Cooking with coconut oil, olive oil, or sunflower oil can release high levels of toxic chemicals known as aldehydes, which have been linked to cancer, heart disease, and dementia, scientists claim.

Led by Martin Grootveld, a professor of bioanalytical chemistry and chemical pathology at De Monfort University in the UK, a new series of experiments on corn and sunflower oil showed that “a typical meal of fish and chips” fried in these vegetable oils contained up to 200 times more aldehydes than are considered safe. Olive oil, butter, and coconut oil, on the other hand, contained far lower levels of aldehydes.

The research might be particularly surprising due to the fact that butter and lard contain far more saturated fats than vegetable oils, which contain polyunsaturated fats. Most vegetable oils (take Crisco, for example) boast far less saturated fat than butter or other animal fats. Health officials have pushed the public to avoid butter and lard, and replace it with vegetable oils to reduce their fat intake.

“For decades, the authorities have been warning us how bad butter and lard was,” Grootveld told The Telegraph. “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… 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.”

During his appearance on BBC’s Trust Me, I’m a Doctor, Grootveld gave volunteers sunflower oil, vegetable oil, corn oil, cold-pressed grapeseed oil, olive oil, butter, goose fat, and lard, and asked the participants to cook foods with them every day. Leftover oil from their cooking was collected and sent to the lab at De Montfort University. Grootveld and his research team found that sunflower oil and corn oil produced aldehydes at the highest levels, and that it was actually safer to use olive oil, butter, lard, or coconut oil.

“Sunflower and corn oil are fine as long as you don’t subject them to heat, such as frying or cooking,” Grootveld said in a press release. “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.” Olive oil, meanwhile, is much better “because lower levels of these toxic compounds are generated, and secondly, the compounds which are formed are less threatening to the human body.”