PROTEIN METABOLISM 



479 



variations in the levels fed will also fix the minimum requirement 

 for each essential amino acid. 



Nattirally there are complications. To some extent the results 

 depend upon the physiological state of the animal. Requirements 

 lor the growth of young animals differ from those for the mainte- 

 nance of adults. Also requirements for pregnancy, lactation, recovery 

 from illness, and wound repair will differ from those for normal 

 animals. In fact, there is evidence that certain amino acids may not 

 be essential in the diet in some cases but may be required in others 

 when the synthetic capacity is not sufficient to meet special demands. 

 In addition, there are species differences even among mammals. 

 Finally there are purely experimental problems. A diet containing 

 only the essential amino acids in the amounts minimally required is 

 inadequate for any purpose. Such a diet does not contain enough 

 nitrogen to permit a suitable synthesis of those amino acids the 

 animal can make. Therefore minimum levels of essential amino 

 acids must be met for physiological needs, and minimum total nitro- 

 gen (or protein) levels must also be met by the diet. 



Table 20-1 lists the amino acids essential for the growth of infants 



TABLE 20-1. Essential Amino Acids and the Levels Required Daily 



Human Beings, 



and rats. Chicks require the same compounds, with the addition of 

 glycine according to some reports. A recjuirement for glycine is 

 stuprising in view of the presumably widespread metabolism of 



