288 AN AMERICAN TEXT-BOOK OF PHYSIOLOGY. 



proteid need be given to cover actual tissue-waste. This point will be referred 

 to again in speaking of the value of the albuminoids. 



Luxus Consumption. The fact that normally more proteid is eaten, even 

 in a mixed diet, than is necessary to cover the actual tissue-waste led some of 

 the older physiologists to speak of the excess as unnecessary, a luxus, and the 

 rapid destruction of the excess in the body was described as a "luxus con- 

 sumption." There can be no doubt about the fact that proteid may be, and 

 normally is, eaten in excess of what is necessary to repair tissue-waste, or in 

 excess of what is requisite to maintain nitrogenous equilibrium at a low 

 level. But it is altogether improbable that the excess is really a " luxus." 

 It has been stated, in speaking of nitrogenous equilibrium, that an animal 

 may be kept in this condition upon a certain minimal amount of pro- 

 teid, or upon various larger amounts up to the limit of the power of the 

 alimentary canal to digest and absorb ; but it has also been shown (Munk 1 ) 

 that if an animal is fed upon a diet containing quantities of proteid barely 

 sufficient to maintain N equilibrium, it will after a time show signs of mal- 

 nutrition. It seems to be necessary, as Pfluger pointed out, that the tissues 

 should have a certain excess of proteid to destroy in order that their nutri- 

 tional or metabolic powers may be kept in a condition of normal activity. 

 Hence we find that well-nourished individuals habitually consume more proteid 

 than would theoretically suffice for N equilibrium. For example, the average 

 diet of an adult contains, or should contain, from 100 to 118 grams of protei<J 

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

 may be maintained, for short periods at least, upon 40 grams of proteid a day, 

 provided large amounts of fats or carbohydrates are eaten. It is scarcely neces- 

 sary 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 chemical 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 CO 2 , H 2 O, and 

 urea or some related nitrogenous product. Gelatin serves, then, as a source 

 1 DuBois-Reymond's Archiv filr Physiologic, 1891, p. 338. 



