Cooking with vegetable oils releases toxic chemicals linked to cancer and other diseases, according to leading scientists, who are now recommending food be fried in olive oil, coconut oil, butter or even lard.
The results of a series of experiments threaten to turn on its head official advice that oils rich in polyunsaturated fats - such as corn oil and sunflower oil - are better for the health than the saturated fats in animal products.
Scientists found that heating up vegetable oils led to the release of high concentrations of chemicals called aldehydes, which have been linked to illnesses including cancer, heart disease and dementia.
Martin Grootveld, a professor of bioanalytical chemistry and chemical pathology, said his research showed "a typical meal of fish and chips", fried in vegetable oil, contained as much as 100 to 200 times more toxic aldehydes than the safe daily limit set by the World Health Organisation.
In contrast, heating up butter, olive oil and lard in tests produced much lower levels of aldehydes. Coconut oil produced the lowest levels of the harmful chemicals.
Concerns over toxic chemicals in heated oils are backed up by separate research from an Oxford professor.
Professor John Stein, Oxford's emeritus professor of neuroscience, said that partly as a result of corn and sunflower oils, "the human brain is changing in a way that is as serious as climate change threatens to be". Because vegetable oils are rich in omega 6 acids, they are contributing to a reduction in critical omega 3 fatty acids in the brain by replacing them, he believes.
"If you eat too much corn oil or sunflower oil, the brain is absorbing too much omega 6, and that effectively forces out omega 3," Professor Stein said. "I believe the lack of omega 3 is a powerful contributory factor to such problems as increasing mental health issues and other problems such as dyslexia."
Professor Grootveld, of De Montfort University in Leicester, who carried out a series of experiments, said: "People have been telling us how healthy polyunsaturates are in corn oil and sunflower oil. But when you start ... subjecting them to high amounts of energy in the frying pan or the oven, they undergo a complex series of chemical reactions which result in the accumulation of ... toxic compounds."