CHEMISTRY OF DIGESTION AND NUTRITION. 349 



diet of an adult male contains, or should contain, from 100 to 118 grams of 

 proteid per day, but it has been shown that nitrogen and body equilibrium in 

 man may be maintained, for short periods at least, upon 40 or even 20' grama 

 of proteid a day, provided large amounts of i'ats or carbohydrates are eaten. 

 It is scarcely necessary to add that this beneficial excess has a limit, and that 

 too great an excess of proteid food may cause troubles of digestion as well as 

 of general nutrition. 



Nutritive Value of Albuminoids. — The albuminoid most frequently oc- 

 curring 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; O, 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 formation of new protoplasm. 

 Experiments, however, have demonstrated clearly that this is not the case. 

 Animals fed upon albuminoids together with fats and carbohydrates do not 

 maintain N equilibrium; a certain proportion of tissue breaks down, giving 

 an excess of nitrogen in the urine. The final result of such a diet would be 

 continued loss of weight and, finally, malnutrition and death. Gelatin, how- 

 ever, is readily digested, gelatoses and gelatin peptones being formed ; these 

 are absorbed and oxidized in the body, with the formation of C0 2 , H 2 0, 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 X equilibrium may be reduced greatly. I Fpon 

 the theory of circulating proteids, this is explained by saying that these sub- 

 stances are burnt in place of proteid, and that the proportion of this latter 

 material which undergoes the fate of circulating proteid is thereby diminished. 

 Actual experiments have shown that gelatin is more efficacious than either Hits 

 or carbohydrates in protecting the proteid in the body, and it has been sug- 

 gested, therefore, that it may take the place, partly or completely, of the circu- 

 lating proteid, according to the amount icd. If this suggestion is true, we 

 may say that gelatin has a nutritive value the same as that of the proteids, 

 except that it cannot be constructed into living proteid. The relative value 

 of fats, carbohydrates, and gelatin in protecting proteid from destruction in 

 the body is illustrated by the following experiment, reported by Voit. A dog- 

 weighing .32 kilograms was led alternately upon proteid ami sugar, proteid 

 and fat, and proteid and gelatin : 



■Sivt'n: Skandinavischu Arehiv fur Physiologic 1899, Bd. 10, S. 91. 



