140 BIOLOGY AND HUMAN LIFE 



118. Food and health. All the chemical changes and activi- 

 ties that are constantly going on in living protoplasm are called 

 metabolism. Some of these processes are constructive, leading 

 to the formation of new protoplasm and tissues. Other changes 

 are destructive, resulting in the breaking down of proteins and 

 other complex substances. The activity of the protoplasm dur- 

 ing the rest or sleep of the body is fairly constant, and is called 

 the basic metabolism. We can measure the metabolism or ac- 

 tivity in an organism by measuring the amount of heat that the 

 body gives off, since all the chemical and mechanical processes 

 end in this form of energy. Even the cold-blooded animals and 

 plants give off heat.^ Now the first function of food is to supply 

 the materials used up in metabolism— proteins and fuels (fats 

 and carbohydrates)— in proportion to the daily needs of the 

 body. These needs vary with the basic metabolism and with 

 the amount of external work done. Two men of the same weight 

 have about the same food requirements so long as they both 

 keep quiet ; but if one is more active than the other when not 

 resting, he needs more food. 



It is possible to determine pretty accurately (i) how much 

 protein is needed for a growing body, (2) how much protein and 

 fuel food is needed for basic metabolism, and (3) how much 

 fuel food is needed for the additional work of the body. 



In addition to the nutrients (see page 102) the body needs cer- 

 tain mineral substances (chiefly calcium, phosphorus, and iron) 

 and supplies of vitamins (see page 102). A deficiency in protein 

 cannot be made up by an excess of fats or carbohydrates, al- 

 though fats and carbohydrates may replace each other as fuels. 

 Vitamins, while needed in but very small quantities, cannot be 

 excluded from the diet without bringing about serious disturb- 

 ances in metabolism. In the same way, a lack of calcium may 

 bring about softening of the bones in the case of a growing child, 



iThe unit of energy used in measuring heat is the calorie. This is the 

 amount of heat that is used up in raising the temperature of a kilogram of 

 water (a little more than a quart) from 0° to 1° C. For very delicate work 

 the "small calorie" is used; this is one thousandth of a "large calorie." 



