BUILDING MATERIAL 63 



where else. Thus the energy of a sleeping child is poten- 

 tial and becomes changed to various forms of kinetic 

 energy when he wakes. Finally, all energy liberated in 

 the body ultimately manifests itself either in action or 

 heat, and hence we may say that our actions and our 

 body temperature are merely manifestations of energy 

 whose source is the oxidation of the nutrients, carbo- 

 hydrate and fat. Since some part of our body is always 

 in action, and all parts must be kept at a constant temper- 

 ature, it follows that there must always be on hand a 

 sufficient quantity of fuel to supply this energy. It is 

 this fact that makes it so important that our daily diet 

 contain a certain proportion of carbohydrate and fat, 

 and explains why people whose work is largely muscular 

 require more of these nutrients than those whose labors 

 are less active. 



Other substances than fats and carbohydrates may be 

 used as fuel, notably both proteids and albuminoids, as 

 both contain oxidizable carbon. The carbohydrates and 

 fats, however, are the cheapest and easiest sources of this 

 energy. It is interesting to note that the carbohydrates 

 are capable of transformation into fat, and that it is in the 

 latter state that the reserve energy of the body is stored. 

 In starvation it is this stored fat that maintains life. 



Another striking feature that can be deduced from the 

 above statements is that proteid is the only universal food, 

 since it is the only nutrient that combines in itself both 

 tissue building and fuel qualities. 



Value of common foods. — A study of Chart II., shows us 

 that an important factor in determining the value of a 

 food is its composition. A food like meat, for example, 

 owing to its high percentage of proteid, is at once recog- 



