Seed Oils: What the Concerns Are and What the Evidence Shows

Few food topics have become as polarized as seed oils. One camp treats soybean, corn, sunflower and canola oil as a major driver of chronic disease. Another treats the idea as a social media myth. The evidence does not support either extreme, and this article keeps the original concern in view while checking each claim against the research. The title reflects this article’s original editorial stance toward heavily processed oils; the sections below set out what the studies actually found.

What seed oils are

Seed oils are refined oils taken from seeds such as soybean, corn, sunflower, safflower and canola (rapeseed). They are high in polyunsaturated fat, particularly the omega-6 fatty acid linoleic acid (LA). They are cheap, shelf-stable and widely used in packaged foods, restaurant frying and home cooking. Because they are so common in the food supply, much of the debate is really about modern, heavily processed eating, which makes it hard to isolate the oil itself.

Concern 1: Omega-6 and inflammation

The theory is that high omega-6 intake, relative to omega-3, pushes the body toward chronic inflammation. That is biologically plausible, since LA is a precursor to arachidonic acid, which the body uses to make signaling molecules. But when this has been tested directly, the results are not what the theory predicts. A systematic review of 15 randomized trials in healthy people found that none reported significant increases in a wide range of inflammatory markers, including C-reactive protein, fibrinogen and several cytokines, when dietary LA was raised (Johnson & Fritsche, 2012).

The omega-6 to omega-3 ratio is also less settled than it is often presented. One critical appraisal argued that the ratio has theoretical and practical difficulties and suggested that the main shortfall in Western diets is long-chain omega-3 (EPA and DHA), not an excess of omega-6 (Harris, 2018). That fits with a practical point we cover in omega-3 vs. omega-6: eating more fatty fish is a better-supported move than obsessing over a number.

Concern 2: Heat and oxidation

This is the strongest part of the case against seed oils. Polyunsaturated fats are chemically more fragile than saturated or monounsaturated fats. Laboratory work shows that heating polyunsaturated-rich oils to frying temperatures generates aldehydes and other lipid oxidation products, and that these penetrate into fried food. The researchers who describe this call for human trials, because the direct evidence that eating such compounds causes disease in people is still missing (Grootveld et al., 2020). In one laboratory comparison, a monounsaturated-rich algae oil produced far fewer aldehydes than polyunsaturated-rich oils, although that study was funded by the algae oil’s manufacturer (Moumtaz et al., 2019).

So the concern is real chemistry with unproven human consequences. It argues for practical caution: do not reuse frying oil, do not overheat oil until it smokes, and treat repeatedly fried restaurant food as the highest-exposure source, rather than treating a salad dressing or a sauté as equivalent.

Concern 3: Trans fats

The original article linked seed oils to trans fats. Industrial trans fat came from partial hydrogenation, not from liquid seed oils themselves. In 2015 the FDA determined that partially hydrogenated oils are no longer generally recognized as safe, and manufacturers were barred from adding them to most foods after June 2018, with a final compliance date of January 1, 2021 (FDA). The FDA notes that small amounts of trans fat also occur naturally in dairy and meat from ruminant animals. Trans fats are a good reason to read labels, but they are not a property of seed oils.

Concern 4: Heart disease and metabolic health

This is where the evidence is mixed in an interesting way.

In favor of linoleic acid. The American Heart Association’s presidential advisory concluded that randomized trials that replaced saturated fat with polyunsaturated vegetable oil reduced cardiovascular disease by about 30% (Sacks et al., 2017). A pooled analysis of 13 cohorts (310,602 people) found that the highest versus lowest dietary LA intake was associated with 15% lower risk of coronary heart disease events (Farvid et al., 2014). A harmonized analysis of 30 studies using blood and tissue biomarkers found that higher LA levels were associated with lower risk of total cardiovascular disease, cardiovascular mortality and ischemic stroke (Marklund et al., 2019). A pooled analysis of 20 cohorts found LA biomarkers associated with a lower risk of type 2 diabetes (risk ratio 0.65 per interquintile range) (Wu et al., 2017).

Cautionary findings. Two older trials were re-analysed using recovered data. In the Sydney Diet Heart Study, men with recent heart events who swapped saturated fat for safflower oil and margarine had higher death rates than controls (all-cause mortality 17.6% versus 11.8%) (Ramsden et al., 2013). In the Minnesota Coronary Experiment, a corn-oil diet lowered cholesterol but showed no mortality benefit, and a meta-analysis of the comparable trials found no benefit on coronary or all-cause mortality (Ramsden et al., 2016). The authors of the Minnesota re-analysis concluded that incomplete publication has led to the benefits of replacing saturated fat with linoleic acid being overestimated. A Cochrane review of saturated fat reduction found a 21% reduction in cardiovascular events but little or no effect on all-cause mortality (Hooper et al., 2020); see our piece on saturated fat and heart health for that debate in full.

Cohort studies cannot prove cause, and neither can biomarker studies. Still, a large US cohort of 221,054 adults followed for up to 33 years found that higher total plant-based oil intake was associated with 16% lower total mortality, with higher soybean, canola and olive oil individually associated with lower mortality, while the highest butter intake was associated with 15% higher mortality (Zhang et al., 2025). If seed oils were broadly harmful at ordinary intakes, these studies would be expected to show it, and they do not.

The context problem: ultra-processed food

A large share of the seed oil in a typical diet comes from chips, fast food, pastries and packaged snacks. That is a reason to be careful with those foods, though the oil itself may not be the main problem. Seed oil is often a marker of a diet high in ultra-processed food, which carries its own risks; see what the NOVA classification shows. Cutting back on seed oil by eating fewer fried and packaged foods is a good idea for many reasons. Swapping the oil in an otherwise unchanged ultra-processed diet may help much less.

A sensible, evidence-based approach

• Make whole foods the default. Most of the benefit comes from cooking at home and eating fewer fried and packaged foods.
• Use extra-virgin olive oil as your main cooking and dressing oil. It has the most supporting evidence and is more resistant to oxidation than most seed oils.
• Avoid reusing frying oil and limit repeatedly fried food. This is where the oxidation concern is best supported.
• Do not rely on butter as a health upgrade. In the large US cohort, butter was the oil associated with higher mortality, though that was observational.
• Eat fatty fish regularly for long-chain omega-3, which matters more than the ratio.

The honest takeaway

The case against seed oils has a strong core and an overextended edge. The strong core is oxidation when heated repeatedly, and the fact that seed oils are a hallmark of ultra-processed food. The overextended edge is the claim that ordinary amounts of linoleic acid cause inflammation, heart disease, diabetes or cancer: randomized and cohort evidence mostly point the other way, with a few older trials as an unresolved caution. The reasonable position is to limit deep-fried and ultra-processed foods, cook mainly with olive oil, and not to treat a bottle of canola oil as a poison.

References

• Johnson, G. H., & Fritsche, K. (2012). Effect of dietary linoleic acid on markers of inflammation in healthy persons: a systematic review of randomized controlled trials. Journal of the Academy of Nutrition and Dietetics, 112(7), 1029–1041. PubMed
• Harris, W. S. (2018). The omega-6:omega-3 ratio: a critical appraisal and possible successor. Prostaglandins, Leukotrienes and Essential Fatty Acids, 132, 34–40. PubMed
• Grootveld, M., et al. (2020). Potential adverse public health effects afforded by the ingestion of dietary lipid oxidation product toxins: significance of fried food sources. Nutrients, 12(4), 974. PubMed
• Moumtaz, S., et al. (2019). Toxic aldehyde generation in and food uptake from culinary oils during frying practices: peroxidative resistance of a monounsaturate-rich algae oil. Scientific Reports, 9, 4125. PubMed
• U.S. Food and Drug Administration. Trans fat: partially hydrogenated oils no longer generally recognized as safe; compliance dates. FDA
• Sacks, F. M., et al. (2017). Dietary fats and cardiovascular disease: a presidential advisory from the American Heart Association. Circulation, 136(3), e1–e23. PubMed
• Farvid, M. S., et al. (2014). Dietary linoleic acid and risk of coronary heart disease: a systematic review and meta-analysis of prospective cohort studies. Circulation, 130(18), 1568–1578. PubMed
• Marklund, M., et al. (2019). Biomarkers of dietary omega-6 fatty acids and incident cardiovascular disease and mortality. Circulation, 139(21), 2422–2436. PubMed
• Wu, J. H. Y., et al. (2017). Omega-6 fatty acid biomarkers and incident type 2 diabetes: pooled analysis of individual-level data for 39,740 adults from 20 prospective cohort studies. The Lancet Diabetes & Endocrinology, 5(12), 965–974. PubMed
• Ramsden, C. E., et al. (2013). 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, 346, e8707. PubMed
• Ramsden, C. E., et al. (2016). Re-evaluation of the traditional diet-heart hypothesis: analysis of recovered data from Minnesota Coronary Experiment (1968–73). BMJ, 353, i1246. PubMed
• Hooper, L., et al. (2020). Reduction in saturated fat intake for cardiovascular disease. Cochrane Database of Systematic Reviews, (5), CD011737. PubMed
• Zhang, Y., et al. (2025). Butter and plant-based oils intake and mortality. JAMA Internal Medicine, 185(5), 549–560. PubMed

Related reading

• Omega-3 vs. Omega-6: Getting the Ratio Right When Everything Is Fried in Seed Oil
• Saturated Fat and Heart Health: Revisiting an Old Debate With New Evidence
• Ultra-Processed Foods: What the NOVA Classification Actually Shows

This article is for general educational purposes and is not a substitute for personalized medical advice. Anyone managing heart disease, diabetes or cholesterol should speak with a clinician about their diet.