An Exploratory Study of the Role of Dietary Proteins in the Regulation of Intestinal Glucose Absorption

Abstract

Several studies have demonstrated that high protein diets improve glucose homeostasis. Nevertheless, the mechanisms underlying this effect remain elusive. This exploratory study aims to screen and compare the acute effects of dietary proteins from different sources on intestinal glucose absorption. Six dietary proteins from various sources were thus selected and digested thanks to the INFOGEST static gastrointestinal digestion protocol. The digested proteins were able to decrease intestinal glucose absorption in vitro and ex vivo. Moreover, acute ingestion of casein and fish gelatin led to improved glucose tolerance in Wistar rats without significant effect on insulin secretion. In parallel, GLUT2 mRNA expression in enterocytes was decreased following short-term incubation with some of the digested proteins. These results strengthen the evidence that digested protein-derived peptides and amino acids are key regulators of glucose homeostasis and highlight their role in intestinal glucose absorption.

Keywords: GLUT2; SGLT1; digested proteins; glucose tolerance; intestinal glucose absorption.

Figure 1

Some digested dietary proteins decrease intestinal glucose absorption in vitro and ex vivo. (A) After 1-h apical pre-incubation with 5 mg/ml digested proteins, Caco-2/TC7 cells differentiated on transwells were apically exposed for 40 min to the same digested proteins supplemented with ¹⁴C-α-methyl-D-glucopyranoside (AMG). Glucose uptake was then quantified by measuring radioactivity in the cellular compartment. Results are expressed as % of control (blk SGID) – mean ± SD (**, p < 0.01; ***, p < 0.001; and ****, p < 0.0001 compared to control – one-way ANOVA followed by Dunnett's test). (B) Rat jejunal sacs were filled with 31.25 mg/mL digested proteins (hemoglobin, ovalbumin, casein, pea proteins, fish gelatin, and gluten) and ³H-glucose. Glucose transport was quantified by measuring radioactivity in the incubation medium for 20 min and AUC was then calculated. Results are expressed as % of control (blk SGID) – mean ± SD (**, p < 0.01 compared to control – one-way ANOVA followed by Dunnett's test).

Figure 2

Acute ingestion of casein and fish gelatin improves glucose tolerance in rats. Overnight fasted rats were first given either water or protein (hemoglobin, ovalbumin, casein, pea proteins, or fish gelatin) at a dose of 1 g/kg bodyweight by oral gavage (n = 8); 30 min later, glucose was orally administered at a dose of 2.5 g/kg bodyweight to start OGTT. (A) Mean plasma glucose increase during OGTT expressed in mg/dL (*, p < 0.05; **, p < 0.01; and ****, p < 0.0001 compared to control (water) – two-way ANOVA followed by Dunnett's test) and (B) the corresponding iAUC values expressed as % of control (water) – mean ± SD (**, p < 0.01 compared to control – one-way ANOVA followed by Dunnett's test). (C) Mean plasma insulin increase during OGTT expressed in ng/mL (*, p < 0.05 and **, p < 0.01 compared to control (water) – two-way ANOVA followed by Dunnett's test) and (D) the corresponding iAUC values expressed as % of control (water) – mean ± SD (*, p < 0.05 compared to control – one-way ANOVA followed by Dunnett's test).

Figure 3

Some digested dietary proteins decrease GLUT2 mRNA expression in vitro. (A,B) SGLT1 (A) and GLUT2 (B) mRNA relative levels normalized to TBP in Caco-2/TC7 cells incubated for 4 h with 5 mg/mL digested proteins (hemoglobin, ovalbumin, casein, pea proteins, fish gelatin, and gluten). Control condition (Blk SGID) is set at 1. Mean ± SD (***, p < 0.001 and ****, p < 0.0001 compared to control – one-way ANOVA followed by Dunnett's test). (C,D) SGLT1 (C) and GLUT2 (D) mRNA relative levels normalized to HPRT1 in Caco-2/HT29-MTX co-culture incubated for 4 h with 5 mg/ml digested proteins (hemoglobin, ovalbumin, casein, pea proteins, fish gelatin, and gluten). Control condition (Blk SGID) is set at 1. Mean ± SD (**, p < 0.01 and ***, p < 0.001 compared to control – one-way ANOVA followed by Dunnett's test). (E,F) SGLT1 (E) and GLUT2 (F) mRNA relative levels normalized to HPRT1 in rat jejunal scraped mucosa, 30 min after gavage with 1 g/kg of bodyweight of dietary proteins (hemoglobin, ovalbumin, casein, pea proteins, and fish gelatin). Control condition (water) is set at 1. Mean ± SD (*, p < 0.05 compared to control – one-way ANOVA followed by Dunnett's test).

Figure 4

Proposed mechanism of digested protein effects on intestinal glucose absorption. Once digested, dietary proteins decrease intestinal glucose absorption and GLUT2 mRNA expression in enterocytes.