Suppressive Effect of Polyphenols from the Seed Coat of Scarlet Runner Beans on Blood Glucose Levels

Zhaohong Ci^{1, 2}, Chengyu Jiang^{1, 2} and Michiyuki Kojima^{2, 3,}

¹Department of Food Production Science, Obihiro University of Agriculture and Veterinary Medicine, 11, Nishi-2-sen, Inada-machi, Obihiro, Hokkaido 080-8555, Japan

²United Graduate School of Agricultural Sciences, Iwate University, 3-18-8, Ueda, Morioka, Iwate 020-8550, Japan

³Department of Human Sciences, Obihiro University of Agriculture and Veterinary Medicine, 11, Nishi-2-sen, Inada-machi, Obihiro, Hokkaido 080-8555, Japan

Cite this paper:
Zhaohong Ci, Chengyu Jiang and Michiyuki Kojima. Suppressive Effect of Polyphenols from the Seed Coat of Scarlet Runner Beans on Blood Glucose Levels. Journal of Food and Nutrition Research. 2018; 6(3):182-186. doi: 10.12691/jfnr-6-3-7

Abstract

Scarlet runner beans (SRB) are a valuable source of many nutrients, including proteins, starch, dietary fiber, and oligosaccharides, and are used in various foods in Japan. In this study, we analyzed their polyphenol and procyanidin contents, DPPH radical scavenging activity, and reducing power. SRB (purple) had a higher polyphenol content and DPPH radical scavenging activity than those of SRB (black). The reducing power of SRB (black) (5.1 mg/g) was greater than that of SRB (purple) (4.1 mg/g). Both SRB (purple) and SRB (black) had greater levels of polymeric polyphenols (total Fra.II and Fra.III) than monomeric polyphenols (Fra.I), and inhibited the activity of α-glucosidase in a dose-dependent manner. SRB (black) showed higher α-glucosidase inhibitory activity (IC₅₀, 26.4 µg/mL) than that of SRB (purple) (IC₅₀, 39.7 µg/mL), and a mixed pattern of inhibition (non-competitive and uncompetitive). The α-glucosidase inhibitory activity was greater for polyphenols from the seed coat (>91%) than from the cotyledon (<0.1%) for SRB (purple) and SRB (black). Both 250 mg/kg and 750 mg/kg polyphenols from the coat of SRB (purple) effectively suppressed the elevation of blood glucose levels after the oral administration of starch in mice. These results suggest that the seed coat of SRB has useful properties and, in particular, has potential applications for the treatment of diabetes.

Keywords:
scarlet runner bean seed coat polyphenol α-glucosidase blood glucose

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