NUTKITION 221 



determined solely by appetite and the feeding customs of 

 their home or community, usually consume each day food 

 of a fuel value of 20 calories per pound of body weight. 

 It is exceptional to find less than 16 calories or more than 

 24 so long as only moderate amounts of exercise are taken; 

 and many students of this subject have assumed that one 

 requirement of diet is that the daily intake of food should 

 have approximately this fuel value. 



This view has, however, been seriously questioned by 

 careful and competent observers, and their work seems to 

 show that a fuel value of 13.5 calories per pound of body 

 weight more nearly represents the actual needs of the body. 

 In other words, the usual diet, with its three hearty meals a day, 

 has a fuel value one and one half times as great as the minimum 

 requirement of the body. Whether the excess is or is not 

 harmful to the body will be discussed later (see p. 239). 



The chief factor which influences the amount of this 

 minimum fuel value is the amount of muscular exercise 

 taken. Men at hard labor require from 20 to 25 calories per 

 pound of body weight, or even more; on the other hand, 

 during the marked muscular relaxation of sleep the require- 

 ment is reduced to from 10 to 11 calories. Exposure to cold, 

 when not counteracted by warm clothing, similarly increases 

 the fuel requirement. 



If, then, we generally eat more food than the fuel needs 

 of the body require, what becomes of the excess ? This 

 question can be answered only incompletely in the present 

 state of our knowledge. A portion of the food eaten leaves 

 the body undigested in the feces ; and the more abundant 

 the diet, the greater is the amount lost in this way. Part 

 of the food also is destroyed in the alimentary canal, es- 

 pecially in the large intestine, by microbic action, and this 

 similarly increases with the diet. This microbic food destruc- 

 tion involves the liberation of heat within the body but does 

 not yield power for work, the excess of heat being dissipated 



