THE ENERGY REQUIREMENTS OF THE BODY 453 



shown in the amount of heat and mechanical work, is the same as would be 

 developed if the same bodies were completely oxidized outside the body. 

 If one gram of fat be taken into the body and is completely oxidized, result- 

 ing in the production of a definite amount of carbon dioxide and water, it 

 may be supposed to have produced the same amount of heat as it would have 

 produced outside the body. In the case of protein food it is a little different, 

 since it is never completely oxidized within the body, but may be supposed 

 to give rise to a definite amount of urea and other lower nitrogen compounds 

 not completely oxidized bodies. In this case the gram of protein may be 

 considered to liberate the same amount of heat as the protein would outside 

 the body minus the amount which would be obtained from the complete 

 oxidation of the resulting urea, etc. 



The actual amount of heat produced per diem has been experimentally 

 ascertained in the case of man and animals by the aid of an apparatus, the 

 calorimeter. An animal is enclosed in a metal cage completely contained 

 in a second cage containing water. Air is let into and out of the inner box 

 by means of metal tubes so arranged that the inlet tubes maintain a con- 

 stant temperature and the outlet tubes pass through water between the two 

 chambers. The heat given out by the animal warms the water in the outside 

 box, and may be estimated by the rise- of its temperature, the amount of which 

 is known. At the same time the carbon dioxide output is measured. 



The amount of heat evolved by the oxidation of various food stuffs has 

 been carefully measured by numerous observers; the figures calculated by 

 Rubner being perhaps most satisfactory, \vhich are: 



HEAT VALUE TO THE BODY. 



i gram carbohydrate 4.1 calories. 



i gram fat 9.3 calories. 



i gram protein 4.1 calories. 



One gram of dry protein has a total heat value of 5 . 754 (Rubner), hence 

 it is obvious that protein is not completely oxidized by the body. Each 

 gram of protein yields at least one-third of a gram of urea, which has a heat 

 value of 2 . 5 calories per gram. 



Atwater has checked the energy value of the foods actually consumed 

 against the actual liberation of heat and work energy of the human body. 

 He finds a wonderfully close agreement both for periods of rest and for periods 

 of work. Atwater's estimate for the energy needs of man are as follows: 



Man without muscular work 2,700 calories. 



Man with light muscular work 3,000 calories. 



Man with moderate muscular work 3,5 calories. 



Man with severe muscular work 4,500 calories. 



