Plasma Amino Acid Response Curve and Amino Acid Requirements in Young Men: Valine and Lysine

Abstract

A total of 14 young men were studied to determine the relationships between the concentration of free valine or lysine in blood plasma, and the dietary intake and daily requirement for each of these amino acids. The diet, containing an L-amino acid mixture, which was patterned as in egg protein, supplied nitrogen equivalent to about 0.5 g egg protein (N × 6.25)/kilogram body weight/day. In three separate experiments with a total of 10 subjects, dietary valine intake varied between 4 and 20 mg/kilogram body weight, and in another with four subjects, lysine intake ranged from 1 to 24 mg/kilogram body weight. The free valine level in fasting plasma was relatively constant at valine intakes below 14 to 16 mg/kilogram body weight/day and increased above this intake level. Crude nitrogen balances in these subjects approached equilibrium or were positive at valine intakes of about 12 to 14 mg/kg. Considering integumental and sweat nitrogen losses, it appears that fasting plasma valine levels begin to rise at an intake of 14 to 16 mg/kg, which is presumably the minimum requirement for these subjects. This technique should be useful to determine valine requirements under various conditions. In the lysine study three subjects remained in nitrogen balance throughout. Mean fasting plasma lysine levels were relatively constant at and below an intake of 6 mg/kilogram body weight and were 13 and 18% higher at the 7 and 24 mg/kg lysine intakes, respectively. In comparison with the valine findings, fasting plasma lysine levels would not appear as useful a response criterion to assess the human maintenance requirement for this amino acid.