Contributions of fat and protein to the incretin effect of a mixed meal

Abstract

Background: The relative contributions of fat and protein to the incretin effect are still largely unknown.

Objective: This study assessed the incretin effects elicited by a mixed meal, and by its fat and protein components alone, with the use of a hyperglycemic clamp combined with oral nutrients.

Design: Eight healthy volunteers were studied over 6 h after ingestion of a sandwich containing 1) dried meat, butter, and white bread; 2) dried meat alone; 3) butter alone; or 4) no meal (fasting control). Meals were ingested during a hyperglycemic clamp, and the incretin effect was calculated as the increment in plasma insulin after food intake relative to the concentrations observed during the control study.

Results: A significant augmentation of postprandial insulin secretion, independent of plasma glycemia, occurred after ingestion of the mixed nutrients and the lipid component of the mixed meal (203 ± 20.7% and 167.4 ± 22.9% of control, respectively; both P < 0.05), whereas the protein component did not induce a significant incretin effect (129.0 ± 7.9% of control; P = 0.6)

Conclusions: Fat ingestion, in an amount typical of a standard meal, increases insulin secretion during physiologic hyperglycemia and thus contributes to the incretin effect. In contrast, ingestion of protein typical of normal meals does not contribute to the augmentation of postprandial insulin secretion. This trial was registered at clinicaltrials.gov as NCT00869453.

Figures

FIGURE 1.

Mean (±SEM) glycemia (A) and insulinemia (B) over time after meal ingestion (0 min). Two-factor ANOVA was used for comparison between the 4 conditions. Mean glycemia was not different between the 4 conditions (P = 0.3). There was a significant effect of time (P < 0.001) and no significant interaction between meal and time (P = 0.4). The insulin response, expressed as AUC, was significantly different from the “no meal” condition after ingestion of the sandwich meal (P < 0.001) and the butter meal (P < 0.05). Ingestion of dried meat alone did not induce a significant insulin response (P = 0.6). There was a significant effect of meal (P < 0.001) and time (P < 0.001) and a significant interaction between meal and time (P < 0.001) on insulinemia.

FIGURE 2.

Mean (±SEM) incretin effect observed after ingestion of the sandwich, butter, or dried meat meal. The incretin effect is expressed as the percentage of insulin concentrations observed with no meal. ANOVA with Tukey's post hoc test was used for comparison between the 3 conditions. A significant incretin effect was observed after sandwich and butter ingestion. *Significantly different from no meal, P < 0.05.

FIGURE 3.

Mean (±SEM) GIP concentrations over time after meal ingestion (0 min). Two-factor ANOVA was used for comparison between the 4 conditions. There was a significant effect of meal (P < 0.0001) and time (P < 0.0001) and a significant interaction between meal and time (P < 0.0001). GIP, glucose-dependent insulinotropic polypeptide.

FIGURE 4.

Mean (±SEM) GLP-1 concentrations after meal ingestion (0 min). Two-factor ANOVA was used for comparison between the 4 conditions. There was no significant difference between the 4 conditions (P = 0.07), but there was a significant effect of time (P < 0.001) and a significant interaction between meal and time (P = 0.02). GLP-1, glucagon-like peptide 1.

FIGURE 5.

Mean (±SEM) CHOX (A), LOX (B), and EE (C) over time after meal ingestion (0 min). Two-factor ANOVA was used for comparison between the 4 conditions. A: There was a significant effect of meal (P < 0.0001) and time (P < 0.0001) but no significant interaction between meal and time (P = 0.08). B: There was no significant effect of meal (P = 0.08), there was a significant effect of time (P < 0.0001), and there was no significant interaction between meal and time (P = 0.9). C: There was a significant effect of meal (P < 0.0001) and time (P < 0.0001) and a significant interaction between meal and time (P < 0.0001). CHOX, carbohydrate oxidation; EE, energy expenditure; LOX, lipid oxidation.