Do seed oils promote oxidative stress?

Seed oils are oils extracted from the seeds of various plants. You’ll recognise them as rapeseed (or canola), corn (maize), cottonseed, grapeseed, soybean, sunflower, safflower, and rice bran oil. They’re affordable, have a high smoke point (meaning they are stable at high temperatures, which makes them suitable for frying), and many are rich in vitamins E and K. So, why are many on social media advocating to ditch seed oils because they supposedly promote oxidative stress? Let’s debunk this myth.

What is oxidative stress?

Oxidative stress happens when there’s an imbalance between free radicals and antioxidants in the body, leading to cell damage. Over time this may lead to the development of diseases such as cancer or cardiovascular diseases. Antioxidants protect the body cells from harmful free radicals. Free radicals are produced naturally in the body during metabolism but also come from sources like smoking and alcohol.

Typical dietary antioxidants are vitamin C, vitamin E, and carotenoids, while minerals such as zinc or selenium are key components of antioxidant enzymes in the body.^1,2 Seed oils are particularly high in vitamin E.^3-5

Fig. 1 – Amount of vitamin E in seed oils. ^3-5

Why are seed oils believed to cause oxidative stress?

Seed oils are high in polyunsaturated fatty acids (PUFAs). Besides seed oils, PUFAs are also found in foods like fish and nuts. These fats are called “polyunsaturated” because they have more than one double bond in their structure. Think of these double bonds as weak links in a chain – they’re areas that can react more easily with other substances, like oxygen, heat, or light. This reactivity can lead to oxidation.

Oxidation happens when oxygen reacts with the double bonds in PUFAs. Imagine cutting an apple and leaving it out – it turns brown because oxygen in the air reacts with the apple’s surface. Similarly, oxidation in oils can produce free radicals.

Although PUFAs are more likely to oxidise than other fats, they come with natural protection. Seed oils, for example, are high in vitamin E themselves, which has antioxidant actions (meaning they prevent oxidative stress). Vitamin E protects PUFAs from breaking down and neutralises free radicals. Eating seed oils is a way to increase our intake of vitamin E, which helps combat oxidative stress rather than promote it.⁶

This vitamin E content is one reasons as to why randomised controlled trials consistently show no significant differences in markers of oxidative stress from seed oils.^6-9 When two diets were compared, one rich in rapeseed oil, and another high in saturated fats (e.g., found in butter), it found that the rapeseed oil diet did not increase the degree of oxidative damage in the body.⁶Similarly, another study looked at whether taking linoleic acid (the main fat in seed oils) would increase oxidative stress in healthy young women.⁷ They had 30 volunteers take either a high dose of linoleic acid (15 grams per day, about 2 tablespoons of sunflower oil), a lower dose (7.5 grams per day), or a control supplement made of palmitic acid (another type of fat) for six weeks, but they found no significant changes after linoleic acid supplementation.

How to prevent oxidation of seed oils?

The risk of oxidation is higher when oils are exposed to heat, light, and air for long period, not just because they contain PUFAs.⁹ Storing seed oils properly – in a cool, dark place and tightly sealed – helps maintain their freshness and prevents oxidation. Using seed oils within their smoke point and avoiding overheating can also prevent the formation of harmful byproducts.

Takeaways

• Seed oils are heart-healthy oils with many benefits: they are affordable, high in vitamin E and, and are stable at high temperatures.
• Seed oils are rich in vitamin E, which acts as an antioxidant and helps protect the body from oxidative stress by preventing free radical damage. Studies show no evidence that eating seed oils increases markers of oxidative stress in the body.
• Oxidative stress can come from many sources, such as smoking, excessive alcohol consumption, and a poor diet overall. Blaming seed oils ignores these larger contributors to oxidative damage. Eating a balanced diet rich in fruits, vegetables, and whole grains (which contain antioxidants) is far more important than avoiding seed oils.
• Most dietary guidelines and international and European health bodies, including the World Health Organization and the European Society of Cardiology, recommend we aim to get less than 10% of our total daily energy from saturated fats (e.g., found in butter, palm oil, and coconut oil) to reduce our chronic disease risk and reductions should be achieved by replacing them with unsaturated fats, particularly polyunsaturated fats found in soybean, rapeseed (canola), corn, safflower, and sunflower oil.^10,11

This article is part of a series on seed oil myths:

Do seed oils cause inflammation?

Do seed oils cause chronic diseases?

Does the processing of seed oils pose a health risk?

References

1. Stanner S, Weichselbaum E. (2013). Antioxidants. In Caballero B (ed.) Encyclopedia of Human Nutrition. Vol. 1, 3rd edition, pp. 88-99. doi: 10.1016/B978-0-12-375083-9.00013-1.
2. Hollman PCH, et al. (2011). The biological relevance of direct antioxidant effects of polyphenols for cardiovascular health in humans is not established. Journal of Nutrition 141:989S-1009S.
3. Composition of foods integrated dataset (CoFID).
4. Dutch Food Composition Database (NEVO). (2021). NEVO-online version 2021/7.1. Accessed 22 January 2025.
5. U.S. Department of Agriculture, Agricultural Research Service, Beltsville Human Nutrition Research Center. FoodData Central. https://fdc.nal.usda.gov/.
6. Södergren E, Gustafsson IB, Basu S, Nourooz-Zadeh J, Nälsén C, Turpeinen A, et al. (2001). A diet containing rapeseed oil-based fats does not increase lipid peroxidation in humans when compared to a diet rich in saturated fatty acids. European Journal o
7. De Kok TMCM, Zwingman I, Moonen EJ, Schilderman PAEL, Rhijnsburger E, Haenen GRMM, Kleinjans JCS. (2003). Analysis of oxidative DNA damage after human dietary supplementation with linoleic acid. Food and Chemical Toxicology 41(3):351-358.
8. Freese R, Dragsted LO, Loft S, Mutanen M. (2008). No effect on oxidative stress biomarkers by modified intakes of polyunsaturated fatty acids or vegetables and fruit. European Journal of Clinical Nutrition 62(9):1151-1153.
9. Foster R, Williamson CS, Lunn J. (2009). BRIEFING PAPER: Culinary oils and their health effects. Nutrition Bulletin 34:4–47.
10. World Health Organisation (WHO)., 2018. Draft Guidelines: Saturated fatty acid and trans-fatty acid intake for adults and children.
11. Piepoli, Massimo F., et al. "2016 European Guidelines on cardiovascular disease prevention in clinical practice: The Sixth Joint Task Force of the European Society of Cardiology and Other Societies on Cardiovascular Disease Prevention in Clinical Pract