A comparison of the insulinotropic and enterogastric response to ingestion of an equivalent quantity of maltodextran and whey protein

In the absence of carbohydrate, the insulinotropic effect of whey protein ingestion induces a rise in plasma insulin and lowering of blood glucose during the first 30 min after ingestion (Power et al. 2009) ⁽¹⁾. It was hypothesized that the insulinotropic action of whey protein may, in part, be mediated via the secretion of the enterogastric hormones, represented by glucagon-like peptide-1 (GLP-1).

The present paper reports on the post-prandial response of plasma glucose, insulin and GLP-1 to feeding maltodextran and an equivalent quantity of whey protein. With ethical approval and informed consent four young, healthy subjects (n=4, age 25(2.8) y, BMI 21.8(1.1) kg/m²) undertook a randomized trial of two treatments, either a protein (WHEY 0.33 g/kg, 10% w/v water) or maltodextran (MALT; 0.33 g/kg, 10% w/v water), each treatment separated by 5 days. Subjects fasted overnight (10 h) prior to participation. Serial blood samples were withdrawn prior to and post ingestion for a period of 2 h and batch analysed for glucose, insulin and total GLP-1. Data are presented as mean (SEM; n=4). Area under the curve for plasma insulin or total GLP-1 (AUC _0–120) was calculated by trapezoidal integration. Difference in the mean response was analysed by paired Student's t-test.

Figure 1. Mean plasma insulin, glucose and total GLP-1 for maltodextran (MALT) and whey protein concentrate (WHEY) during the post-prandial period. Values are mean (SEM; n=4).

Following ingestion of the MALT, plasma insulin increased rapidly in the first 30 min and reached a maximal concentration (C_max) of 299(48) pM which directly correlated with the change in plasma glucose (Fig. 1). Ingestion of WHEY resulted in a C_max for plasma insulin of 182(23) pM within 30 min and was independent of a change in plasma glucose (Fig. 1). The differing insulin responses for MALT and WHEY were reflected in the AUC, 8506(2059) vs. 6512(1368) pM.2 h (P=0.29). In contrast, ingestion of MALT gave a transient increase in total GLP-1 in the first 30 min and a C_max of 7.4(3) pM but ingestion of WHEY gave a prolonged increase with a C_max of 11.4(2) pM at 75 min. Total GLP-1 AUC was −64(40) vs. 536(102) pM.2 h (P=0.013) for MALT and WHEY, respectively.

The enterogastric response, GLP-1, was greater following ingestion of an equivalent protein (WHEY) versus carbohydrate (MALT) load. This contrasts with previous reports that GLP-1 secretion is glucose dependent ⁽²⁾. There was a shift in the time course of maximal plasma insulin and GLP-1 response and this data does not support the hypothesis that GLP-1 directly mediates the insulinotropic action of protein. The magnitude of the protein mediated insulin response can be augmented by hydrolysis of the protein ⁽³⁾; therefore future studies are warranted to examine the effect of protein hydrolysis and those of peptide components upon GLP-1 secretion.