64 FOOD 



and needing repair, are constantly requiring replacement by 

 new cells, and, in the case of the child, are continually in- 

 creasing in number. The repair of an ordinary machine, 

 an engine, for example, is made at the expense of money, 

 but the repair and replacement of our human cell machinery 

 are accomplished at the expense of food. More than one 

 third of all the food we eat goes to maintain the body cells, 

 and to keep them in good order. It is for this reason that 

 we consume a large quantity of food. If all the food we eat 

 were utilized for energy, the housewife could cook less, and 

 the housefather could save money on grocer's and butcher's 

 bills. If you put a ton of coal in an engine, its available 

 energy is used to run the engine, but if the engine were like 

 the human body, one third of the ton would be used up by 

 the engine in keeping walls, shafts, wheels, belts, etc., in 

 order, and only two thirds would go towards running the 

 engine. When an engine is not working, fuel is not con- 

 sumed, but the body requires food for mere existence, regard- 

 less of whether it does active work or not. When we work, 

 the cells break down more quickly, and the repair is greater 

 than when we are at rest, and hence there is need of a larger 

 amount of food ; but whether we work or not, food is necessary. 



60. The Different Foods. The body is very exacting in its 

 demands, requiring certain definite foods for the formation 

 and maintenance of its cells, and other foods, equally definite, 

 but of different character, for heat; our diet therefore must 

 contain foods of high fuel value, and likewise foods of cell- 

 forming power. 



Although the foods which we eat are of widely different 

 character, such as fruits, vegetables, cereals, oils, meats, eggs, 

 milk, cheese, etc., they can be put into three great classes : 

 the carbohydrates, the fats, and the proteids. 



61. The Carbohydrates. Corn, wheat, rye, in fact all 



