Cooking polyunsaturated plant oils at high temperatures causes chemical changes that produce free radicals, which can change how the body processes these oils.

We analyzed the available evidence and found that cooking polyunsaturated plant oils at high temperatures appears to cause chemical changes that produce free radicals, which may alter how the body processes these oils [1]. All seven studies or assertions we reviewed support this idea, with none contradicting it. When oils like sunflower, soybean, or flaxseed are heated to high temperatures—such as during frying or searing—their molecular structure can break down. This breakdown releases free radicals, which are unstable molecules that can interact with other compounds in the body. These changes might affect how the body metabolizes the fats, potentially shifting their biological effects compared to when the oil is consumed raw or lightly heated. We don’t know exactly how significant these changes are in real-world eating patterns, or whether they lead to measurable health outcomes. The evidence we’ve reviewed doesn’t tell us how much exposure is needed, or if the body can compensate for these changes over time. But the consistent pattern across all seven reports suggests that high-heat cooking does trigger chemical shifts in these oils. What this means for everyday cooking is simple: if you’re using oils high in polyunsaturated fats—like those from seeds or nuts—it may be better to avoid heating them to very high temperatures. For high-heat methods like frying, choosing more stable oils like olive or avocado oil could be a practical alternative. For lower-heat uses like salad dressings or light sautéing, polyunsaturated oils remain a reasonable choice.