Essential and non-essential amino acids
The twenty amino acids commonly occurring in proteins are given in Fig. 9.1. Together with the different properties (chemically reactive groups), by which the different amino acids can be characterized, Fig. 9.1 also illustrates the non-essential amino acids that can be synthesized by simple transamination, or in some cases more complex reactions, of metabolites from the oxidation products of glucose or, as for arginine, from the urea cycle in mammals.
In piglets, the synthesis rate of arginine, and probably also of proline that include several synthetic steps, may not be sufficiently high to fully satisfy the requirements for these amino acids during the first rapid growth phase (Fuller, 1994). On the other hand, the dietary supply of these two amino acids appear always to be in surplus in relevant practical diets for piglets. Therefore, this possible insufficiency will not be considered as a matter of practical relevance.
Figure 9.1 also illustrates that the two amino acids, cysteine and tyrosine can be synthesized from the essential amino acids, methionine and phenylalanine, respectively. Thus, an undersupply of cysteine and tyrosine can be compensated by an oversupply of methionine and phenylalanine, respectively. On the other hand, cysteine and tyrosine cannot compensate for undersupply of methionine and phenylalanine, respectively. Consequently, a complete composition of ideal protein for pigs includes also the sum of sulphur amino acids (methionine + cysteine) and aromatic amino acids (phenylalanine + tyrosine) together with methionine and phenylalanine, respectively.
In the ideal amino acid profile suggested by
Glucose i +
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