338 HOW DOES MAN DETERMINE THE VALUE OF FOOD? 



use carbohydrates for this purpose, as they are economical and 

 easily digested. The inhabitants of cold countries get their 

 heat-producing foods largely from fats. In tropical countries 

 and in hot weather a considerable amount of fresh fruit should 

 be used in the diet. 



The relation of size and age to diet. Age is a factor in determin- 

 ing 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 size and weight until 

 young manhood or womanhood, and then its size and weight 

 remain nearly stationary, varying with health or illness. It is 

 evident that adults require food only to repair the waste of cells 

 and to release 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. This seems to be due, first, to 

 the more active muscular life of the man, and, second, 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. Food must be digested 

 in order to be used in the body. Animal foods in general can 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 thus break these woody walls. This is one 

 reason for the thorough cooking of vegetable foods. Cereals 

 and legumes are less digestible foods than milk and eggs. The 

 agreement or disagreement of food with an individual is largely a 

 personal matter. Jack Spratt, for example, cannot eat raw toma- 

 toes without suffering from indigestion, while Mrs. Spratt can 

 digest tomatoes but not strawberries. Each individual should 

 learn early in life the foods that disagree with him and leave such 



