Cooking with vegetable oil releases chemicals largely associated with cancer and other diseases, research suggests.
In a string of experiments, scientists discovered that heating vegetable oils such as corn oil and sunflower oil resulted in high levels of aldehydes being released.
This toxic chemical has previously been linked to cancer, heart disease and dementia.
One of the researchers, Professor Martin Grootveld, said a "typical" fish and chips meal that is fried in vegetable oil contains 100-200 times more aldehydes than the safe daily limit recommended by the World Health Organisation (WHO).
Conversely, heating up coconut oil produced the lowest levels of aldehydes. Heating butter, olive oil and lard also produced lower amounts of the chemical.
Professor Grootveld, professor of bio-analytical chemistry and chemical pathology at De Montfort University, told The Telegraph: "For decades, the authorities have been warning us how bad butter and lard was. But we have found butter is very, very good for frying purposes and so is lard.
"People have been telling us how healthy polyunsaturates are in corn oil and sunflower oil. But when you start messing around with them, subjecting them to high amounts of energy in the frying pan or the oven, they undergo a complex series of chemical reactions which results in the accumulation of large amounts of toxic compounds."
Interest in the link between vegetable oil and cancer sparked following a BBC 2 series aired in July called 'Trust Me I’m A Doctor', where Professor Grootveld helped investigate the “healthiest” oils to cook with.
As part of the show, a group of researchers aimed to answer the question "which fats and oils are best to cook with?".
They gave a group of volunteers in Leicester a variety of fats and oils - including sunflower oil, vegetable oil, corn oil, cold pressed rapeseed oil, olive oil (refined and extra virgin), butter and goose fat - and asked them to use the products in their everyday cooking.
The volunteers were also asked to collect left over oil after cooking, which was then analysed by a research team at Leicester School of Pharmacy at De Montfort University.
Writing on the BBC's website, one of the researchers Michael Mosley explained that when oils and fats are heated in frying or cooking, their molecular structure changes.
"They undergo what's called oxidation - they react with oxygen in the air to form aldehydes and lipid peroxides. At room temperature something similar happens, though more slowly. When lipids go rancid they become oxidised," he said.
"It's a simple chemical fact that something which is thought to be healthy for us is converted into something that is very unhealthy at standard frying temperatures."
Instead, he recommended olive oil for frying or cooking.
"Firstly, because lower levels of these toxic compounds are generated, and secondly the compounds which are formed are less threatening to the human body," he explained.
Consumers are also encouraged to try coconut oil, butter and even lard, as a replacement.
"Sunflower and corn oil are fine as long as you don't subject them to heat, such as frying or cooking," said Professor Martin Grootveld. "If I had a choice between lard and polyunsaturates, I'd use lard every time."