A Taiwanese research group published on the effects of high oxidized frying oil on insulin secretion (reduces insulin secretion)in 2007. In this study they (Chiang et al. 2010) attempt to determine the cause by experimenting with mice. They used three treatment groups:
Low Fat Diet
High Fat Diet
High Oxidized Frying Oil Diet.
The mice fed high oxidized frying oil exhibited reduced insulin secretion and high blood glucose levels. Very important here: their islets of langerhans (the tissue that produces insulin, as well as glucagon) showed evidence of oxidative damage. Glucagon and insulin work together to keep blood sugar stable. It seems likely that oxidative damage would occur throughout the body, so its probably best to avoid fried food even if diabetes or metabolic syndrome is not one of your concerns.
The High Fat diet mice did not show such changes. This implies that a diet high in fried food may put people at risk of diabetes or metabolic disorder by interfering with production of hormones regulating blood sugar, while a high fat diet of unoxidized oil might be just fine. Vitamin E is an important anti-oxidant and is protective against the effects of High Oxidized Frying Oil. Chiang et al. found that adding Vitamin E to the diet reduced the effects of the Oxidized Fat diet. Thanks to all for their hard work.
Ya-Fan Chiang, Huey-Mei Shaw, Mei-Fang Yang, Chih-Yang Huang, Cheng-Hsien Hsieh and Pei-Min Chao (2010). Dietary oxidised frying oil causes oxidative damage of pancreatic islets and impairment of insulin secretion, effects associated with vitamin E deficiency British Journal of Nutrition : 10.1017/S0007114510005039
Chao, P., Huang, H., Liao, C., Huang, S., & Huang, C. (2007). A high oxidised frying oil content diet is less adipogenic, but induces glucose intolerance in rodents British Journal of Nutrition, 98 (01) DOI: 10.1017/S000711450769000X