FOOD 99 



are converted into sugar by digestive ferments before they 

 can be absorbed ; so that in the herbivora an enormous 

 amount of sugar is constantly passing from the intestines 

 into the blood. In spite of this only mere traces of sugar 

 are found in the blood, and one explanation is that it is 

 rapidly destroyed by oxidation. 



The influence of starch on the metabolism of proteid is 

 very remarkable, it preserves it from destruction, and hence 

 less proteid is required when the diet contains a due pro- 

 portion of carbo-hydrate than when it is deficient. Carbo- 

 hydrates produce a greater economy of proteid than fat 

 does, but it must exist in a definite proportion to proteid in 

 order to have the most economical effect. 



During work it is the fat, but more particularly the 

 carbo-hydrates, which are used up ; with ordinary work the 

 using up of proteid in the body is no greater than during 

 rest. This only holds good provided the proper proportion 

 of carbo-hydrate is present in the diet, if it is insufficient 

 the proteid is drawn upon and the animal loses weight. 



Cellulose. — The fibre found in plants consists of two 

 kinds, the hard or Lignin fibre, and the digestible fibre 

 known as Cellulose. 



Much discussion has occurred respecting the digestion of 

 cellulose, and we will refer again to the subject in dealing 

 with the question of the digestibility of food. It is quite 

 certain that the herbivora can digest it, but whether by a 

 cellulose dissolving enzyme, or by the process of putrefac- 

 tion in the digestive canal is unknown. 



When cellulose is dissolved it is converted into sugar, 

 and passes into the blood as such. The lignin of all plants 

 is indigestible. 



Excepting fat, we may regard all the non-nitrogenous 

 substances which are capable of absorption, viz., starch, 

 sugar or sugary substances, gums, and cellulose, as havmg 

 the same composition as starch, and producing the same 

 effect when absorbed. _ . 



The organic acids, viz., acetic, butyric and propionic, 

 produced during digestion from starch, sugar, and cellulose, 



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