FUNCTIONS OF THE NUTRIENTS 183 



ENERGY RELATIONS. HEAT REGULATION 



As has been pointed out, the animal body is the field 

 of numerous mechanical activities. What is the rela- 

 tion of the several nutrients to these manifestations of 

 vital energy is an interesting and in some ways an in- 

 tensely practical matter. 



264. Relation of protein to muscular activity. — ^The 

 belief prevailed at one time that muscular contraction 

 caused a wasting of the muscle substance which must be 

 replaced by the protein compounds of the food; in other 

 words, protein alone was believed to sustain the work of 

 the animal body, both internal and external. It would 

 follow from this that the more work is done the more 

 protein is needed. This view is no longer held. The more 

 exact methods of modern research have revealed the 

 fact that an increase of muscular effort, even up to a 

 severe point, increases but little, if any, the nitrogen 

 compounds of the urine, these being the measure of the 

 protein that is destroyed. 



265. Energy chiefly from carbohydrates and fats. — 

 There has come to light a corresponding fact that the 

 consiunption of fuel in the body, other than proteins 

 increases proportionately with the increase of work. 

 This means that as animals are ordinarily fed mechanical 

 work is largely sustained through the combustion of 

 carbohydrates and fats, although a fairly generous 

 amount of protein seems to promote the well-being of 

 a draft animal. 



266. Heat regulation. — ^As no energy is ever lost, 

 into what is the energy converted that is applied to 

 muscular contraction? It is concluded that mupcular 

 energy used by the animal is partly transformed into 



