METABOLISM 13 



teins, carbohydrates, and fats are alone of importance. 

 It is true that small amounts of energy are contained in 

 other ingredients of the diet, such as gelatin, but for 

 practical purposes only the three first named need be 

 considered. The energy yielded by a unit (1 gramme) 

 of each of these constituents in the body, expressed in 

 Calories, is as follows : 



Protein ... ... ... 4*1 Calories. 



Carbohydrate -.. ... 4*1 



Fat ... ... ... 9-3 



Now, the total amount of energy expended by a man 

 of average weight doing a moderate amount of muscular 

 work is something between 2,500 and 3,000 Calories, 

 or the amount which would be produced by the con- 

 sumption of 1 pound of good coal ; and it follows that, if 

 equilibrium is to be maintained, the total intake in the 

 form of food must equal this sum, and if it does not do 

 so chronic malnutrition and inanition result. 



What proportion of the total energy required should 

 be supplied by each food constituent is an important 

 question in practical dietetics. 



We have already seen that protein should be regarded 

 as only accidentally a source of energy, its chief use 

 being to replace the loss of nitrogenous material in the 

 tissues, and that by far the largest proportion of the 

 body's energy must be derived from carbohydrates and 

 fats. 



Amount of Carbohydrate and Fat required. As 

 to the proportion of the total intake of energy which 

 should be supplied by carbohydrates and fats respec- 



