Postprandial plasma lysine as an indicator of dietary lysine adequacy in infants

Postprandial plasma free amino acids were studied as an index of the level of dietary lysine. Fasting blood was obtained from 11 children who were then fed a meal in which wheat provided 100% of 0.4 g protein and 30% to 50% of 25 kcal/kg body weight. Blood samples were obtained 3 and 4 hours postprandially. Fasting blood samples were obtained from these same children after 9, 18 and 27 (n=6) days of consuming the same diet (2 g protein, 125 kcal/kg/day). Postprandial blood samples were taken from four additional children after consuming a meal containing 25 kcal and 0.75 g protein (0.5 g from wheat, 0.25 g casein)/kg. This meal was expected to provide adequate lysine while still maintaining lysine in lowest concentration among the essential amino acids relative to needs for protein synthesis. After consuming the unsupplemented wheat diet postprandial values showed a significant rise of total amino acids at 3 hours with a return to fasting levels at 4 hours. Total essential amino acids did not change at 3 or 4 hours. Lysine concentration was significantly lower at 3 and 4 hours. The ratio of lysine to total essential amino acids decreased at 3 and, more so, at 4 hours. In 3 of 4 children studied the postprandial decrease of plasma lysine and its molar fraction was not manifest after consuming wheat + casein. The fasting lysine concentration during continued wheat consumption showed no change at 9 days and then a progressive fall at 18 and 27 days. The results with wheat + casein lend support to the concept that a postprandial fall in the concentration of lysine and its molar fraction indicates not merely that lysine is present in lowest concentration but that it is present in inadequate concentration relative to needs for protein synthesis.