The Fat On Cooking Oils – What To Use When

Whether Or Not An Oil Is Healthy Depends On How It Is Used.

In the field of Nutrition, probably one of the most contentious subjects is  dietary fats – which types, how much, how often and where to find the ones best for our health.

A lot of misinformation about dietary fats has been fuelled by food manufacturers grasping onto very outdated health science for the purpose of marketing.  However, the truth about the health benefits of dietary fats is finally emerging.  Saturated and monounsaturated fats are no longer the ‘bad guys’.  Coconut oil is enjoying enormous popularity and butter is making its comeback as healthier than its chemical cocktail counterpart – margarine.

However when it comes to cooking oils, there is some really important information that is overlooked when considering which oil is healthier to use.  Many health benefit claims are actually referring to the oil in its cold, uncooked form.  What happens to different oils once heat is applied is a totally different story, and the majority of time, we are applying heat to our oils!


In an investigation by Michael Mosely 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 to assess which ones were healthier options.

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.

Here are their findings:


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.”

“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.

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.

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.

Source:  BBC News

In line with the findings of Prof Grootveld’s team, I have always followed this general rule of thumb when it comes to cooking with oils:

Saturated Fats (eg butter, coconut oil) are most stable – use for high heats.

Monounsaturated fats (eg olive oil, nut oils) are moderately stable – use for moderate heats

Polyunsaturated fats (ie plant and seed oils) are very unstable and should not be subjected to heat at all.  For example, flaxseed oil is a highly nutritious polyunsaturated oil but should never be used in cooking – but great to drizzle over a salad or add to a smoothie!  Sesame seed oil has amazing flavour and is best added to your stir fries at the end of cooking, just before serving, rather than actually cooking in it.


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