GENERAL METHODS HISTORY OF PROTEIN FOOD. 819 



of the connective tissue. Collagen of bones or of connective tissue 

 takes up water when boiled and becomes converted into gelatin. 

 We eat gelatin, therefore, in boiled meats, soups, etc., and, besides, 

 it is frequently employed directly as a food in the form of table 

 gelatin. Collagen has the following percentage composition: C, 

 50.75 per cent.; H, 6.47; N, 17.86; O, 24.32; S, 0.6. It resembles 

 the protein molecule closely ih percentage composition, and it would 

 seem that the tissues might use it as they do protein for the for- 

 mation of new protoplasm. Experiments, however, have demon- 

 strated clearly that this is not the case. Animals fed upon albu- 

 minoids together with fats and carbohydrates do not maintain 

 nitrogen equilibrium. The final result of such a diet would be 

 continued loss of weight and malnutrition and death. Some light 

 is thrown upon the inability of the gelatin to act as a tissue fcrmer 

 from a consideration of the split products formed from it in hydro- 

 lytic cleavage. While it yields a number of the products usually 

 given by the proteins, there are others which are lacking, such as 

 cystein (thioaminopropionic acid), tyrosin (oxyphenylaminopro- 

 pionic acid), serin (oxyaminopropionic acid), and tryptophan (indol- 

 aminopropionic acid). On the hypothesis that proteins during 

 digestion are normally split more or less completely into their 

 constituent parts, and that the characteristic body-protein of the 

 animal is reconstructed synthetically by a new arrangement of these 

 groups, it is apparent that the gelatin, when used without protein, 

 may fail to furnish some of the groupings necessary to such a 

 synthesis. Gelatin is readily digested, gelatoses and gelatin 

 peptones and eventually some split products being formed; these 

 are absorbed and oxidized in the body, with the formation of 

 C0 2 , H 2 O, and urea. Gelatin serves, then, as a source of energy 

 to the body in the same sense as do carbohydrates and fats. When 

 any one of these three substances is used in a diet, the proportion of 

 protein necessary for the maintenance of nitrogen equilibrium may 

 be reduced greatly. Actual experiments have shown that gelatin 

 is more efficacious than either fats or carbohydrates in protecting 

 the protein in the body. The relative value of fats, carbohydrates, 

 and gelatin in protecting protein from destruction in the body is 

 illustrated by an experiment reported by Voit: A dog weighing 

 32 kgms. was fed alternately upon protein and sugar, protein and 

 fat, and protein and gelatin, with the following result: 



NOURISHMENT (GMS.) CALCULATED DESTRUCTION OF 



MEAT. GELATIN. FAT. SUGAR. FLESH IN BODY (GMS.). 



400 200 450 



400 250 439 



400 200 356 



Practically, however, the use of gelatin in diets is restricted by its 

 unpalatableness when employed in large quantities. Whatever 



