Meju, unsalted soybeans fermented with Bacillus subtilis and Aspergilus oryzae, potentiates insulinotropic actions and improves hepatic insulin sensitivity in diabetic rats

BACKGROUND: Although soybeans have the ability to attenuate insulin resistance, it is insufficient to alleviate type 2 diabetic symptoms and different types of fermented soybeans may have even better anti-diabetic effects. Meju, unsalted fermented soybeans exhibited better insulin sensitizing and insulinotropic actions than unfermented cooked soybeans (CSB). We investigated whether meju fermented in the traditional (TMS) manner for 60 days and meju fermented in the standardized (MMS) method inoculating Bacillus subtilis and Aspergillus oryzae for 6 days modulated insulin resistance, insulin secretion, and pancreatic β-cell growth and survival in 90% pancreatectomized (Px) diabetic rats, a moderate and non-obese type 2 diabetic animal model. METHODS: Diabetic rats were divided into 3 groups: 1) TMS (n = 20), 2) MMS (n = 20) or 3) casein (control; n = 20). Rats were provided with a high fat diet (40 energy % fat) containing assigned 10% meju for 8 weeks. At the end of experiment insulin resistance and insulin secretion capacity were measured by euglycemic hyperinsulinemic clamp and by hyperglycemic clamp, respectively. Additionally, β-cell mass and islet morphohometry were determined by immunohistochemistry and insulin signaling in the liver was measured by western blot. RESULTS: TMS and MMS increased isoflavonoid aglycones much more than CSB. CSB and TMS/MMS improved glucose tolerance in diabetic rats but the mechanism was different between treatments (P < 0.05). CSB enhanced peripheral insulin sensitivity including hepatic insulin sensitivity better than the control but TMS and MMS enhanced only hepatic insulin sensitivity through activating insulin signaling in diabetic rats (P < 0.05). However, TMS and MMS, but not CSB, potentiated glucose-stimulated insulin secretion and β-cell mass (P < 0.05). MMS had better insulinotropic actions than the control (P < 0.05). CONCLUSIONS: The anti-diabetic action of MMS, especially when fermented with Bacillus subtilis and Aspergillus oryzae, was superior to CSB by increasing isoflavonoid aglycones and small peptides with regard to type 2 diabetic rats.