular reliability of the appetite. 

 This is the agent which prompts so surely to the taking of 

 extra food when one exchanges an inactive life for one of 

 bodily activity. The most radical changes in the total 

 metabolism are unlikely to lead to lasting variations in 

 body weight beyond slight gains and losses, which, by the 

 way, are often the reverse of what was anticipated. Ex- 

 ercise, which is supported by large oxidation, may even 

 result in some increase of weight, showing that the appe- 

 tite has rather more than met the precise need of the body. 



Carbon Retention. When a quantitative comparison 

 is made between the compounds in the diet and those ex- 

 creted it is not infrequently found that carbon is being 

 stored, though the nitrogen of income and outgo may be 

 balanced. What can be inferred as to the nature of the 

 substance added to the tissues? Just as in the previous 

 case where we desired to interpret the meaning of the 

 carbon loss during fasting, we have to consider the re- 

 spective share taken by carbohydrate and by fat. As be- 

 fore, it is important to know the condition of the subject 

 prior to the trial. If the day is the first of feeding after 

 a fast there will be some recruiting of the glycogen in the 

 body, and a part of the carbon retention may be attributed 

 to a gain of this material. Otherwise, when carbon is 

 stored in the midst of a period of liberal feeding, the prob- 

 ability is that fat rather than carbohydrate has been de- 

 posited. 



Applying the same factors as in the earlier instance, we 

 multiply the retained carbon by 1.3 if the circumstances 

 point to its having been held as fat. An excess of 10 

 grams of carbon in the income over the outgo would be 

 assumed to indicate the addition of 13 grams of fat to the 

 supply in the body. An amount of this magnitude would 

 not show itself decisively in the weight, being easily dis- 

 guised by the temporary gain or loss of water that might 

 occur at the same time. 



