Black carrots fermented with Lactobacillus plantarum or Aspergillus oryzae prevent cognitive dysfunction by improving hippocampal insulin signalling in amyloid-β infused rats

•Fermented black carrots with A. oryzae (FBAO) and L. plantarum (FBLP) were rich in aglycated anthocyanins.•FBAO and FBLP suppressed hippocampal amyloid-β deposition by potentiated insulin signalling.•FBAO and FBLP improved cognitive function better than black carrot.•FBLP most effectively improved both whole body and hepatic insulin sensitivity.•FBLP potentiated first phase insulin secretion and insulin sensitivity in the hyperglycaemic state. We evaluated whether long-term consumption of black carrot (BC), black carrot fermented with Aspergillus oryzae (FBAO) and Lactobacillus plantarum (FBLP) extracts improved cognitive dysfunction and glucose homeostasis in type 2 diabetic rats with dementia. Male diabetic rats received hippocampal CA1 infusions of 3.6 nmol amyloid-β (25–35; AD) or amyloid-β (35–25, non-plaque forming; Normal) per day, for two weeks. AD rats received either 2% lyophilized BC, FBAO, and FBLP or dextrin (AD-CON) for 8 weeks. FBAO and FBLP, rich in aglycated anthocyanins, suppressed amyloid-β deposition in the hippocampus and potentiated insulin signalling, compared to the BC and the control. FBAO and FBLP had better cognitive function and less visceral fat mass than BC. FBLP most effectively improved both whole body and hepatic insulin resistance and first phase insulin secretion and insulin sensitivity in the hyperglycaemic state. Thus, FBLP and FBAO (1 g/kg bw/day; 0.5–1 mg cyanidin and malvidin aglycone/kg bw/day) may have the potent efficacy for improving cognitive function by preventing hippocampal insulin resistance.