• Extra virgin olive oil contains more health-protecting antioxidants than oils such as sunflower or vegetable
  
• But these are broken down when the oil is heated
• The oil is still at least as healthy as other oils, but cooking with it could be a waste of money
  

By Emma Innes

PUBLISHED: 06:32 EST, 28 June 2013 | UPDATED: 06:33 EST, 28 June 2013

It is the gourmet staple favoured by celebrity chefs for its health  benefits and distinctive taste.

But shoppers who cook with extra virgin olive oil could be wasting their money - because it loses most of its benefits when it is heated.

Olive oil has more antioxidants than other oils and also contains bioactive components - called phenolic compounds - thought to help ward off cancer and other diseases.

However, when oils are heated they degrade in quality, say researchers from Portugal’s University of Porto.

Writing in the journal Food Research International, they said their research showed that any kind of heating reduced the power of the phenolic compounds in the oil.

They said: ‘Virgin olive oil consumption, as final seasoning or within cooked foods, is increasing worldwide, mainly due to its recognised nutritional benefits.

‘However, different cooking practices, from common frying, to boiling and microwave cooking, undoubtedly modify the olive oil chemical profile.’

Even after being cooked, olive oil will still be at least as healthy as vegetable oil so it is not worth replacing one with the other, said the report.

But it could be ‘economically advantageous’ to use standard olive oil rather than pay more for the extra virgin variety, it added.

And there are other ways to get the most out of olive oil when cooking, they added.

One is to keep heating to a minimum, another is to keep adding a splash of olive oil during the cooking process so that it does not get so hot it loses its healthy properties.

The report said: ‘Olive oil performance under prolonged thermal processing is usually equal or superior to other refined vegetable oils.

‘However, as most of its bioactive components, including phenolic compounds, are gradually lost, it is economically advantageous to use lower olive oil grades and frequent replenishment under prolonged thermal processing.

'After a short heating period, most olive oil advantages, in comparison with other vegetable oils, are lost.'