Are Seed Oils Actually Bad for You? Heart Health, Frying, and Hexane
By Anne Wilfong, RDN, LD, CEDS | Wilfong Nutrition, Austin, Texas
Seed oils like soybean, sunflower, corn, and canola come up a lot online. You may have heard that they cause heart disease, that they're the problem with processed food, that they turn toxic when you cook with them, or that the chemicals used to make them aren't safe. Some of those concerns don't hold up. Others point to real questions, and a couple come with easy fixes.
Are Seed Oils Bad for Your Heart?
The short answer: the evidence doesn't show that seed oils harm your heart, but it hasn't fully settled whether they help. Most of the debate centers on omega-6, a type of fat these oils contain.
Here's what we know. The American Heart Association recommends replacing saturated fat, like butter, with unsaturated fats, like vegetable oils. People who eat more omega-6 fat tend to have less heart disease. In clinical trials that began in the late 1950s and early 1960s, swapping saturated fat for vegetable oil, mostly soybean oil, lowered heart disease by about 30 percent.
Here's where it gets less clear. Soybean oil contains omega-6 plus a small amount of omega-3. Two trials used oils with almost no omega-3, safflower oil and corn oil, and neither lowered deaths. In one of them, the Sydney Diet Heart Study, the group using safflower oil had more deaths.
Those two trials don't settle it either. In the Sydney study, the margarine people ate was high in trans fat, which harms the heart, and the American Heart Association says that could explain the results. The AHA also questioned the corn oil trial because it was short and many people dropped out.
Bottom line: getting your fats from a mix of sources, including ones with omega-3, has the strongest support. Whether omega-6 on its own helps your heart is still an open question.
What About "Ultra-Processed" Food?
Seed oils also come up in conversations about processed food because they show up in so many packaged snacks and fast foods. In those foods, the oil comes along with refined starch, sugar, and salt, which makes it hard for studies to tell what's causing what. That's a limit of the research, not a judgment on any food or the person eating it. Packaged and convenience foods are often more affordable and easier to get, and they're a normal part of how people eat.
A study at the National Institutes of Health shows how tricky this is. Twenty adults spent two weeks on an "ultra-processed" diet and two weeks on a minimally processed diet. The meals were matched for calories, sugar, salt, fiber, fat, carbs, and protein, and people could eat as much or as little as they wanted. Even with the nutrients matched, people ate more on the "ultra-processed" diet. The study didn't test the oil on its own, so it can't tell us whether the oil played any part.
Where the Real Concerns Are
Reusing oil at high heat. Heating oil breaks it down into compounds called aldehydes, which can damage cells. Levels climb the longer oil is heated and each time it's reused. In one study comparing common cooking oils, sunflower and corn oil, which are high in omega-6, formed more of these compounds than canola oil. Oil that's reused over and over, like in a restaurant fryer, builds up more of these compounds than fresh oil.
How the oil is made. Some seed oils are made using a solvent called hexane, and European law allows small amounts to remain in the finished oil. Europe's last full safety review of hexane was in 1996 and relied on a 90-day study in rats. In 2024, Europe's food safety agency said that review needs updating, and a new one is underway. If this concerns you, look for oils labeled cold-pressed or expeller-pressed, which are made by pressing the seeds instead of using a solvent.
What This Means for You
The heart disease question is a real, ongoing scientific debate. A mix of fat sources, like olive oil, nuts, seeds, fish, and the oils you already cook with, has the strongest support. Strict rules about any single ingredient tend to create more stress around eating than they solve.
Want Help Making Sense of Nutrition Headlines?
If headlines about seed oils, or any single food, are making meals feel stressful, you don't have to sort it out alone. At Wilfong Nutrition, our registered dietitians use a weight-neutral, non-diet approach to help you feel more confident about food. We see clients in person in West Lake Hills and by telehealth across Texas, and we're in-network with BCBS, Aetna, Cigna, United, and Curative.
Frequently Asked Questions
Is canola oil bad for you? Canola oil contains omega-6 along with some plant-based omega-3, so it's different from the safflower and corn oils in the two trials above, which had almost no omega-3. Using a variety of fats has the strongest evidence behind it.
Should I avoid frying with seed oils? Fresh oil builds up fewer harmful compounds than oil reused many times, like in a deep fryer. In one study, canola oil formed fewer of these compounds during frying than sunflower or corn oil.
Do seed oils contain hexane? Small amounts can remain after processing. Europe is re-evaluating hexane's safety because its last full review is decades old. Cold-pressed and expeller-pressed oils skip hexane entirely.
Are seed oils bad because they're in so much processed food? Researchers can't separate the oil from everything else in those foods. The NIH study found people ate more on an "ultra-processed" diet even when nutrients were matched, but it didn't test the oil on its own. That isn't a reason to avoid convenience foods.
Sources
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Farvid MS, et al. Dietary Linoleic Acid and Risk of Coronary Heart Disease: A Systematic Review and Meta-Analysis of Prospective Cohort Studies. Circulation. 2014. https://pubmed.ncbi.nlm.nih.gov/25161045/
Ramsden CE, et al. Use of dietary linoleic acid for secondary prevention of coronary heart disease and death: evaluation of recovered data from the Sydney Diet Heart Study and updated meta-analysis. BMJ. 2013. https://pubmed.ncbi.nlm.nih.gov/23386268/
Ramsden CE, et al. Re-evaluation of the traditional diet-heart hypothesis: analysis of recovered data from Minnesota Coronary Experiment (1968-73). BMJ. 2016. https://pubmed.ncbi.nlm.nih.gov/27071971/
Ramsden CE, et al. n-6 fatty acid-specific and mixed polyunsaturate dietary interventions have different effects on CHD risk: a meta-analysis of randomised controlled trials. British Journal of Nutrition. 2010. https://pubmed.ncbi.nlm.nih.gov/21118617/
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Seppanen CM, Csallany AS. Formation of 4-hydroxynonenal, a toxic aldehyde, in soybean oil at frying temperature. Journal of the American Oil Chemists' Society. 2002. https://link.springer.com/article/10.1007/s11746-002-0598-z
European Food Safety Authority. EFSA's assessment of the need to re-evaluate the safety of hexane as an extraction solvent in food and food ingredient production. Toxicology Letters. 2025. https://www.sciencedirect.com/science/article/abs/pii/S0378427425019277
European Food Safety Authority. Extraction solvents (hexane re-evaluation status). https://www.efsa.europa.eu/en/topics/extraction-solvents
European Parliament and Council. Directive 2009/32/EC on extraction solvents used in the production of foodstuffs and food ingredients. https://eur-lex.europa.eu/legal-content/EN/TXT/?uri=CELEX:32009L0032
Codex Alimentarius. Standard for Named Vegetable Oils (CXS 210-1999). https://www.fao.org/4/y2774e/y2774e04.htm
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