METABOLISM AFTER INGESTION OF FOOD 109 



of less value in the process of digestion, for it may well be supposed that all the 

 digestive products taken together are of more service in metabolism than are 

 single ones eaten alone. 



2. On few physiological questions have opinions changed so much as on the 

 food value of gelatin. It was once supposed that gelatin is the most important 

 food constituent of meat, because it alone could be dissolved. Then the pendu- 

 lum swung to the opposite extreme, and it was claimed that gelatin is of no 

 food value whatever. Continued investigation has shown that both views were 

 equally overdrawn. 



Since gelatin, like proteid, is not completely oxidized in the body, its physio- 

 logical heat value is less than that determined directly by the calorimeter. One 

 gram of ash-free gelatin yields to the body 3.884 Cal. i. e., 21.2 Cal. per 1 g. of N. 



Voit, Oerum, and others have found that in its combustion in the body 

 gelatin spares proteid to a considerable extent, acting in this way much more 

 powerfully than equal quantities of fat or of carbohydrates (see page 120). 

 Gelatin cannot completely replace proteid, partly at least because it lacks the 

 tyrosin and indol groups. But by feeding gelatin the supply of proteid can 

 be reduced considerably without disturbing N-equilibrium. Thus by feeding a 

 quantity of proteid-f ree gelatin sufficient to cover one hundred and one per cent 

 of the daily requirements of energy Krummacher found that the proteid destruc- 

 tion was reduced by about thirty-seven per cent of the amount destroyed in 

 starvation. 



[Murlin has recently shown that when the full calorific requirements of the 

 body are made up with nonnitrogenous foods (of which a large percentage is 

 carbohydrates), nitrogen equilibrium can be maintained in dogs and man, if 

 two-thirds of the starvation requirements for nitrogen are supplied in the form 

 of gelatin and the other one-third in the form of meat. 



Kauffmann also has made a most beautiful experiment on himself. He es- 

 tablished nitrogen equilibrium on a diet containing 42 Cal. per kilogram with 

 casein as the source of proteid nitrogen. He then replaced the casein with a 

 mixture of gelatin and certain amino acids, tyrosin, cystin, and tryptophan, 

 which are lacking in the gelatin. The mixture contained exactly the same 

 quantity of nitrogen as the casein and was distributed as follows : Gelatin, ninety- 

 three per cent; tyrosin, four per cent; cystin, two per cent; and tryptophan, one 

 per cent. Perfect equilibrium was maintained for a period of five days. ED.] 



Gelatin spares fat and carbohydrate as well. Thus a dog fed on 200 g. of 

 gelatin lost only 15 g. proteid and 38 g. fat from his body per day, while on the 

 eighth day of starvation the same dog lost 29 g. proteid and 102 g. of fat. 



Gelatin and glutm-forming substances (which behave just like gelatin, 

 Etzinger and Voit), play but a subordinate part, however, in the normal nutri- 

 tion. They occur in the ordinary articles of diet in relatively small quantities, 

 and to this extent have exactly the same importance as an equal quantity of 

 proteid. When gelatin is fed in larger quantities to an animal, he soon refuses 

 to eat. It must then be given forcibly by hand and soon causes indigestion. 

 [Kauffmann complains of great languor and general indisposition for work dur- 

 ing his gelatin experiment effects which he ascribes to the lack of the extractive 

 substances necessary to give the diet the proper flavor and to stimulate the 

 nervous system. ED.] 



3. Fat is split up in digestion into fatty acids and glycerin (cf. Chapter 

 VII). The former when fed alone have exactly the same effect on metabolism 

 as a corresponding quantity of fat. I. Munk placed a dog in N-equilibrium 

 with 800 g. meat and 70 g. fat. Then instead of the fat he gave the fatty acids 

 derived from 70 g. of fat : the animal continued in N-equilibrium. 



9 



