278 ELEMENTARY PHYSIOLOGY 



embodied in the expression "sugar is fattening." At 

 the same time analysis of the liver shows that a large 

 amount of glycogen is stored up in it, as previously ex- 

 plained (p. 215). 



If fats be given in increasing quantity they also finally 

 lead to a laying on of fat, but by no means so readily as 

 does an increase of carbohydrates. At the same time, 

 no storage of glycogen is observed in the liver. Fats are 

 therefore not as fattening as might at first sight have been 

 expected. 



We have already mentioned gelatin as a food-stuflf 

 which consists of the same four elements as are present 

 in proteins and in somewhat similar proportions. But 

 the nitrogen in gelatin cannot completely replace that 

 of a protein for the repair of nitrogenous waste. An 

 animal fed with gelatin dies ultimately of "nitrogen 

 starvation," though not as soon as it would without this 

 constituent of food. Thus gelatin reduces the waste of 

 the body and, so far, may be a useful article of food. 



Although we have laid stress on the necessity of protein 

 in diet, we should point out the possibility supporting 

 nitrogenous equilibrium on a diet which does not actually 

 contain them. In discussing the digestion and absorption 

 of proteins we saw that they were ultimately absorbed as 

 amino-acids. Of these each protein breaks up into a great 

 many of different characters. If we could collect all these 

 acids and mix them in exactly the correct proportions we 

 could no doubt supply the needs of the body. At present 

 this can be accomplished by artificial digestion of protein 

 and the use of the products as a diet. If, however, such a 

 diet fell short by this amino-acid or that, it would become 

 inadequate. In this fact we find the key to the incom- 

 petence of gelatin to support life. When it is digested 

 there are lacking in this mixture of acids produced, two 

 important bodies which are yielded by protein. 



Some of these bodies necessary ai'e only required in 

 very small quantities. Orientals can live on a diet which 



