114 PHYSIOLOGY FOR DENTAL STUDENTS. 



teins are no doubt the most important ingredients of cells, hut 

 fats and carbohydrates are indispensable also. 



As reserve materials, striking differences exist between the 

 three foodstuffs. Proteins are of little value in this regard for, as 

 we have seen, very little, if any, can become laid down in the 

 tissues when excess is taken as food; on the contrary, all that is 

 not required is thrown out of the body, and when the food sup- 

 ply is cut off, as in starvation, the protein is spared as much as 

 possible (see p. 92). Carbohydrates are very readily depos- 

 ited as a starch-like substance, called glycogen, and this reserve 

 is the first to be called on, not only in starvation, but also when 

 muscular work is performed. It may be considered as the most 

 immediately available material for combustion in the organism, 

 but the limits of its storage are restricted in man to some hun- 

 dreds of grammes, which, as we have seen, soon becomes used 

 up in starvation. Fat is pre-eminently the storage material, and 

 the supply may serve in man to furnish, along with a little pro- 

 tein, enough fuel for several weeks' existence. 



The relative importance of the three foodstuffs is shown in the 

 extent to which each is used in the metabolism during muscular 

 exercise. When there is an abundant store of glycogen, the 

 energy is entirely derived from this source ; when there is little 

 glycogen but much fat, it is fat that is burned, and when neither 

 of these is abundant but much protein is being taken witli the 

 food, or the animal is reduced to living on its own tissues, as in 

 starvation, it is protein. In other words, the type of metabolism 

 occurring during muscular work is the same as that which imme- 

 diately preceded it ; the only change is in the extent of the com- 

 bustion, not in the nature of the fuel employed. 



