464 



METABOLISM, NUTRITION, AND DIET 



The Energy Value of Fats in Metabolism. Fats, in comparison 

 with other food principles, are of especial value as sources of energy. They 

 are completely oxidized in the body to carbon dioxide and water, and yield, 

 therefore, as much energy to the body as they yield upon oxidation outside 

 the body. The energy equivalent of i gram of fat is 9 . 3 large Calories, 

 more than twice that of starch, which in the body yields only 4.1 Calories 

 per gram, or of protein with 4.1 Calories available yield of energy. 



A study of the elimination of nitrogen and of carbon during fasting shows 

 that the fats contribute to energy formation for many days. This is illus- 

 trated by the following computation by Voit: 



METABOLISM IN A DOG DURING FASTING. (VoiT.) 



The amount of fat metabolized is sharply influenced by the amount and 

 kind of other food. For example, if the amount of fat metabolized per day 

 in fasting is first determined, then a ration of protein given for a few days, 

 followed by a second fasting period, it will be found that the metabolism of 

 body fat is sharply increased in the second period, due to the stimulating 

 influence of the protein. This is demonstrated by the following determi- 

 nation of Rubner: 



The fat of the ordinary daily diet is absorbed into the blood and no doubt 

 contributes directly to oxidation processes. In some of the lower carnivor- 

 ous vertebrates fats are undoubtedly the primary and direct source of the 

 energy of daily activity. Just the steps in this oxidation process cannot at 

 present be given. If the fat of the food is insufficient, then the body store 

 is immediately drawn upon; if in excess, then it is stored in the body. This 



