900 NUTRITION AND HEAT REGULATION. 



needs of the body, the proteins are, so far as we know, pretty much 

 of the same vahie, but from the standpoint of supplying material for 

 tissue construction they may differ a great deal (see p. 894). Some 

 proteins, when fed together with an adequate supply of non-pro- 

 tein material and water and salts, furnish all the nitrogenous com- 

 pounds necessary for maintenance and growth; others under the 

 same conditions fail to support growth or maintenance, or both. 

 The former group may be spoken of as complete or adequate pro- 

 teins, the latter as incomplete or inadequate proteins. The differ- 

 ence between the two kinds seems to lie in the character of the 

 amino-acids of which they are composed. Some of the amino- 

 acids which the body tissues need for repair or growth may be made 

 in the body from other amino-acids, glycin, for example, but others 

 apparently must be furnished in the protein of the food, and if they 

 are lacking, tissue construction is not possible. 



It has long been known that gelatin is an inadequate protein 

 in this sense. It is digested easily and absorbed and eventu- 

 ally undergoes oxidation in the body with the production of carbon 

 dioxid, water, and urea. The energy liberated by this metabolism 

 is utilized no doubt in the body, and the gelatin constitutes an 

 "energy-food" similar in a general way to the carbohydrates and 

 fats, although its various amino-acids must give it to some extent 

 a special significance. The important point in this connection is 

 that gelatin alone or with carbohydrates or fats does not suffice to 

 maintain nitrogen equilibrium. It does not supply fully the 

 nitrogenous material needed for the repair of tissue. This defi- 

 ciency is explained by the fact that in the composition of the gelatin 

 certain important amino-acids are lacking, tryptophan (indolami- 

 nopropionic acid), tyrosin (oxyphenylaminopropionic acid), and 

 cystein (thioaminopropionic acid) . It is stated that if a dog is fed 

 upon a diet in which the nitrogenous material is represented onh^ 

 by the split products of a gelatin-hydrolysis he will show a minus 

 nitrogen balance, but if the above-named missing amino-acids are 

 added, particularly the tryptophan, he will then maintain his 

 nitrogen equilibrium. 



The history of gelatin as a food is very interesting and, indeed, instructive, 

 since it serves or should serve as a warning against a premature application 

 of the results of scientific investigation. A condensed account of the subject 

 is given by Voit in Hermann's Handbuch der Physiologie, vol. vi., p. 396. 

 It would seem that on account of the high nitrogen content of the gelatin, and 

 the fact that it is soluble, there was a tendency to attribute to it an unusual 

 nutritive value. The fact, too, that the gelatin could be obtained from bones 

 which otherwise were burned or thrown away was important in suggesting 

 a means for the economical feeding of the poor. The matter was inquired 

 into by a committee during the French Revolution and subsequently by a 

 commission of the French Academy, who made favorable reports. The success 

 of d'Arcet, in making gelatin economically by a new process, led the Philan- 

 thropic Society of Paris to request the Academy of Medicine to investigate 



