THE ABSORPTION AND USE OF FOODS 503 



experiment was tried of dividing the protein into six equal parts 

 and administering one part every four hours. When this was 

 done, so that there was a practically continuous though small 

 absorption of protein into the Body, it was found possible to make 

 good completely the tissue break-down with an amount of protein 

 very little in excess of the theoretical requirement. 



The Replacement Value of Different Proteins. Obviously to 

 make good the wear and tear of the tissues with the least possible 

 amount, the proteins ingested must correspond as closely as pos- 

 sible in composition with those of the Body itself. We have 

 already noted (p. 11) that the differences between different pro- 

 teins are in the number or relative amounts of amino acids present. 

 These differences are in some cases very pronounced. In general 

 meat proteins resemble those of man more closely than do pro- 

 teins of vegetable origin. Any protein that contains only a small 

 proportion of some amino -acid that is present in human protein 

 in large proportion must evidently be fed in sufficient amount to 

 satisfy the requirement for that particular amino acid. The other 

 constituents, meanwhile, are in excess and afford a surplus to be 

 used as fuel. It appears that the Body possesses a limited ability 

 to convert some kinds of amino .acids into others, but this is ap- 

 plicable to so few of the many which make up the protein molecule 

 as to have little practical bearing. 



Maintenance Proteins and Growth Proteins. Evidently, from 

 what has been said above, any protein that is completely lacking 

 in some essential amino acid or acids cannot serve to replace worn- 

 out tissues. Such a protein is ordinary table gelatin. This is a 

 protein derived from bone and connective tissue (p. 49). It is 

 deficient in three of the amino acids which are essential to living 

 protein (tryptophan, tyrosin, and cystein). No matter how much 

 gelatin may be included in the diet, if there is not provided also 

 some protein which contains these essential acids there will be a 

 wasting of the tissues. 



Related to this fact is the even more remarkable discovery that 

 there are certain proteins which are fully adequate for the main- 

 tenance of the Body, but will not suffice for the formation of new 

 tissues. Young animals fed upon diets whose protein components 

 are of this character will maintain a constant weight, but will 

 not grow. A good example of such a protein is gliadin, one of the 



