Fig 2: Titration of the capture antibody and HRP-streptavidin solution for the developed GLP-1 s-ELISAs. The optimal concentrations were determined using the checkerboard titration method as described in Materials and Methods section. (A) Optimal concentrations of the capture antibody. (B) The optimal concentration of streptavidin-HRP solution.

Fig 3: Cinnamtannin A2 promotes the secretion of GLP-1 from STC-1 cells. STC-1 cells were treated with flavan-3ols at the indicated concentrations. l-glutamine (10 mM) and DMSO (0.1% as final concentration) were used for the positive and vehicle controls, respectively for 4 h. The concentration of ‘active’ and ‘total’ GLP-1 in the medium was measured using the developed sensitive active and total s-ELISAs, respectively. (A, B) (−)-epicatechin. (C, D) Procyanidin B2. (E, F) Procyanidin C1. (G, H) Cinnamtannin A2. Data are shown as means ± SE (n = 3). *Indicates significant difference from the vehicle control-treated cells (p<0.05 by Dunnett’s multiple comparison test). All compounds treatment (except cinnamtannin A2) were conducted simultaneously using the same plate and the values of the vehicle control in panels (A), (C), and (E) and that of positive control in panels (B), (D), and (F) were shared, respectively. l-gln, l-glutamine.

Fig 4: PCBE increases the secretion of GLP-1 in plasma of mice. Male C57BL/6J mice (6 weeks old) were orally dosed with BE or PCBE in water at 300 mg/kg body weight. Plasma GLP-1 levels were measured 60 min after administration of BE or PCBE using the developed total GLP-1 s-ELISA. Data are shown as means ± SE (n = 5). *Indicates significant difference from the water only-treated group (p<0.05 by Dunnett’s multiple comparison test). BE, black soybean seed coat extract; PCBE, procyanidin-rich black soybean seed coat extract.