Cooking with vegetable oils such as corn oil and sunflower oil was thought to be better for health as these polyunsaturates contain fewer saturated fats.
However, experts now say that heating up these oils leads to a release of toxic chemicals, known as aldehydes, which have been linked to cancer, dementia and heart disease.
Butter, lard and olive oil tend to produce far lower levels of these toxic by-products, said Martin Grootveld, a professor of bioanalytical chemistry and chemical pathology at De Montfort University in Leicester. Prof Grootveld’s research has found that a typical fish and chip supper could contain up to 200 times the level of aldehydes advised as safe by the World Health Organisation.
He said: “I agree that we should not be consuming excessive amounts of saturated fats but when you mess around with them in a frying pan then they are completely unreactive, unlike sunflower oils.
“The more unsaturated the oils they are, the more toxic products are generated during cooking.
“If I had a choice between cooking using lard and cooking using polyunsaturates, then I would definitely go for cooking in lard.”
There is also growing evidence that cooking with vegetable oils can also give off harmful fumes, said Prof Grootveld.
NHS guidance says it is better to use low-fat versions compared to foods high in saturated fat and warns that men should eat no more than 30g of saturated fat each day and women should have no more than 20g of saturated fat.
Prof Grootveld added: “This information has been around for a long time but the food industry does not seem to want to hear it.
“The NHS is still recommending that people cook with polunsaturates, which our research shows are not good for you when heated up.”
Scottish nutritionist Emma Conroy told The Scotsman that changing people’s habits in this area could not come soon enough.
Ms Conroy, owner of Edinburgh Nutrition, said: “I could not agree more with what these experts are saying.
“For years I have been telling people that it is much better to use traditional foodstuffs such as lard, beef dripping and even ghee, a kind of clarified butter used in Indian cooking.
“It is so much better for you and does not bring the risks that processed things do.
“We need to go back to previous generations where they actually knew what they were doing.”