METABOLISM 193 



bon dioxide and water produced. Results of the most extra- 

 ordinary accuracy have been obtained by Atwater, Benedict, 

 Lusk, du Bois, and others, who have worked with apparatus 

 constructed on this general principle. 



One of the most important and far reaching results obtained 

 in this way is that the law of conservation of energy holds for 

 the animal body. The animal body can neither create nor 

 destroy energy. In a series of experiments representing an 

 exchange of over 590,000 calories, the difference between the 

 theoretical calculation and the amount actually measured was 

 only 501 calories, an error of only about 0.1%. Thus an animal 

 works on the same principle as an engine, a candle or an alcohol 

 flame. The heat which it produces comes from the oxidation of 

 organic substances, either those of the food, or those of the body 

 tissues,. 



In calculating the amount of heat which will be produced in 

 the body by a given foodstuff, it must be borne in mind that 

 only the portion of the food which is absorbed will be available 

 for burning in the cells, and also that if the food is not com- 

 pletely burned to carbon dioxide and water, this fact must be 

 taken into account. The carbohydrates ordinarily are com- 

 pletely burned to C0 2 and H 2 0, as are also the fats. The pro- 

 teins, however, are not completely oxidized, for uric acid, urea, 

 and other substances of protein origin are excreted in the urine. 

 The figures usually used for the average energy value of the 

 three foodstuffs are carbohydrates 4.1 large calories per gram, 

 proteins 4.1 and fats 9.3. It will be seen that the fats yield by 

 far the most heat per gram. (It will be recalled that a large 

 calorie is the amount of heat required to raise 1 kilo of water 

 through 1 C. of temperature, usually from 15 to 16 C. or 

 from to 1 C.) 



Accurate and extended studies have been made of the amount 

 of heat produced (and thus the amount of fuel required) by 

 individuals in health and in various diseases, at rest and during 

 exercise, waking and sleeping, during periods of mental ease 

 and of great mental exertion, at different ages from infancy to 



