A recent study showed that cooking in vegetable oil for just 20 minutes, produced 20 times the permitted levels of adelhyde recommended as a maximum limit by the World Health Organisation.
When I see someone who is otherwise healthy frying healthy kale and tofu in sunflower oil, I despair how good intentions can go so wrong.
For years, we have thought that vegetable oils, including sunflower oil and corn oil, were better than butter and animal based fats.
But the tide of opinion has changed and the latest scientific evidence reveals that dairy products actually protect against heart disease and type 2 diabetes.
Sadly, the news has come too late for the millions of people who now shun full-fat milk and butter because they think it is bad for them.
I now always advise my patients to actively avoid eating any industrial vegetable oils.
Cold pressed extra virgin olive oil however will protect your heart and give you a bit of a boost when it comes to antioxidants which help to mop up damaging free radicals in the blood.
But the vast majority of vegetable oils are not going to help, despite claims that they help reduce cholesterol.
A recent BMJ analysis found that even if cholesterol levels do decline on a diet of vegetable oil margarines, there is no knock-on benefit to heart health and a more worrying trend to increasing the risk of death.
So my message is drizzle your cold pressed olive oil on your chorizo to your (healthy) heart’s content but stick to butter (and ghee) when you want to create a bit of a sizzle.
The dangers of turning up the heat
It is this assumption that has consistently seen health experts warn people not to cook with olive oil and advise that vegetable and sunflower oil are safer for cooking as they have a higher smoke point, so can resist greater heat without producing cancer-causing agents.
That point is 225c for sunflower oil and 230c for corn oil, compared to 160c to 190c for extra virgin olive oil.
That was the theory when we began our research.
To test it out we asked some volunteers, local residents in Leicester, to use a variety of fats and oils, provided by us, in their everyday cooking.
The volunteers were asked to collect any leftover oil after cooking, which we would then have analysed.
They were given sunflower oil, vegetable oil, corn oil, cold-pressed rapeseed oil, olive oil (refined and extra virgin), butter, goose fat and lard.
Samples were then sent to De Montfort University in Leicester, where Martin Grootveld, professor of bio-analytical chemistry and chemical pathology, got to work in the lab analysing their contents.
His team also ran a parallel experiment where they heated up these oils and fats to frying temperatures.
The findings were surprising, to say the least.
Indeed, for many who have followed traditional advice on cooking with oils and fats it will prove to be a case of 'everything you think you know is wrong'.
What about olive oil?
Is cooking with olive oil a no-no? Not according to Professor Grootveld.
In fact, it is, on balance, unquestionably the best compromise.
Sunflower oil, supposedly the healthier alternative, is far worse. Even lard, so demonised it has entered common usage as an insult, is preferable to sunflower oil and its closely related cousin, corn oil.
To understand why, we must look closely at what happens to fats and oils when heated to a high temperature.
They undergo what is called oxidation: they react with oxygen in the air to form substances such as aldehydes and lipid peroxides. At room temperature something similar happens, though more slowly.
When fats go rancid they have been oxidised, and it results in the same by-products. It is these aldehydes they form that are the problem.
Consuming or inhaling them, even in small amounts, has been linked to increased risk of cancer and heart disease.
'We found that oils which were polyunsaturated-rich - corn oil and sunflower oil - generated very high levels of aldehydes,' Professor Grootveld told me.
This surprised me. Like so many others, I had always thought of sunflower oil as being healthy.
'Sunflower and corn oil are fine, but only as long as you don't subject them to heat, such as frying or cooking,' said Professor Grootveld.
'It's a simple chemical fact that something which is thought to be healthy for us is converted into something that is unhealthy at frying temperatures.'
Aldehydes, which are known promoters of cancer, heart disease and dementia when eaten or inhaled, were present in levels up to 20 times higher than recommended by the World Health Organisation.
Not only that, but Professor Grootveld's team also identified two previously unknown aldehydes in the samples of these oils - a world first and cause of some excitement for scientists, but bad news for consumers.
Put simply, cooking with these oils is producing even more toxic compounds than has ever before been realised. In contrast, the olive oil and cold-pressed rapeseed oil produced far fewer aldehydes, as did butter and goose fat.
The reason being that these fats are richer in monounsatured and saturated fats, and are much more stable when heated.
'Far lower levels of toxic compounds were generated by these oils and the compounds that were are actually less threatening to the human body,' says the professor.
And as one in the eye for those who have eschewed old-fashioned fats, his research also suggests that when it comes to cooking, frying in animal fats may be preferable to sunflower or corn oil.
His personal cooking choice is unequivocal. 'If I had a choice between lard and polyunsaturates, I'd use lard every time,' he says.
Staying true to the science, it must be stressed that as long as they are not heated, polyunsaturated fats (vegetable and sunflower oils) are still seen as a healthy option.
These fats reduce bad cholesterol and lower the risk of stroke and heart disease.
And while the most stable fats of all are the saturated ones, such as butter, these should still be used sparingly as they can increase bad cholesterol and the risk of narrowed arteries and heart disease.
So far, so interesting, but the discoveries were not over.
Along the way our study had thrown up additional bad news about the destructive compounds hidden in our food - even storing oils in the wrong way can change them, as sunlight creates the same damaging chemical reactions as heat, only more slowly.
So storing oils in a dark cupboard is highly recommended to avoid formation of these dangerous by-products.
The research hasn't just changed my thinking on oils - it's changed the way I use them, too. I am considering giving lard a try - and I never thought I would hear myself say that.
Source Mail Online