BIOLOGY IN HUMAN AFFAIRS 



proteins. The farm pig fed only wheat proteins can trans- 

 form about 23 per cent of them into pig protein, whereas 

 it can transform about 65 per cent of the proteins of milk 

 into pig protein. Since proteins differ so widely in their 

 composition, it is frequently found that combinations of 

 two or more proteins from different sources have a higher 

 biological value than either one fed singly. This is because 

 they are not equally deficient in any one amino-acid. 

 Feeding tests have shown that the proteins of the different 

 cereal grains tend to be deficient in the same digestion 

 products, and, accordingly, the combination of wheat 

 and corn, of wheat and oats, or of corn and oats proteins 

 does not enhance the value of the mixture. The protein 

 mixture is not enhanced by combining pea or bean proteins 

 with any cereal grains or by combining pea and bean 

 proteins alone. On the other hand, the proteins of eggs, 

 meat, and milk are especially rich in those digestion 

 products which are furnished in but small amounts by 

 most vegetable proteins. Hence, combinations of these 

 proteins of animal origin with cereal or other vegetable pro- 

 teins make mixtures of high biological value. The proteins 

 of the leaves of plants are so constituted as to enhance 

 markedly the proteins of cereals, peas, beans, and other 

 seeds of plants. 



There has long been much speculation concerning the 

 optimum content of protein in the dietary. The vegetarian 

 generally eats a relatively low protein diet; the meat eater 

 a high protein diet. Experiments have shown that even a 

 single large dose of histidine, tyrosine, or cystine — three 

 digestion products common to most proteins — when in- 

 jected into the blood of a dog will seriously injure the 

 kidneys. Experiments on rats have shown that high 

 protein dietaries, continued over a considerable period, 

 tend to produce nephritis. Hinhede states that the oc- 

 currence of nephritis parallels the protein consumption in 



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