830 METABOLISM. 



proteins) decomposed in the body of rabbits in starvation. According 

 to these investigations, the physiological heat of combustion in calories 

 for each gram of substance is as follows : 



i gram, of the dry substance. Calories. 



Protein from meat 



Muscle 4.0 



Protein in starvation '. 3 



Fat (average for various fats) 9.3 



Carbohydrates (calculated average) 4.1 



The physiological combustion value of the various foods belonging to 

 the same group is not quite the same. It is, for instance, 3.97 calories 

 for a vegetable protein, conglutin, and 4.42 calories for an animal protein 

 body, syntonin. According to RUBNER the normal heat value per 1 

 gram of animal protein may be considered as 4.23 calories, and of vegetable 

 protein as 3.96 calories. When a person on a mixed diet takes about 

 60 per cent of the proteins from animal foods and about 40 per cent from 

 vegetable foods, the value of 1 gram of the protein of the food is equivalent 

 to about 4.1 calories. The physiological value of each of the three chief 

 groups of organic foods, by their decomposition in the body, is in round 

 numbers as follows: 



Calories. 



1 gram protein 4.1 



1 gram fat 9.3 



1 gram carbohydrate 4.1 



1 gram alcohol 7.1 



These figures are generally used in the calculation of the energy con- 

 tent of various foodstuffs and diets. 



The extent of gas exchange and the so-called respiratory quotient 

 is, besides the extent of nitrogen elimination, of the greatest importance 

 in the calculation of the extent of energy metabolism and the division 

 of the energy between the protein, fat and carbohydrate. 



On comparing the inspired and the expired air we learn, on measuring 

 them when dry and at the same temperature and pressure, that the volume 

 of the expired air is less than that of the inspired air. This depends 

 upon the fact that not all of the oxygen appears again in the expired 

 air as carbon dioxide, because it is not only used in the oxidation of car- 

 bon, but also in part in the formation of water, sulphuric acid, and other 

 bodies. The volume of expired carbon dioxide is regularly less than the 



CO 



volume of the inspired oxygen, and the relation ^, which is called the 



respiratory quotient, is generally less than 1. 



The magnitude of the respiratory quotient is dependent upon the kind 

 of substances destroyed in the body. In the combustion of pure carbon 

 one volume of oxygen yields one volume of carbon dioxide, and the 

 quotient is therefore equal to 1. The same is true in the burning of 



