After feeding rats with either soy (HF) or oxidized frying oil (HO) over a period of 8 weeks the researchers found:
[...] mice in the HO group showed glucose intolerance and hypoinsulinaemia, and their islets showed impaired glucose-stimulated insulin secretion (P,0·05; HO group v. LF and HF groups). Significantly higher oxidative stress and a lower mitochondrial membrane potential were observed in the islets in the HO group compared with the LF and HF groups. Immunoblots showed that the reduction in insulin levels in HO islets was associated with activation of the c-Jun NH2-terminal kinase and a reduction in levels of pancreatic and duodenal homeoboxFurthermore, they found that, "when dietary OFO-induced tissue vitamin E depletion was prevented by high-dose vitamin E supplementation (500 IU(1·06 mmol all-rac-a-tocopherol acetate)/kg diet; [...] the OFO-mediated reduction in islet size and impairment of glucose tolerance and insulin secretion were significantly attenuated."
So, obviously it is the interplay of oxidation and lack of antioxidants (in this case vitamin E) which collectively initiate these metabolic changes, which consequently are at the bottom of what is often called "the metabolic syndrome". Bottom line: Beware of oxidized fats and support your antioxidant capacity by healthy food choices and supplements.