I’ve been getting this question a LOT from readers recently, and it’s no wonder: This message is everywhere. In fact, one of my favorite beauty experts, nutritionist Kimberly Snyder, R.D., states in her book Beauty Detox Foods, “Polyunsaturated vegetable oils [like canola, corn, cottonseed, grapeseed, safflower, soybean, and sunflower oils] are extremely unstable, and this instability gives way to reactivity, meaning they become rancid and oxidize when exposed to heat, light, and air, and simply as time passes and they sit in your cupboard. Rancid and oxidized oils are aging bombs you don’t want to ingest in your body, if staying young and beautiful is one of your goals!”
So let’s break this down into the science behind this claim.
Polyunsaturated Oils Do Oxidize Faster Than Monounsaturated Oils
First, polyunsaturated vegetable oils will oxidize faster than monounsaturated vegetable oils. It’s the very nature of their chemical structure — polyunsaturated means that the oil is missing more than one hydrogen bond, so the oil tends to be more reactive. So it is a wise recommendation to buy polyunsaturated oils in smaller quantities, to use them quickly after opening, and to heat them at lower temperatures.
However, All Oils Must Oxidize
(Game of Thrones fans: All men must die. All oils must oxidize). Now that that funny is out of the way, the truth of the matter is, when you expose oils of any kind — polyunsaturated, monounsaturated, or saturated — to heat and oxygen, they will lose electrons and go through a process called oxidation.
The key here is the smoke point. Apply enough heat, past an oil’s smoke point, and oil forms byproducts called “cooking oil polar compounds.” Studies on cooking oil polar compounds are very limited. Preliminary research shows they could raise blood pressure, cholesterol, and heart disease risk (AOCS.org)—but there are still very few human studies.
Oxidized Oils May Accelerate the Development of Age Spots
According to endocrine physiologist Dr. Ray Peat, “In the 1960s, Hartroft and Porta gave an elegant argument for decreasing the ratio of unsaturated oil to saturated oil in the diet (and thus in the tissues). They showed that the “age pigment” is produced in proportion to the ratio of oxidants to antioxidants.
This “age pigment” production is multiplied by the ratio of unsaturated oils to saturated oils. More recently, a variety of studies have demonstrated that ultraviolet light induces peroxidation in unsaturated fats, but not saturated fats, and that this occurs in the skin as well as in vitro.
Rabbit experiments, and studies of humans, showed that the amount of unsaturated oil in the diet strongly affects the rate at which aged, wrinkled skin develops. The unsaturated fat in the skin is a major target for the aging and carcinogenic effects of ultraviolet light.” (source)
Watch for Any Oil’s Smoke Point
Smoke point is the temperature at which oils start to break down, lose nutrients, and develop off flavors. (You’ll know it’s happening if the oil is letting off wisps of smoke.) Some oils, like corn, sunflower, and soybean, have higher smoke points, so they’re better for high-heat cooking like deep frying and searing. Other oils have low smoke points, and should probably be reserved for applications like dressing.
Check out this list of common polyunsaturated oils and their smoke points:
- Canola 400ºF
- Corn 450°F
- Cottonseed 420°F
- Grapeseed 390ºF
- Safflower 450ºF
- Soybean 450ºF
- Sunflower 440ºF
Free Radical Destruction from Oxidized PUFAs is Not Massive, But Subtle and Cumulative
The claim that free radicals within oxidized oils cause DNA/RNA damage and cellular destruction sounds like it could have some validity behind it, but most of us consume enough antioxidants in our diet so they are not an issue. Furthermore, truth be told, those who are overweight tend to consume more fried, fatty foods and higher amounts of polyunsaturated oils overall — and they look younger, with less wrinkles, and firmer skin (Renee Rouleau).
If each trip to McDonald’s caused massive destruction to your cellular DNA/RNA and aged you a decade, believe me, they wouldn’t be grossing $27.5 billion annually (Wikipedia). That’s not to say fried, fatty food cooked in polyunsaturated fatty oils can’t do gradual damage to your skin, like it does to your waistline. But it is to say the effect is subtle and cumulative — not the bomb of mass destruction some health and beauty experts are making PUFAs sound like.
Bottom Line: It’s Still the Sun, But Avoid Overheating + Consuming Rancid Oils
People always want to hear their enjoyable habits are good for them. Hence why we have diets claiming you can “eat whatever you want,” or beauty advice like this, telling people it’s not the oh-so-pleasurable-yet-carcinogenic sun accelerating aging, but rather their consumption of polyunsaturated oils.
Truth be told, there is no significant evidence to demonstrate polyunsaturated oils cause or accelerate aging, except if you are exposed to UV light afterwards, and then there are studies that demonstrate the presence of unsaturated fatty acids in your bloodstream cause your skin to create more sunspots than if you did not have unsaturated fatty acids in your bloodstream. But, again, UV light is involved.
Therefore, my recommendation is you can curb your exposure to “cooking oil polar compounds” by cooking with oils that are composed mainly of MUFAs rather than PUFAs, like olive oil, avocado oil, or sesame oil for most cooking. I would also avoid eating oily, fried, fatty food before you go on a sunny beach vacation, or in the summer months in general. But, truth be told, this is one aging concern that seems to be largely overblown based on the scientific and epidemiological evidence at this time.