The Little-Known Side Effect Of Heating Vegetable Oils: A Functional Medicine Doctor Explains

The choices we make in the grocery aisle as well as in the kitchen can make a huge difference when it comes to getting the most from the fats we eat. Fats are a critical component of our diet, but it's not always obvious how to choose them well. Especially when you're hosting or preparing food for Thanksgiving (or any other holiday!), here's how to choose the best-quality fats and oils in your cooking and which to use in different food-preparation scenarios.

One very important factor in determining which fat to use is its smoke point, the cooking temperature at which a fat or oil begins to break down. Do not heat an oil past its smoke point! This is the point at which the oil begins to degrade and the toxic oxidation products begin to accumulate. The quality of oils can vary greatly, so these estimates are more "ballpark" but you know when an oil has reached its smoke point after it starts to smoke and burn when heated.

Specifically, use saturated fats for high-heat cooking and unsaturated fats for low-heat cooking. That's because unsaturated fats, especially polyunsaturated, can become oxidized during the cooking process and degrade into toxic oxidation products when overheated, causing harmful compounds to circulate in our bodies. It's best to stick with saturated fats like coconut oil or ghee when you have to do high-heat cooking.

Why? It reduces the formation of harmful advanced glycation end products1 (AGEs). Olive oil is fine for lower-heat cooking and finishing. With polyunsaturated fats such as nut and seed oils (like flaxseed oil), it's best to avoid heating at all; save these for dressings and finishing oils.

Look for "unrefined," "cold-pressed," and "unfiltered," as they are the least processed ones. Unrefined oils have a better nutrient profile since the "cloudiness" you sometimes see in unfiltered, cold-pressed oils is most often a sign of additional antioxidant and phytonutrient content. While extra refining can raise the smoke point and shelf life of oils, those valuable nutrients are lost in the process.

Certain GMO crops are used to produce oils such as canola, soybean, and corn, so you can still look for the non-GMO or organic label on oils. Oils made from organic crops will reduce your exposure to pesticides and herbicides, and butter and ghee from grass-fed cows will have higher omega-3 content as well as conjugated linoleic acid (CLA). CLA has anti-inflammatory, anti-atherogenic, and anti-obesity properties, and grass-fed beef and dairy are some of the best sources.

These include corn, cottonseed, peanut, and soybean. Here are three big reasons why:

These oils are usually heated at very high temperatures, and/or processed with chemical solvents such as hexane to extract the oil and then processed with more chemicals to improve the color, smell, and shelf life. Hexane and other toxic by-products of oil processing (e.g., aldehyde) have been linked to a host of health problems from cancer to cardiovascular disease to metabolic syndrome.

First, to be clear, omega-6 fatty acids are a necessary part of our diet (e.g., to create prostaglandins, to activate peroxisomes). However, they can cause inflammation when consumed in excess or for the wrong application. Most people need to reduce their omega-6 intake and increase their omega-3 intake.

Most vegetable oils in the United States come from genetically modified organisms (GMOs), crops that have been genetically engineered to have certain "desirable" traits. Eighty-eight percent of corn, 90 percent of cottonseed, and 94 percent of soybeans grown in the United States are GMOs. One of the most convincing arguments for avoiding GMOs is the higher level of pesticides and herbicides they carry. The increased exposure for humans has been linked to detrimental health effects. The Institute for Functional Medicine has a nice list of other reasons why you would also want to avoid GMOs.

Canola oil is another vegetable oil to know about. It is often said to be healthy since it contains a higher amount of omega-3 fatty acids than vegetable oils. However, 90 percent of canola oil in the United States is genetically modified and highly processed, and the plant species itself was created via genetic tinkering. We think there are better ways to get omega-3s into your diet such as including some unheated flaxseed oil. If you do decide to use canola oil, make sure you're choosing organic or non-GMO and avoid heating it since those polyunsaturated omega-3s are very fragile and prone to oxidation.

These are liquid oils that have been chemically processed to make them solid at room temperature, such as margarine and shortening. We recommend actively avoiding trans fats until they have been eliminated from our food supply over the next three years.