FOODS. 141 



THE ENERGY OR HEAT VALUE OF FOOD PRINCIPLES. 



The food consumed not only restores the material metabolized 

 and discharged from the body, but also the energy which has 

 been expended as heat and mechanic motion. The food principles 

 are products of the constructive processes taking place in the vege- 

 table world during the period of growth and activity. At the time 

 of their formation there is an absorption and storing of the sun's 

 energy which then exists in a potential condition. During the metab- 

 olism of the animal body these compounds are reduced through 

 oxidation to relatively simple bodies, such as carbon dioxid, water, 

 urea, etc., with the liberation of their contained energy. All of the 

 energy of the body, whatever its manifestations may be, can be traced 

 to chemic changes going on in the tissues, and more particularly to 

 those changes involved in the oxidation of the food principles. 



The amount of heat or energy which any given food principle will 

 yield can be determined by burning a definite amount (e. g., i gram) 

 to carbon dioxid and water and ascertaining the extent to which the 

 heat thus liberated will raise the temperature of a given amount of 

 water (e. g., i kilogram). The amount of heat may be expressed in 

 gram or kilogram degrees or calories, a gram calorie or kilogram 

 calorie being the amount of heat required to raise the temperature of 

 a gram or a kilogram (1000 grams) of water i C. The apparatus 

 employed for this purpose is termed a calorimeter, and consists 

 essentially of a closed chamber in which the oxidation takes place, 

 surrounded by a water jacket, the rise in temperature of the water 

 indicating the amount of heat produced. 



The results obtained by investigators employing different calorim- 

 eters and different food principles of the same group vary, though 

 within certain limits: e. g., i gram casein yields 5.867 kilogram cal- 

 ories; i gram of lean beef, 5.656 calories; i gram of fat yields 9.353, 

 9.423, 9.686 calories; i gram of carbohydrate, 4.182, 4.479, etc -> 

 calories. These numbers represent the physical heat values of these 

 food principles. 



In the human body as determined by calorimetric methods the 

 oxidation of the food principles yields practically the same amount 

 of heat they yield when oxidized outside the body, with the excep- 

 tion of the proteids, which are oxidized only to the stage of urea. As 

 this compound is capable of further reduction in the calorimeter to 

 carbon dioxid and water with the liberation of heat, the quantity of 

 heat it contains must therefore be deducted from the calorimetric 

 heat value of the proteid. According to Rubner, i gram urea will 

 yield 2.523 kilogram calories. As the urea which results from the 

 oxidation of i gram of proteid is about ^ of a gram, the amount of 

 heat to be deducted from the heat value of the proteid is J of 2.523, or 

 841 calories. It has also been shown that some of the ingested 



