Plasma free amino acids in children consuming lupin protein with and without methionine supplementation

Plasma free amino acids were measured in the fasting state and 3 and 4 hours postprandially in children after 9 days consumption of diets in which lupin provided all of 6.4-6.7% protein kcal, either with supplementation of 2% (grams/gram protein) DL-methionine (L & Met, n = 9) or with an isonitrogenous amount of urea (L & U, n = 10). Fasting concentration of total amino acids (TAA) and of essential amino acids (TEAA) and the TEAA:TAA molar ratio did not differ between diets. Fasting Met concentration (15 +/- 4 mumol/liter) and the Met:TEAA ratio (0.021 +/- 0.005) were markedly depressed after 9 days of L & U. Supplementation with Met caused an expected increase of Met concentration (25 +/- 6 mumol/liter) and the Met:TEAA molar ratio (0.039 +/- 0.007) and a profound decrease of Thr concentration (119 +/- 28 to 77 +/- 22 mumol/liter) and Thr:TEAA (0.165 +/- 0.027 to 0.124 +/- 0.028). Postprandially after L ", Met and Met: TEAA did not change from their low fast ing values. After L rMet, Met and Met:TEAA increased significantly relative to fasting values. Threonine concentration increased but the Thr:TEAA ratio decreased significantly (0.124 +/- 0.028 to 0.111 +/- 0.027). These studies confirm Met as the first-limiting amino acid in lupin protein and suggest that Thr becomes limiting when Met is provided in adequate amounts.