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Phenylalanine Requirements with Different Levels of Tyrosine^1,2,

South Dakota Agricultural Experiment Station, Department of Research in Food and Nutrition, Brookings, South Dakota

The quantitative relationship between phenylalanine and tyrosine was investigated. The subjects were 22 young college men and women. They were fed a semipurified diet containing an amino acid mixture which supplied the essential amino acids in amounts present in 20 g of egg protein plus arginine, histidine, cystine and tyrosine. When nitrogen equilibrium was attained the subjects were shifted to a diet of the same composition except that phenylalanine and tyrosine were omitted. This diet was then supplemented with crystalline tyrosine at 3 levels, zero, 200, and 400 mg, and the amount of phenylalanine required for N equilibrium at each level was determined. Nitrogen balance periods for the different levels of tyrosine and phenylalanine studied were usually 6 days long. Two criteria of adequacy were used, the Leverton "zone of equilibrium" and the zero balance used by Rose as the equilibrium point. With no tyrosine in the diet both criteria indicated a requirement of 600 to 700 mg phenylalanine for the women and 900 to 1000 mg for the men. At 200 mg tyrosine both women and men maintained nitrogen balances that were within the "equilibrium zone" and most of the women were storing nitrogen. With 400 mg tyrosine most of the women stored nitrogen at 300 to 400 mg phenylalanine; the men required 600 to 700 mg to attain the "equilibrium zone" or store nitrogen, or both. Under the conditions of this study 200 mg tyrosine had a replacement value of 35 to 40% for the women and approximately 50% for the men. The replacement value of 400 mg tyrosine was approximately 50% for both women and men.

² Received support under the RRF program NC-49.

Manuscript received 18 November 1963.

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