A lot can go wrong with sunflower oil—”depending on how it’s processed and its fatty acid profile, it can be extremely unhealthy and contribute to significant metabolic dysfunction,” says Shanahan. As mentioned above, you really want to avoid refined sunflower oil varieties that are also high in linoleic acid, an omega-6 PUFA that is quite unstable.
There’s nothing inherently wrong with omega-6 PUFAs, but the damage that occurs to these fats when exposed to high temperatures during refining, and then again during cooking, is a big problem. “When these unstable PUFAs encounter the high heat, they oxidize and break down chemically to produce dangerous new molecules such as aldehydes, 4-hydroxynonenal, cyclic amines, toxic alcohols, and toxic ketones,” says Shanahan. When consumed, this collection of compounds can drive oxidative stress and inflammation in the body and contribute to a range of inflammatory conditions, from joint pain to acne to gut problems to a variety of chronic diseases.
But if heating high-PUFA oils like sunflower oil can create such a nasty cocktail of compounds, you may be wondering why it’s marketed as having a high smoke point. Contrary to popular belief, an oil’s smoke point doesn’t always guarantee stability under heat. In fact, various studies (like this one) show that after repeated frying sessions, a higher level of harmful byproducts are produced in high-linoleic oils such as sunflower oil (smoke point 450 degrees Fahrenheit) than in high-oleic oils such as olive oil (smoke point 350 to 470 Fahrenheit). If you really want to cook with sunflower oil, high-oleic varieties are a more stable choice.
Finally, if you eat a lot of PUFA-rich oils such as sunflower oil (and other common vegetable and seed oils), they can end up stored in your body fat. The problem there, explains Shanahan, is that when your body later burns these fats for fuel, they break down—yet again—into the same dangerous compounds mentioned above, further driving inflammation and overall metabolic dysfunction. Keep in mind, you may unintentionally be ingesting these oils when you consume packaged foods—so read those labels and aim to consume more whole foods in general.