Does using olive oil for frying create trans fats?

THE QUESTION

THE ANSWER

It's true that heating a cooking oil can change its chemical makeup, as well as its taste and nutritional value. But using cooking oil (olive or otherwise) to fry foods doesn't generate trans fats. That said, some oils are better suited for high-heat cooking than others.

Trans fats are formed during partial hydrogenation, a process that creates fats useful to food manufacturers because they have a long shelf life and are able to withstand repeated heating without breaking down. To make partially hydrogenated fats, food chemists pump hydrogen atoms into liquid unsaturated oils to make them semi-solid.

During hydrogenation, some of the healthy unsaturated fatty acids are converted to trans fatty acids. The problem, of course, is that a steady intake of trans fats increases the risk of heart disease.

Even though frying with olive oil doesn't form trans fats, it's important to not heat olive oil – or any cooking oil – past its "smoke point," the temperature at which it starts to burn. After an oil burns, its flavour and nutrient content decrease. Overheating extra-virgin olive oil will also destroy some of its beneficial phytochemicals. (Extra-virgin olive oil is cold pressed from olives using minimal heat and no chemicals. As a result, it retains the highest amount of phytochemicals and nutrients compared to "olive oil" or "light olive oil," which have been refined.)

It's best to choose a cooking oil with a smoke point above 400 degrees, since most foods are fried at a temperature between 350 and 450 F. Extra-virgin olive oil (smoke point 410 degrees), olive oil (436-468 degrees), canola oil (400-475), grapeseed oil (420), peanut oil (440), sunflower oil (440), safflower oil (510) and refined coconut oil (436-468) are all suitable for stir-frying and sautéing.

Cooking oils with a lower smoke point include unrefined coconut oil (350 degrees), flaxseed oil (225), hemp oil (330) and unrefined walnut oil (320). Flaxseed, hemp and unrefined walnut oils should not be used for cooking since heat destroys their essential fatty acids. These oils should be used as a condiment, in salad dressings or is smoothies and stored in the refrigerator

When it comes to choosing a cooking oil, consider nutrition, too. If you want to increase your intake of heart-healthy monounsaturated fats, choose olive, peanut or canola oil. Canola oil is also a source of alpha linolenic acid, an omega-3 fatty acid thought to help protect from heart disease if your diet lacks omega-3's from fish. (Flax, walnut and hemp oils also contain alpha linolenic acid.)

I encourage my clients to get more vitamin E in their diet, an antioxidant plentiful in grapeseed, safflower and sunflower oils. Vitamin E helps maintain immune function and it also protects brain cells from oxidative stress (e.g. damage caused by free radicals). In fact, a 2010 study published in the Archives of Neurology found that men and women whose diets provided the most vitamin E were significantly less likely to develop Alzheimer's disease than their peers who consumed the least.

Whatever your choice of cooking oil, buy only what you'll use within one year. Store oil in a cool, dark cupboard or the refrigerator. Most oils have a limited shelf life and turn rancid quickly if exposed to light, heat and air. You'll know your oil is rancid if it has a characteristic off smell and taste; if it does, throw it out.

Leslie Beck, a Registered Dietitian, is based at the Medisys clinic in Toronto. She can be seen every Thursday at noon on CTV News Channel's Direct lesliebeck.com.

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