L-Phenylalanine and L-Tyrosine Requirements of the Growing Rat

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Abstract
Two experiments were conducted to reevaluate the phenylalanine requirement of the growing rat fed an amino acid diet containing zero, intermediate and excessive levels of tyrosine and to study the interrelationship between phenylalanine and tyrosine in meeting the phenylalanine requirement of the growing rat fed an amino acid diet. The effects of varying the levels of phenylalanine and tyrosine in the diet on plasma free phenylalanine and tyrosine levels were also studied. When the diet contained 0, 0.16, 0.32 and 0.48% tyrosine, the calculated phenylalanine requirements were 0.70, 0.52, 0.38 and 0.38% of the diet, respectively. The minimum phenylalanine requirement was 0.38% of the diet. The calculated total phenylalanine plus tyrosine requirement was 0.69% of the diet, and tyrosine could supply 45% of this total. Plasma free tyrosine was a much better indicator of dietary phenylalanine plus tyrosine adequacy than plasma free phenylalanine. Tyrosine remained at a low and constant level in the plasma when phenylalanine plus tyrosine was deficient in the diet, but increased in a stepwise manner as the level of either phenylalanine or tyrosine was increased beyond the level required for maximum weight gain.