Is it safe to cook with olive oil?
|Let’s check the science: Is it true that olive oil turns toxic at high temperatures?|
A friend was recently told by her GP that she should not cook with olive oil because, he claimed, it becomes toxic when heated. In our house, we cook with olive oil all the time because we know it’s healthier than butter or other saturated fats, so this would have serious implications for us. Do I need to switch to another oil for cooking?
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The promoters’ arguments – science or red flags?
Strangely, I could find very few websites encouraging the view that olive oil is worse than any other oils in this respect. Here’s a typical extract from one of those:
DANGER: Don’t Cook with Olive Oil!
That’s right, you read it correctly, do not cook with olive oil because it’s dangerous to your health. Numerous scientific studies show continually that olive oil is one of the best sources of the Omega oils that YOU NEED for a healthy nervous system and cardiovascular system…BUT what they often neglect to tell you is that those wonderful BENEFITS ARE ALMOST COMPLETELY DESTROYED once you heat the oil! That’s right, the process of heating oils can cause the fats to become carcinogenic; which means causes CANCER! Heating causes enzymes to be destroyed, proteins are denatured, fats become carcinogenic, carbohydrates (sugars) become caramelized, vitamins and minerals become less available, and water is eliminated.
Most websites, though, were quite sensible and gave the advice that all oils deteriorate when heated, and olive oil is about average in this respect. They generally assured me that there was nothing to worry about (e.g. here). Given that my friend’s GP was so concerned, I had expected to find more anti-olive oil information.
Do I see any red flags? Well, the excerpt above misleads because contains exaggerations and straw men and tries to frighten us with them. Heating food does destroy enzymes, denature proteins, caramelize sugars and reduce water, but that is irrelevant because olive oil contains practically none of those substances. In any case, we cook foods precisely because we want these things to happen. That’s what makes them tastier and more easily digested.
My nonsense detectors always go into overdrive at the mention of toxins. They are among the favourite bogeymen of bogus scientists (along with radiation and negative energy). There are lots of toxins in our environment. Many foods contain substances that are toxic, and cooking can produce more. Our bodies have evolved to be quite good at dealing with the odd toxic molecule that manages to find its way inside. It even produces and eliminates toxins of its own.
It’s important to realise that just about anything (yes, even water) can be toxic to us in large enough doses. So there’s no need to panic at the thought of ingesting a small amount of a substance that, in large amounts, could poison us. Our bodies are quite used to dealing with this situation. I suspect this is probably the case with any toxins produced when olive oil is heated.
Still, my friend’s GP would have been aware of all of this, so I’m interested to see what my research reveals.
The scientific evidence
This is a complex issue to investigate. Luckily, I have a background in chemistry, so I was able to follow fairly well most of the papers I discovered. Here’s what they told me:
- Vegetable oils oxidise as they age and are exposed to air and light. This process is accelerated when they are heated (here, here).
- Some of the oxidation products are toxic (here, here, here and here), but (here) we have good natural defences against such substances.
- The ease of oxidation of an oil is influenced by its degree of unsaturation. Polyunsaturated oils oxidise more easily than saturated oils (here).
- Corn oil and sunflower oil contain high proportions of polyunsaturated oils; they oxidise easily. Coconut oil is high in saturated oils; it doesn’t oxidise easily.
- Olive oil contains a high proportion of monounsaturated oil. It oxidises less easily than polyunsaturated oils (here, here).
- Virgin and extra-virgin olive oils also contain antioxidants which help resist oxidation as the oil is heated (here, here and here). Several studies have shown that virgin olive oil produces fewer oxidation products than polyunsaturated oils when heated (here, here)
- If an oil is heated beyond its smoke point, it gives off toxic smoke. The smoke point of olive oil is around 200°C. Some refined oils, such as palm, peanut, safflower and soybean oils can have smoke points around 230°C to 260°C, but unrefined oils can have smoke points in the low hundreds.
- I could find no studies showing evidence of damage to the health of humans resulting from oxidation products of cooking oils.
My initial skepticism was pretty-well confirmed by my research. Olive oil certainly doesn’t deserve to be singled out as a source of toxins in cooking. There are oils that are far worse, and in any case, as long as I don’t use extreme temperatures or cooking times, the amount of oxidation product will be small. My body should be able to deal with it easily. I’ll continue to use olive oil (preferably extra-virgin) for cooking, but I’ll do it sensibly.
Health Check: cooking oils to eat and avoid at The Conversation.
Photo credit: IndyDina with Mr. Wonderful on flickr.
|This is one of ScienceOrNot’s Let’s check the science series.|