Dietary resistant starch upregulates total GLP-1 and PYY in a sustained day-long manner through fermentation in rodents

Abstract

Glucagon-like peptide-1 (GLP-1) and peptide YY (PYY) are anti-diabetes/obesity hormones secreted from the gut after meal ingestion. We have shown that dietary-resistant starch (RS) increased GLP-1 and PYY secretion, but the mechanism remains unknown. RS is a fermentable fiber that lowers the glycemic index of the diet and liberates short-chain fatty acids (SCFAs) through fermentation in the gut. This study investigates the two possible mechanisms by which RS stimulates GLP-1 and PYY secretion: the effect of a meal or glycemic index, and the effect of fermentation. Because GLP-1 and PYY secretions are stimulated by nutrient availability in the gut, the timing of blood sample collections could influence the outcome when two diets with different glycemic indexes are compared. Thus we examined GLP-1 and PYY plasma levels at various time points over a 24-h period in RS-fed rats. In addition, we tested proglucagon (a precursor to GLP-1) and PYY gene expression patterns in specific areas of the gut of RS-fed rats and in an enteroendocrine cell line following exposure to SCFAs in vitro. Our findings are as follows. 1) RS stimulates GLP-1 and PYY secretion in a substantial day-long manner, independent of meal effect or changes in dietary glycemia. 2) Fermentation and the liberation of SCFAs in the lower gut are associated with increased proglucagon and PYY gene expression. 3) Glucose tolerance, an indicator of increased active forms of GLP-1 and PYY, was improved in RS-fed diabetic mice. We conclude that fermentation of RS is most likely the primary mechanism for increased endogenous secretions of total GLP-1 and PYY in rodents. Thus any factor that affects fermentation should be considered when dietary fermentable fiber is used to stimulate GLP-1 and PYY secretion.

Fig. 1.

Total glucagon-like peptide-1 (GLP-1), total peptide YY (PYY), dipeptidylpeptidase-4 (DPP-4) activity, blood glucose, and insulin in rats fed control diet (□) or resistant starch (RS) diet (▪) ad libitum for 10 days in study 1. Total GLP-1 and total PYY in RS-fed rats were significantly increased compared with controls (P < 0.01) at every sample collection time point over a 24-h period. DPP-4 activity was not significantly different between the groups at all time points. Blood glucose was similar between the groups, except at 1 time point. Insulin in RS-fed rats was lower at 2 time points than in controls. OD, optical density. Values are means ± SE (n = 6–8 for each group at each time point). *P < 0.05 vs. control at the same time collection point.

Fig. 2.

Proglucagon, PYY, and ghrelin and CCK gene expression in epithelial cells collected from different parts of the gut of rats fed control (open bars), nonfermentable fiber control (gray bars), and RS (filled bars) diets in study 2. Values are means ± SE for groups of 8–11 rats. Values that do not share a common letter (a, b) are significantly different (P < 0.05).

Fig. 3.

Direct in vitro stimulating effects of short-chain fatty acids (SCFAs) on proglucagon gene expression in STC-1 cells in study 3. STC-1 cells were incubated in normal-pH (open bars) or low-pH (filled bars) medium with different concentrations of butyrate, propionate, and acetate for 24 h. Data are from 3–5 independent experiments with 3–9 replicates for each experiment. *P < 0.05; **P < 0.01 vs. control (0 mM in normal medium).

Fig. 4.

HEK-293 cells in study 4 were transiently transfected with rat PYY promoter fused to the luciferase reporter gene and incubated with different concentrations of SCFA for 20 h (A) or SCFA at different time periods (B). Values are expressed relative to luciferase (Luc) activity in cells transfected with control solution without promoter and incubated for the same period. Values are means ± SE for 2–5 independent experiments with 2–5 replicates in each experiment.

Fig. 5.

Glucose tolerance was improved in streptozocin (STZ)-induced diabetic mice fed RS in study 5. A: oral glucose tolerance test (OGTT) in normal mice and diabetic mice fed control or RS diet. Glucose concentration at time 0 was obtained after a 5-h fast. B: area under the curve (AUC) of OGTT for mice fed control (open bar) and RS (solid bar) diets. Values that do not share a common letter are significantly different (P < 0.05). Values are means ± SE (n = 8–9 for each group).