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METABOLISM AND NUTRITION 



conclusion, and other observations tend in the same direction. It is evident 

 from fasting experiments that the body offers great resistance to the disso- 

 lution of proteid once it has been organized into living protoplasm (cf. page 

 97) ; and the following experiment by Siven shows that the body can main- 

 tain its status of proteid on a very small amount of proteid in the daily ration. 



The subject was a man, thirty years of age, whose ordinary diet contained 

 about 16 g. N daily (=100 g. proteid). By giving a correspondingly larger 

 quantity of nonnitrogenous food, his proteid was gradually reduced to 6.3 g. N 

 per day. The results are summarized briefly in the following table : 



During this experiment the body not only did not lose proteid, but during 

 the first three series it actually gained 20.16 g. N, and even in the fourth 

 series lost but 0.58 g. That is to say, by proper adjustment of the diet the 

 supply of proteid can be reduced to a very low level, 1 without entailing any 

 loss of the body's own proteid. 



Recently several authors, notably Loewi, have published observations accord- 

 ing to which the final end products of proteolytic digestion not only can replace 

 proteid 2 in the metabolism, but are able to bring about a N-retention in the body. 

 If these observations should be confirmed in their entirety, the fact would be of 

 the greatest significance for our conception of the metabolic processes. For the 

 present we would not venture to express any definite opinion on the subject. 



9. STORAGE OF CARBOHYDRATES IN THE BODY 



In 1848 Cl. Bernard and Barreswill reported that the liver differs from 

 all the other organs in that it contains a large amount of sugar, whatever the 

 character of the food. Some years later Bernard demonstrated that this sugar 

 is produced by the liver from a substance difficultly soluble in water, and in 

 1857 he isolated this mother-substance as glycogen. 



Glycogen is very widely distributed in organic nature and probably occurs 

 in all animals. In the vertebrates it has been found in almost all organs 

 where it has been sought, which must mean that glycogen is of great physio- 

 logical importance in the body. 



The amount of glycogen in the different organs varies considerably. It 

 occurs most abundantly in the liver and the muscles, but in the latter it is 

 to be observed that different muscles in the same animal may have a very 

 different percentage of glycogen. Likewise, corresponding muscles on the two 



1 For discussion of the optimum amount of proteid in the diet see page 142. 



2 Cf. also page 109. 



