REASONS FOR DIFFERENT DIETS 



129 



tropical countries and in hot weather a considerable amount of 

 fresh fruit should be used in the dietary. 



The Relation of Body Size and Age to Diet. — A large person, of 

 course, needs more food than a small one. Why? Age is a factor 

 in determining not only the kind but also the amount of food to 

 be used. Young children require a large proportion of protein in 

 their diet in order to grow. They are also more active than older 

 persons and so use a large amount of food as fuel in proportion to 

 their weight. The body constantly increases in weight until 

 young manhood or womanhood, and then its weight remains 

 nearty stationary, varying with health or illness. It is evident that 

 food in adults simply repairs the waste of cells and releases energy. 

 Elderly people need much less protein than do younger persons. 



The Relation of Sex to Diet. — As a rule, boys need more food 

 than girls, and men than women, 

 the more active muscular life of 



This seems to be due, first, to 



the man, and, secondly, to a 

 layer of fatty tissue directly 

 under the skin of the woman, 

 which acts as an insulating 

 layer against loss of heat 

 from the body. Larger bodies, 

 because of greater surface, give 

 off more heat than smaller ones. 

 Men are usually larger than are 

 women, — another reason why 

 they require more food. 



The Relation of Digestibility 

 to Diet. — Animal foods in gen- 

 eral may be said to be more 

 completely digested within the 

 body than plant foods. This is 

 largely due to the fact that plant cells have woody walls that the 

 digestive juices cannot dissolve. Heat causes the starch grains to 

 swell and break these woody walls, and that is the reason for the 

 thorough cooking of vegetable foods. Cereals and legumes are less 

 digestible foods than are milk and eggs. Plant proteins in general 

 do not have as many useful amino-acids as do animal proteins. 



Siarch^rd/h 



Cellivall 



Protoplasm 



In the potato, the starch grains are 

 formed inside of the cell. 



