NUTKITION 219 



abundant mixed diet, both the nitrogen and the carbon of 

 the excretions were accurately determined. These have 

 shown that while muscular exercise does not necessarily increase 

 protein disintegration, it invariably increases the production of 

 carbon dioxide. If the carbon of the carbon dioxide came 

 from protein, it would be accompanied by increased excretion 

 of nitrogen derived from the broken-down protein. The fact 

 that it is not so accompanied can only mean that it came 

 from the oxidation of something which did not contain 

 nitrogen, that is, from fat or carbohydrate, or both. 



But while muscular work does not necessarily or even 

 usually increase protein decomposition, and the power for 

 the same may be derived largely, if not entirely, from car- 

 bohydrates and fats, it has been shown conclusively that 

 under certain conditions this power may come entirely from 

 protein. In one experiment a large and very lean dog was 

 fed for several weeks on an abundant diet of lean meat, 

 containing practically no carbohydrate or fat; during this 

 time the dog did large amounts of work in a treadmill and 

 in other ways; and since it was found that this work could 

 be done for weeks at a time on the meat diet, we conclude 

 that the protein must have been the sole source of power 

 for the work ; it must also have served as the source of 

 heat production, for the normal temperature of the animal 

 was maintained. 



5. Summary of considerations on the supply of power for 

 work. . These and other experiments show (1) that the 

 animal body can get its energy for mechanical work and for 

 the production of heat from protein, or from carbohydrate, 

 or from fat; (2) that when the animal is on an abundant 

 mixed diet, even vigorous muscular work does not increase 

 the oxidation of protein, 1 but does enormously increase that 



1 Under the abnormal conditions of excessive muscular work (for ex- 

 ample, six-day walking matches or bicycle races) the protein oxidation is 

 often increased. 



