n6 HUMAN PHYSIOLOGY 



The salts of the foodstuffs not only serve to replace the 

 same salts in the body, but also to form other salts present 

 in the organism which are not taken up with the food (e.g. 

 the alkali carbonates). Continued lack of salt in the food 

 (salt-hunger) results in death, even though food be given 

 in sufficient quantity. 



Of the combustible foodstuffs, protcids serve to replace the 

 body-proteids destroyed by the physiological combustion. 

 There is no nitrogenous substance, besides proteid, which can 

 supply the body with material for building up its proteids. 

 Hence proteids are absolutely necessary for building up the 

 tissues. But all proteids are not capable of doing this, for 

 the albuminoids (gelatin) cannot entirely replace the proteids 

 of the body. Otherwise it appears that the various true 

 proteids (simple and combined proteids, proteoses) are of 

 equal value as food for replacing the body proteids. At all 

 events, it is not necessary that the same kinds of proteids 

 found in the body should be found in the food. For exam- 

 ple, the haemoglobin and nucleo-albumins in the body, 

 originate, no doubt, from the union of other proteids with 

 iron or phosphoric acid. The albuminoids of the body are 

 also formed from the true proteids, not from the albuminoids 

 of the food. 



Proteids contain all the elements needed for replacing 

 organic substances in the body; fats and carbohydrates con- 

 tain only a part, viz. carbon, oxygen, and hydrogen. 

 Hence proteids alone must be sufficient to satisfy the demand 

 for combustible food for the body. We can, indeed, feed 

 the carnivorous animals on an exclusive proteid diet. This 

 cannot be done with man and vegetable-eating animals, for 

 the quantity of proteid necessary to support life cannot be 

 digested by them. 



The fats and carbohydrates serve as material for combus- 

 tion which furnishes the body with energy for heat production 

 and for work. In metabolism, gelatin plays the same part. 

 It is often stated that fat serves mainly for heat production, 



