140 METABOLISM 



by means of a calorimeter, and the number of calories of heat. 

 obtained is known as the combustion equivalent. This, in round 

 numbers, for proteins, has been found to be 4100 calories; 

 for fats, 9300 calories; and for carbohydrates, 4100 calories. 

 These foodstuffs, so far as their potential energy is concerned, 

 are interchangeable, so that if carbohydrates are to take the 

 place of fats, they must be furnished in the ration of 9300 

 to 4100 or as 2.2 to 1. In other words, it takes more than 

 twice as much carbohydrate material to render the same energy 

 as any given amount of fat. This ration of 1 to 2.2 is known 

 as the isodynamic equivalent. 



The energy produced by the body in twenty-four hours may 

 be measured as heat in two ways : 



1. It may be measured directly by placing the animal in a 

 calorimeter. 



2. It may be obtained by feeding on a given diet, determining 

 from the excreta the amount of food destroyed, and multiply- 

 ing by the proper combustion equivalent. These methods 

 are known respectively as direct and indirect calorimetry. The 

 nutritional value of foodstuffs cannot be estimated from their 

 contained energy alone, and it is necessary to follow the changes 

 they undergo in their metabolism as far as it is possible. 



Metabolism of Proteins. The decomposition of the pro- 

 tein molecule during digestion may be said to have a number 

 of purposes: (1) It facilitates the absorption of this foodstuff; 

 (2) it is, very probably, desirable that such digestion products 

 as proteoses and peptones do not appear in the blood, for 

 if injected into the circulation they cause profound disturb- 

 ances and are rapidly excreted by the kidneys; (3) food proteins 

 cannot be used directly in the formation and repair of proto- 

 plasm, since the tissue proteins differ from the food proteins, 

 as well as from each other, in the amount and kinds of amino- 

 acids entering into the structure of their molecules; (4) under 

 ordinary conditions meals contain a surplus of nitrogen which 

 must be gotten rid of because unnecessary to the needs of the 

 tissues. This is done by the conversion of superfluous amino- 

 acids and poisonous ammonia compounds into innocuous 

 urea. 



It follows, ' therefore, that there are distinguishable two 

 paths of protein metabolism. The metabolism that takes 



