Image copyright ALAMY

Choosing the right oil to cook with is a complicated business, writes Michael Mosley.

When it comes to fats and oils, we are spoiled for choice. Supermarket shelves are heaving with every conceivable option. But these days it is extremely confusing because there is so much debate about the benefits and harm that come from consuming different types of fats.

On Trust Me, I'm a Doctor we decided to look at things from a different angle by asking: "Which fats and oils are best to cook with?"

You might think it is obvious that frying with vegetable oils has to be healthier than cooking with animal fat, like lard or butter. But is it really?

To find out, we gave some Leicester residents a variety of fats and oils and asked our volunteers to use them in their everyday cooking. The volunteers were also asked to collect any leftover oil which would then be analysed.

The fats and oils they used included sunflower oil, vegetable oil, corn oil, cold pressed rapeseed oil, olive oil (refined and extra virgin), butter and goose fat.

Find out more Trust Me I'm A Doctor is broadcast on BBC Two at 20:00 BST on Wednesday 29 July Watch the programme on BBC iPlayer

Samples of oil and fat, after cooking, were collected and sent to Leicester School of Pharmacy at De Montfort University in Leicester, where Prof Martin Grootveld and his team ran a parallel experiment where they heated up these same oils and fats to frying temperatures.

When you are frying or cooking at a high temperature (at or close to 180C or 356F), the molecular structures of the fats and oils you are using change. 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.

Consuming or inhaling aldehydes, even in small amounts, has been linked to increased risk of heart disease and cancer. So what did Prof Grootveld's team find?

"We found," he says, "that the oils which were rich in polyunsaturates - the corn oil and sunflower oil - generated very high levels of aldehydes."

I was surprised as I'd always thought of sunflower oil as being "healthy".

Image copyright Thinkstock Image caption Lard has a reputation as being unhealthy

"Sunflower and corn oil are fine," Prof Grootveld says, "as long as you don't subject them to heat, such as frying or cooking. 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."

The olive oil and cold-pressed rapeseed oil produced far less aldehydes, as did the butter and goose fat. The reason is that these oils are richer in monounsaturated and saturated fatty acids, and these are much more stable when heated. In fact, saturated fats hardly undergo this oxidation reaction at all.

Prof Grootveld generally recommends olive oil for frying or cooking. "Firstly because lower levels of these toxic compounds are generated, and secondly the compounds that are formed are actually less threatening to the human body."

His research also suggests that when it comes to cooking, frying in saturate-rich animal fats or butter may be preferable to frying in sunflower or corn oil.

"If I had a choice," he says, "between lard and polyunsaturates, I'd use lard every time."

Lard, despite its unhealthy reputation, is actually rich in monounsaturated fats.

Our study also threw up another surprise because Prof Grootveld's team identified in some of the samples sent in by our volunteers a couple of new aldehydes that they had not previously seen in the oil-heating experiments.

"We've done some new science here," he says with a smile on his face. "It's a world first, I'm very, very pleased about it."

I'm not sure that our volunteers would have been quite so thrilled to discover their cooking had managed to generate new, potentially toxic compounds.

So what is Prof Grootveld's overall advice?

Firstly, try to do less frying, particularly at high temperature. If you are frying, minimise the amount of oil you use, and also take steps to remove the oil from the outside of the fried food, perhaps with a paper towel.

To reduce aldehyde production go for an oil or fat high in monounsaturated or saturated lipids (preferably greater than 60% for one or the other, and more than 80% for the two combined), and low in polyunsaturates (less than 20%).

He thinks the ideal "compromise" oil for cooking purposes is olive oil, "because it is about 76% monounsaturates, 14% saturates and only 10% polyunsaturates - monounsaturates and saturates are much more resistant to oxidation than polyunsaturates".

When it comes to cooking it doesn't seem to matter whether the olive oil is "extra virgin" or not. "The antioxidant levels present in the extra virgin products are insufficient to protect us against heat-induced oxidation."

His final bit of advice is always keep your oils in a cupboard, out of the light, and try not to reuse them as this also leads to the accumulation of nasty side-products.

Know your fats

Image copyright Thinkstock

Polyunsaturated fats Contain two or more carbon-carbon double bonds. When eaten in as food such nuts, seeds, fish and leafy greens, they have clear health benefits. However, the benefits of consuming sunflower oil and corn oil, although rich in polyunsaturates, are much less clear.

Contain two or more carbon-carbon double bonds. When eaten in as food such nuts, seeds, fish and leafy greens, they have clear health benefits. However, the benefits of consuming sunflower oil and corn oil, although rich in polyunsaturates, are much less clear. Monounsaturated oils Contain just one carbon-carbon double bond. They are found in avocados, olives, olive oil, almonds and hazelnuts, and also in lard and goose fat. Olive oil, which is approximately 76% monounsaturated, is a key component in the Mediterranean diet, which has been shown to significantly reduce the risk of heart disease.

Contain just one carbon-carbon double bond. They are found in avocados, olives, olive oil, almonds and hazelnuts, and also in lard and goose fat. Olive oil, which is approximately 76% monounsaturated, is a key component in the Mediterranean diet, which has been shown to significantly reduce the risk of heart disease. Saturated fats have no double bonds between carbon atoms. Although we are encouraged to switch from eating saturated fats, particularly dairy and other fats derived from animals, the benefits of doing so are being challenged.

have no double bonds between carbon atoms. Although we are encouraged to switch from eating saturated fats, particularly dairy and other fats derived from animals, the benefits of doing so are being challenged. The percentages of each in the oils below varies somewhat but these values are typical

Type of oil or fat Polyunsaturated (%) Monounsaturated (%) Saturated (%) Coconut oil 2 6 86 Butter 3 21 51 Lard 11 45 39 Goose fat 11 56 27 Olive oil 10 76 14 Rapeseed oil 28 63 7 Sesame oil 41 40 14 Corn oil 54 27 12 Sunflower oil 65 20 10

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What's the right diet for you?

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Image copyright Various

After years of being told, and telling others, that saturated fat clogs your arteries and makes you fat, there is now mounting evidence that eating some saturated fats may actually help you lose weight and be good for the heart, writes Michael Mosley.

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Trust Me I'm A Doctor continues on BBC Two at 20:00 BST on Wednesday 29 July - or watch on BBC iPlayer

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