Karel's Nutrition Blog

Phenylalanine

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A relationship similar to that presented for methionine and cystine exists for phenylalanine and tyrosine, two important aromatic amino acids. Tyrosine is considered dispensable because it can be synthesized by the fish from the indispensable amino acid phenylalanine. If tyrosine is included in the diet, it reduces the amount of phenylalanine needed in the diet. Thus, fish have a total aromatic amino acid requirement.

The phenylalanine or total aromatic amino acid requirement values for fish are presented in Table 23.9. Most requirement values fall within the range of 50-60 g kg-1 of dietary protein except for the lower value for rainbow trout and the higher value for common carp.

Since the fish has a metabolic need for both phenylalanine and tyrosine, and only a certain portion of the phenylalanine can be converted into tyrosine and still meet the animal's need for phenylalanine, it is important to determine how much of the total aromatic amino acid requirement can be provided by dietary tyrosine. Growth studies indicate that tyrosine can replace or spare about 60% of the phenylalanine requirement in common carp (Nose, 1979), 50% in channel catfish (Robinson et al., 1980a), 48% in rainbow

trout (Kim, 1993) and 46% for milkfish (Borlongan and Goloso, 1993).