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Olive oil is often presented as a healthy alternative to other oils for cooking. The problem is, depending who you ask, it sometimes is presented as something to avoid, toxic, dangerous.

For example, this article claims:

Many people believe that it is unsuitable for cooking because of the unsaturated fats. I used to believe in this myth, but after doing more research I realized that I was mistaken. Today, I’d like explain why olive oil is an excellent choice for cooking, even for high heat methods like frying.

And this one claims:

With scientists advising against cooking with olive oil - it is claimed to produce toxic chemicals when heated - what should you use instead?

What's the actual scientific consensus? Does it really affects the health negatively? Should we be worried and avoiding it?

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I seem to perceive some confusion in your question, and I'll try to clarify all doubts as much as I can. Olive oil is one of the best oils you can use to cook. It is mainly composed by monounsaturated fatty acids (1), that are neutral to cardiovascular risk and blood cholesterol. There are some oils that are better, by this point of view, like canola oil, that has a higher proportion of polyunsaturated fatty acids, that are beneficial to cardiovascular risk. Some oils are worse, like palm oil, because of the higher proportion of saturated fatty acids, that are detrimental to cardiovascular risk. For the same reason you should also avoid margarines and butter.

Olive oil has also a high smoke point (2), that makes it suitable for frying. Smoke point is the temperature at which toxic compounds are formed; it means that you should never heat beyond smoke point of any oil. This is the reason why the dangerousness of very hot olive oil is not something specific of olive oil, but of any oil that is heated beyond its smoke point.

Finally, something that you should also be aware is that most of cooking oils are extracted with the use of hexane (3), a chemical solvent. Although the oil is subsequently refined, "cleaned", and the industry claims it's safe enough to be consumed, this procedure has risen a lot of concern. "Virgin" and "extra virgin" olive oil do not involve the use of any solvent during the production; this characteristic is shared with other oils that are cold pressed. Cold pressing is an extraction technique that, additionally, preserves the chemical content of the polyphenols, antioxidants, and vitamins present in the oil, that are reduced by high temperatures. The regulation of the definition of "virgin" oils and cold pressing is different between countries.

I apologize for my english, it is not my mother language. Here in Italy we speak a different language, and we know about oil.

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    The reference in Wikipedia about smoke point is weird. Refined olive oil has a high smoke point, but virgin and extra virgin has lower than what they claim. Note that their 191 C has "citation needed". See for example seriouseats.com/2014/05/…, the article there uses numbers from Modernist Cuisine, one of the most scientifically thorough books in cooking. Also, I have personally seen olive oil pyrolising at temperatures much below 191. – rumtscho May 18 '15 at 10:50
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    So, your answer is true, but it is beside the point - the OP asked about the health problems created when the oil is "heated" (it is likely that the citation assumed heating above the smoke point). You simply said "you shouldn't" but in practice, everybody overheats olive oil when cooking with it - I have observed it in my kitchen, with friends, and with recipes which just cannot stay below the smoke point when cooked properly. So a good answer would not assume that the oil is not heated above the smoke point, but explain what happens when it is heated above it. – rumtscho May 18 '15 at 10:52

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