GENERAL METHODS HISTORY OF PROTEID FOOD. 791 



With regard to the non-nitrogenous portion of the proteid mole- 

 cule the evidence to show that it may be converted to glycogen 

 (sugar) has already been given (p. 733), while the views regarding 

 its conversion to fat will be referred to in the following chapter under 

 the head of Origin of the Body Fat. 



Nutritive Value of Albuminoids. The albuminoid most fre- 

 quently occurring in food is gelatin. It is derived from collagen 

 of the connective tissues. 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; 0, 24.32; S, 0.6. It resembles 

 the proteid molecule closely in percentage composition, and it would 

 seem that the tissues might use it as they do proteid 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. Gelatin, 

 however, is readily digested, gelatoses and gelatin peptones and event- 

 ually some split products being formed; these are absorbed and 

 oxidized in the body, with the formation of CO 2 , H 2 O, and urea or 

 some related nitrogenous product. 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 proteid necessary for the maintenance of nitrogen 

 equilibrium may be reduced greatly. Actual experiments have 

 shown that gelatin is more efficacious than either fats or carbohy- 

 drates in protecting the proteid in the body. The relative value 

 of fats, carbohydrates, and gelatin in protecting proteid from de- 

 struction in the body is illustrated by an experiment reported by 

 Voit: A dog weighing 32 kgms. was fed alternately upon proteid 

 and sugar, proteid and fat, and proteid and gelatin, with the follow- 

 ing 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 may 

 be the physiological cause of this peculiarity, there seems to be no 

 doubt that when used largely in the diet both animals and 

 men soon develop such an aversion to it that it is necessary to dis- 

 continue its use. ^Munk has attempted to, determine how far the 



