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Cooking Oils: Friend or Foe?

By on November 19th, 2014 Categories: Uncategorized

All of the cooking oils I use daily come from plants. There are so many types and uses, from olive oil for salads and roasted vegetables, peanut oil for high-heat stir-frying, and toasted sesame oil to add flavor. Plus, I use organic coconut oil as a skin moisturizer.

While oils all have about the same amount of calories (120 per 1-tbsp serving) and fat (14 grams per serving), they have many differences. On the store shelf you see oils described as “partially hydrogenated” or “extra virgin.” On television you hear stories about some oils being super good for you and hear claims that other kinds are dangerous. But what do these labels really mean? And, how do you know if the stories are true?

Something that seems as simple as cooking oil turns out to be pretty complex. Because I’m too busy to weigh all the risks and benefits, I like to stick with what the research says when choosing which oils to keep in my cupboard.

This column focuses on oils that are extracted from fruit and vegetable seeds and nuts. (I’m saving oils used for baking for another column.)

Fat and oil: The good, the bad, and the so-so

When I say “oil” I mean fat in liquid form. You need certain types of fat to be healthy. Fats are one of the three major nutrients our bodies need. The others are protein and carbohydrates.

How healthy a fat is depends on a number of things, including nutritional content, processing, and how the fat reacts to heat and other factors.

Each kind of oil contains a combination of natural chemicals called fatty acids. Fatty acids play many important roles in the body, including helping us store energy and absorb vitamins. We talk about fatty acids in oil in terms of how “saturated” they are. The more saturated the oil, the more stable it is. More stable oils are less likely to break down over time when exposed to air, light, and heat. The more unsaturated an oil is, the more unstable it is.

You can find the types of fat in an oil by checking the label. Look for the nutrition facts near the ingredients list. Oils typically contain a combination of saturated, monounsaturated, and polyunsaturated fats. Oils are classified by the type of fat they have the most of.

Saturated fats are found most often in animal products used for cooking such as lard, bacon fat, or butter. Mostly saturated plant oils include coconut and palm kernel. These fats tend to be very stable and are solid at room temperature.

Recent research supports a limited role for certain saturated fats in our diets, particularly untreated (virgin) coconut oil. It contains a type of fatty acid called lauric acid that seems to boost levels of HDL, the “good” cholesterol. It’s a good idea to limit other types of saturated fats.

Monounsaturated fats are high in monounsaturated fatty acids, or MUFAs. MUFA-strong oils include the familiar favorites, olive and canola, along with more exotic types such as avocado, macadamia nut, and even peanut oil. “High-oleic” versions of seed oils also tend to be higher in MUFAs. They are liquid at room temperature and moderately stable.

Research shows monounsaturated oils are the healthiest. They are a staple of the Mediterranean diet. Some studies have shown that certain compounds in extra-virgin olive oil may have anti-cancer properties. More studies are being done to figure out exactly how these compounds may help reduce cancer risk.

Polyunsaturated fats, or PUFAs, are found in most vegetable and seed oils. These include corn, soybean, safflower, flax seed, sunflower, walnut, and grape seed oils, along with brand-name oil blends. They are the least stable.

Reviews are mixed on polyunsaturated oils, in part because they tend to break down more easily than other fats into toxic byproducts. “Partially hydrogenated” PUFAs, or trans fats, are the ones to avoid.

Trans fats are oils that have been transformed and stabilized by adding hydrogen to them. They’re found in stick margarine, fast foods, and commercial baked goods. Trans fats both lower HDL and raise LDL, the “bad” form of cholesterol. High blood-cholesterol levels have been linked to heart disease and the spread of breast cancer.

Because of their link to heart attack and other diseases, the U.S. Centers for Disease Control says people shouldn’t eat trans fats.

Finding the balance

There are other things to consider when selecting an oil. Cooking oils contain different amounts of alpha-linolenic acid (omega 3) and the similar-sounding linoleic acid (omega 6). Omega-3 and omega-6 fatty acids are essential fatty acids — our bodies need them to function properly. Our bodies can’t make these fatty acids, so we have to get them from food. The ideal ratio of omega 3 to omega 6 in the food we eat is 1:1 (50-50). Still, that can be hard to do. A 1:4 ratio is a realistic goal, but many polyunsaturated oils contain much more omega 6 than omega 3. Some PUFAs, such as corn and safflower, contain little to no omega 3.

It’s not clear whether a diet heavy in omega 6 is a bad or a good thing. Some studies have found that omega-6 fatty acids increase inflammation, which is linked to a host of diseases including cancer. But other research has led the American Heart Association to say omega-6 fatty acids may help prevent heart disease. Most of us don’t eat enough omega-3 fatty acids, so it’s smart to choose monounsaturated oils, such as olive or canola, which are high in omega 3s. (Fish oil also is a good source of omega 3s, but it’s not used for cooking and usually is taken as a supplement.)

How plant oils are produced

Oils also vary in how they are made. Conventionally produced plant oils are extracted from the fruit or seed using an industrial solvent, such as hexane. They undergo other processes as well. Normally there should be no hexane residues in the oil; however, the U.S. Food and Drug Administration doesn’t test for them. Still, some studies have found hexane in soy products, including oil. This is a concern because hexane can affect the central nervous system. The best way to avoid hexane is to use oils that have been processed by machine, described below, rather than by chemicals.

Chemical-free, mechanical processing is called expeller pressing. Nuts or seeds are placed in a machine that uses pressure to squeeze out the oil. The harder the ingredient, the more pressure is needed, which creates a certain amount of heat. These oils are labeled “expeller pressed.” Certain delicate oils, such as flaxseed and olive, need to be pressed under low-heat conditions (under 120 degrees Fahrenheit) to retain their flavor. These and other temperature-controlled oils are labeled “cold pressed.”

Breakdown and byproducts

It’s important to know that oils don’t keep well.

All oils break down over time and will eventually become old, stale, smelly, and unusable. When this happens, they’re said to be rancid. Even healthy oils can become less stable or rancid depending on when you buy them, how you store them, and most importantly, how you cook with them. The more unsaturated the oil, the faster it goes bad. This breakdown process is called oxidation — the fat reacts to oxygen in the air. That chemical reaction can change the flavor, smell, and nutritional value of the oil. It also can cause toxic compounds to form in the oil. Right now, it’s not clear what levels of these compounds may be harmful.

Oxidation also generates free radicals, molecules that may increase inflammation. Some oils, such as sesame and macadamia, contain natural antioxidants that resist this process. Antioxidants can also be added by the manufacturer to slow the oxidation process.

Oxidation can be a problem when cooking because heating oil speeds up oxidation. Studies show that vegetable oils in particular create certain toxic compounds when heated, even at lower temperatures. Some are in the oil itself, which means they’re also in your food. Others are released into the air. This is especially true at high temperatures — for example, when you fry food. Some research shows that vegetable oils (especially canola) oxidize faster when cooked in the microwave.

We do know that once the oil reaches what’s called the “smoke point,” it’s no longer safe to use. The smoke point varies among oils by up to 100 degrees, but the smoke point of many oils is around 450 degrees F. Also, you shouldn’t reheat used oil that has cooled — no matter what you may have seen at fast-food restaurants or on cooking shows!

If you’re going to heat oil for cooking, consider using monounsaturated oils such as olive, canola, avocado, macadamia nut, and peanut. Recent studies have found that they produce fewer toxic compounds than polyunsaturated oils when heated.

If you need to use very high heat, try refined oils. Refined oils are treated to be more stable. Oils can be refined mechanically or chemically. The more expensive, “virgin” olive oils are refined only by machine and then bottled. Conventional, “non-virgin” olive oil may be refined using additives. Refinement removes some nutrients (and flavor) from oils, so while refined oils are good for cooking, the more flavorful, unrefined oils are better cold.


Eating some oil is OK. However, experts say that you should get most of your fatty acids from solid food rather than their oils. So eat olives instead of olive oil. You can control any risks and maximize the benefits of oils by choosing and handling them carefully:

  • Use the freshest oils possible. Check for an expiration date on the label. Some labels list a “harvest date,” which is a better clue to its age.
  • Limit the size of the oil container you buy so it’s less likely to go bad. Buy oils in small quantities if you use it slowly and larger quantities if you use it quickly.
  • Store vegetable oils away from light and heat, and refrigerate them after opening. (They will usually solidify, so allow them to come to room temperature again before using.) If you can, avoid buying vegetable oils in clear packaging.
  • Do not use rancid oil. Sniff the open bottle; if you detect a crayon-like or “off” odor, throw it out.
  • Cook with oil using low to medium heat. Refined vegetable oils are safest to use at higher temperatures.
  • Never use oil after it has smoked. Toss the oil and any food cooked in it. Never reheat used oil.
  • Make sure your kitchen is well-ventilated when you cook with oil. Compounds given off by heated oils are a form of indoor air pollution and can affect the lungs.

Specific oil tips

Make a habit of reading product labels. Once I found the kinds of oils I wanted, shopping became a lot easier.

  • Look for an expeller-pressed oil with no trans fat or genetically engineered seeds. Organic oils usually have all these traits. If price is an issue, buy the best you can. Or, splurge and use sparingly.
  • Use more MUFAs than PUFAs, using mostly extra virgin olive oil and refined canola oil. Both are high in MUFAs, including oleic acid and in omega-3 fatty acids. They also break down less easily than other oils and stand up to heat. Olive oil has a more distinctive taste and canola oil a more neutral one.
  • Look for the high oleic versions of PUFAs. The plants these oils are made from are bred to contain higher levels of oleic acid and are more stable than conventional versions. This trait makes them better for high-heat cooking.
  • If you must deep fry, peanut or sesame oil are good choices (as long as you’re not allergic to peanuts).
  • Saturated fats, particularly virgin coconut oil, are OK for making special treats or adding a splash of flavor in ethnic dishes.
  • Mix it up. It’s unwise (and boring) to stick to just one kind of oil.

What are your favorite oils for cooking with? I’d love to see your recipes.

Marisa Weiss, M.D. is the founder, president, and chief medical officer of Breastcancer.org, the world's most trafficked online resource for medically reviewed breast health and breast cancer information, reaching over 14 million visitors per year. A breast cancer oncologist with over twenty years of active practice in the Philadelphia region, Dr. Weiss is regarded as a visionary advocate for her innovative and steadfast approach to informing, empowering, and treating patients with breast cancer.

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