METABOLISM 275 



(p. 366), so that a greater proportion of the energy of the 

 food is rendered available for metabolism. The stimulus 

 due to taking food is also, of course, decreased as the amount 

 is decreased (p. 272). Hence life is maintained more 

 economically in an under-fed than in a well-fed animal. 



Short periods of under- feeding cause no damage beyond 

 some loss of weight. From the evidence in man it is most 

 probable, however, that in prolonged periods of under- 

 nutrition the power of resisting disease is diminished. 



B. EXCHANGE OF MATERIAL. 



Not only is material necessary as a source of energy, it 

 is also required for the formation of new tissue in growth, 

 and for repair in the full-grown animal. 



In protoplasmic activity (p. 21) there is a continuous 

 breaking down of complex substances to simpler bodies. 

 Some of these products of katabolism can be re-used to 

 build up the living protoplasm, others, however, are excreted 

 as waste. In some cases, too, essential materials are carried 

 off from the body in combination with excretory products 

 (p. 280). New materials must therefore be supplied to 

 make good the waste to ensure that the destructive phase of 

 metabolism shall be balanced by the constructive. 



In addition, certain accessory factors (p. 281) are required 

 to secure the harmonious working of the metabolic processes 

 which is necessary to health. 



Metabolism therefore involves an exchange of material as 

 well as an exchange of energy. 



The substances that take part in metabolism are : — 



(1) Water. 



(2) Amino-acids. 



(3) Salts. 



(4) Accessory factors. 



Water is the chief constituent ; since it is daily given off 

 in large quantities by the kidneys, lungs, skin, and bowels, 

 it must be supplied in sufficient amounts, or the chemical 

 change cannot go on, and death supervenes. 



Amino-acids. — Nitrogenous material is lost to the body 

 in the catabolism of the protein in the tissues and in the 



