THE SIGNIFICANCE OF THE FOODSTUFFS 691 



products of hydrolysis are set free in small quantities at a time and can 

 be therefore absorbed and disposed of in proportion as they are set free. 

 On the other hand, a sudden flooding of the alimentary canal with a large 

 quantity of the products of digestion introduces an abnormal factor which 

 must tend to produce wastage of nitrogen in the body and to disturb the 

 normal chain of processes involved in the regular course of digestion. 



THE BIOLOGICAL VALUE OF DIFFERENT FORMS OF PROTEIN 



In consequence of the wide variation in the composition of the proteins 

 contained in the different tissues of the body, it is necessary that the food 

 shall include all the amino -acids in proper quantities and proportions. 

 Hence it follows that all proteins are not equivalent as regards their capacity 

 for replacing tissue waste or serving for the growth of the animal body. 

 Those, in which nearly all the amino -acids are represented and which contain 

 these in proportions approximating the average of those found in the chief 

 animal proteins, will be more valuable than those in which one or more of 

 the chief amino-acids are absent, or in which there is a large preponderance 

 of one or more of the amino-acids which are required only in small pro- 

 portions for building up the tissues of the animal body. As an example 

 of the first type we may instance collagen and gelatin, and zein the chief 

 protein of maize. Collagen and gelatin on digestion yield the ordinary 

 amino-acids of the fatty series and also a certain amount of phenyl alanine 

 and proline. The oxyphenyl group which occurs in other food proteins 

 in the form of tyrosine, as well as the indol-containing group tryptophane, 

 are absent, so that gelatin if pure gives neither Millon's test nor the Hopkins- 

 Adamkiewicz' test. Hence gelatin cannot entirely replace the proteins of 

 the food. As its capacity for yielding energy to the body equals that of 

 any other protein, and since it supplies also a number of the amino-acids, 

 it can replace to a large extent, but not entirely, other forms of protein 

 which are not deficient in the above-mentioned amino-acids. Physiologists 

 have indeed succeeded in maintaining animals for a short time in a state 

 of nitrogenous equilibrium on a diet containing as its sole nitrogenous con- 

 stituent a mixture of gelatin with tyrosine and tryptophane. In the 

 same way zein yielding no tryptophane nor tyrosine is incapable by itself 

 of supporting life, though this can be accomplished if lysine, tyrosine and 

 tryptophane are administered at the same time. 



As an example of the second type we may instance the protein of wheat 

 flour. A reference to the Table on p. 89 shows that, whereas caseinogen, 

 the protein of milk, contains only 15 per cent, of glutamic acid and serum 

 albumin only 7 per cent., gliadin, the chief protein of flour, contains as much 

 as 36 per cent. If wheat flour is the main or sole source of protein to the 

 body it is evident that a large amount of glutamic acid will be in excess 

 of the amount required to replace tissue waste. These differences in chemical 

 composition between the proteins are of significance when the protein of 

 the food is reduced to the minimal amount required to replace tissue waste, 

 and under such circumstances marked differences are observed between the 



