30 METABOLISM 



Food, then, is not only used to create energy, heat, 

 and activity, but it must replace the loss of tissue in 

 the body which is continually occurring during the 

 performance of the processes of life, from birth to old 

 age. When this waste is not replenished by new tissue, 

 the body cannot properly carry on its functions. 



Every effort of the human body requires a certain 

 amount of energy or force to bring about its perform- 

 ance. Thus a man in his daily work lifts a certain 

 weight; the body cells must reproduce and furnish a 

 proportionate amount of heat and activity necessary 

 to accomplish this effort. This he obtains from the 

 energy developed from a definite amount of food 

 eaten each day. 



Food is held together by a force called potential 

 energy. When broken up into its more absorbable 

 substances by the process of digestion, etc., so that 

 the cells of the tissues can make use of it, to develop 

 energy it is said to have kinetic energy. 



It is essential to determine the heat value of various 

 foods, in other words, to find out how much heat and 

 energy will be derived from the different foods after 

 their ingestion and digestion, etc. This may be deter- 

 mined experimentally by the use of an instrument 

 known as a bomb calorimeter, the result being expressed 

 in calories. A calorie is the amount of heat that is 

 necessary to raise the temperature of 1 kilogram of 

 water 1 C. (It is nearly equal to the amount required 

 to raise one pound of water 4 Fahrenheit.) This 

 expressed in mechanical force, means that a calorie 

 would raise a ton about 1.54 feet, or that it is equal 

 to 1.54 foot-tons. 



The number of calories required to furnish heat and 

 energy sufficient to accomplish a certain amount of 

 activity varies, depending upon the age, sex, amount 

 of work, mental and physical, and climatic conditions. 

 It is essential to know how many calories are required 

 to perform a certain amount of work from the taking 



