Effects of Duodenal Infusion of Lauric Acid and L-Tryptophan, Alone and Combined, on Fasting Glucose, Insulin and Glucagon in Healthy Men

Abstract

The fatty acid, lauric acid ('C12'), and the amino acid, tryptophan ('Trp'), when given intraduodenally at loads that individually do not affect energy intake, have recently been shown to stimulate plasma cholecystokinin, suppress ghrelin and reduce energy intake much more markedly when combined. Both fatty acids and amino acids stimulate insulin secretion by distinct mechanisms; fatty acids enhance glucose-stimulated insulin secretion, while amino acids may have a direct effect on pancreatic β cells. Therefore, it is possible that, by combining these nutrients, their effects to lower blood glucose may be enhanced. We have investigated the potential for the combination of C12 and Trp to have additive effects to reduce blood glucose. To address this question, plasma concentrations of glucose, insulin and glucagon were measured in 16 healthy, lean males during duodenal infusions of saline (control), C12 (0.3 kcal/min), Trp (0.1 kcal/min), or C12+Trp (0.4 kcal/min), for 90 min. Both C12 and C12+Trp moderately reduced plasma glucose compared with control (p < 0.05). C12+Trp, but not C12 or Trp, stimulated insulin and increased the insulin-to-glucose ratio (p < 0.05). There was no effect on plasma glucagon. In conclusion, combined intraduodenal administration of C12 and Trp reduced fasting glucose in healthy men, and this decrease was driven primarily by C12. The effects of these nutrients on postprandial blood glucose and elevated fasting blood glucose in type 2 diabetes warrant evaluation.

Keywords: amino acid; blood glucose; fatty acid; glycaemic control; humans.

Figure 1

Plasma glucose (A), insulin (B), and glucagon (C) concentrations during duodenal administration of lauric acid (“C12”; 0.3 kcal/min), L-tryptophan (“Trp”; 0.1 kcal/min), a combination of C12 and Trp (“C12+Trp”; 0.4 kcal/min), or isotonic saline (“control”), for 90 min and, at t = 120 min, after a buffet-meal. Data (AUCs of glucose and hormone profiles) were analysed using one-way repeated measures ANOVAs, followed, if significant, by post hoc paired comparisons, adjusted for multiple comparisons by Bonferroni’s correction. Post-meal (t = 120 min) and pre-meal (t = 90 min) values were compared using paired t-tests. * p < 0.05 vs. control; # p < 0.05 all treatments vs corresponding values at t = 90 min; α p < 0.05 C12+Trp and Trp vs. corresponding values at t = 90 min. Data are means ± SEM, n = 16, except insulin, n = 15.