46 HUMAN BIOLOGY 



the loss of weight due to abstaining from food because of 

 illness or other reasons. 



It has been found too that when one is engaged in very 

 active exercise, such as playing tennis or football, the loss 

 of weight is greater than when one remains quiet. How, then, 

 can we account for the loss of weight in all the cases that 

 we have been enumerating? We all know that during violent 

 activity considerable quantities of perspiration are given 

 off from the skin, and this has been proved to be true at all 

 other times, though to a less extent. It has also been demon- 

 strated that many waste materials are given off from the 

 lungs, the organs of digestion, and the kidneys. 



We have now accounted for the constant loss of weight 

 in our bodies, but we have still to ask ourselves how these 

 waste substances are produced in the body. The two 

 commonest wastes of the body are carbon dioxid and water. 

 These are produced by the oxidation of the carbon (P. B., 80) 

 and the hydrogen in the foods. This has also been proved 

 to be true in animals and in the human body. 



49. Definition of a food. — The three most important 

 uses of foods have been suggested in the preceding sections. 

 Hence we may say that a food is any substance that yields 

 material for the repair or growth of the body, or that supplies 

 the fuel used by the body for producing heat, or power to do work. 

 It should be understood, however, that no substance should 

 be regarded as a food if it injures the body while supplying 

 materials for growth, repair, or the production of energy. 



III. The Composition of Foods 



50. To determine the food substances present in milk. — 

 Laboratory demonstration. 



1. Shake a bottle containing milk and cream and pour a 



