FOODS 127 



is determined by placing the subject in a respiration chamber provided with 

 appliances containing water, by means of which the heat can be absorbed 

 and measured. (See chapter on Animal Heat.) The unit of heat measure- 

 ment is the Calorie, which is denned as the amount of heat necessary to raise 

 the temperature of one kilogram of water iC. If therefore the volume 

 of the water employed in the experiment expressed in kilograms be multi- 

 plied by the number of degrees of temperature through which it has been 

 raised, the number of Calories will be known. The average number of 

 Calories dissipated by a human being in various ways, e.g., radiation, vapori- 

 zation of water from lungs and skin, warming of foods, air, etc., has been 

 estimated at from 2500 to 3000 each day. The question then to be deter- 

 mined is, whether any given diet scale contains this amount of energy and 

 whether it can be liberated as heat on oxidation in the body. 



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 con- 

 sists 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 calorimeters 

 and different food principles of the same group vary, though within certain 

 limits: e.g., i gram of casein yields 5.867 kilogram Calories; 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 calorimetric 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 exception of the protein, which 

 is 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 protein. According to Rubner, 

 i .gram of urea will yield 2.523 kilogram Calories. As the urea which results 

 from the oxidation of i gram of protein is about J of a gram, the amount of 

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

 0.841 Calories. It has also been shown that some of the ingested protein 

 escapes in the feces, the heat value of which must also be determined and 

 deducted. This having been done, the physiologic heat value becomes 

 4.124 Calories. 



The following estimates give approximately the number of kilogram 

 Calories produced when the food is burned to carbon dioxid, water, and 

 urea in the body : 



i gram of protein yields. . . 4-124 Calories. 



i gram of fat yields 9-353 Calories. 



i gram of carbohydrate yields : : 4-* Calories. 



