220 THE HUMAN MECHANISM 



of carbohydrates and fats. The probable meaning of this is 

 to be sought in the fact that protein decomposition depends 

 primarily not on muscular work but, as we shall see later, 

 on the amount of protein eaten; while the oxidation of fats 

 and carbohydrates depends almost entirely on the demands of 

 the body for energy and is largely independent of the amount 

 of these foods eaten. 



6. The supply of energy for heat production; " heating " 

 foods. In studying the phenomena of heat production in the 

 body we found that when the body needs more heat in order 

 to maintain its normal temperature, this heat is supplied 

 chiefly by greater chemical activity in the muscles (p. 209). 

 The contraction or tone of the muscles increases in response 

 to stimuli from the same motor nerves which stimulate them 

 to activity when they do external mechanical work. Heat 

 production in the body, from the standpoint of nutrition, is 

 therefore, as far as we know, largely a case of increased 

 muscular activity ; and here, as in the case of mechanical 

 work, the energy can be obtained from one kind of food as 

 well as from another. Contrary to popular ideas we have 

 no conclusive evidence that one kind of food supplies heat 

 more readily than another. What is required in cold weather 

 is more food, whether protein or carbohydrate or fat. We 

 shall see that there are good reasons for not unduly increas- 

 ing the protein of the diet under any conditions, and hence 

 in this special case it is probably better to increase the non- 

 nitrogenous foods to a greater extent than the proteins, 

 though not because they are better " heating " foods. 



7. The daily requirement of the body for power and heat. 

 How many calories must be furnished the body to cover its 

 daily needs for work and for the maintenance of its temper- 

 ature ? This question has been studied by several methods, 

 but we must content ourselves with a statement of some of 

 the most important results. Healthy people whose choice 

 of food is not restricted by financial considerations, but is 



