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Seed oils under fire: what the science says
Canola, sunflower and other seed oils have faced growing criticism on social media, with claims they cause heart disease, diabetes and inflammation. Nutrition researchers say the evidence tells a different story.
The omega-6 debate
Critics focus on the high omega-6 fatty acid content in seed oils, arguing these fats trigger chronic inflammation. Yet controlled trials show omega-6 does not raise inflammation, according to Dariush Mozaffarian, director of the Food is Medicine Institute at Tufts University.
"Omega-6 fatty acids produce natural molecules like lipoxins that actually fight inflammation," Mozaffarian said.
A 30-year study of over 200,000 Americans found those who consumed more plant oils, including seed oils, had lower risks of dying from cardiovascular disease or cancer. In contrast, higher butter intake was linked to greater mortality.
Heart health and cholesterol
Observational studies often rely on self-reported diets, which can be unreliable. Matti Marklund, assistant professor of human nutrition at Johns Hopkins, studied blood levels of linoleic acid-an omega-6 fatty acid abundant in seed oils-across 30 studies. He found those with the highest linoleic acid levels had the lowest risk of cardiovascular disease.
"Linoleic acid may improve cholesterol and glucose metabolism, reducing risks for heart disease and type 2 diabetes," Marklund said.
A 2019 analysis of 30 studies concluded that higher linoleic acid levels were associated with a 7% lower risk of heart disease.
Omega-3 vs. omega-6: striking a balance
Western diets often have an omega-6 to omega-3 ratio as high as 50:1, far above the ideal 4:1. A 2022 World Health Organization review linked higher omega-6 to omega-3 ratios with increased risks of cognitive decline and ulcerative colitis. However, the same review found no evidence that omega-6 intake alone raises disease or mortality risks.
"Rather than cutting omega-6, it's better to increase omega-3 intake," Marklund advised, noting both fats offer health benefits.
Processing concerns and cancer risks
Some seed oils are extracted using hexane, a petroleum-derived chemical. While hexane residues could pose risks, the oil undergoes deodorization and bleaching to remove additives. Cold-pressed seed oils avoid this process but are more expensive.
Recent research identified a mechanism by which linoleic acid may fuel the growth of triple-negative breast cancer (TNBC), an aggressive subtype. Nikolaos Koundouros, a postdoctoral associate at Weill Cornell Medicine, found that TNBC cells activate a protein complex linked to tumor growth when exposed to omega-6.
"This doesn't mean everyone should avoid omega-6," Koundouros said. "These fats are essential, and cutting them out entirely could have harmful effects."
Benefits of canola and soybean oils
Canola and soybean oils have been studied extensively. Canola oil has shown anti-inflammatory effects and greater cholesterol improvements than olive oil, long considered the healthiest option. A meta-analysis of 27 trials found canola oil significantly reduced LDL cholesterol and body weight, particularly in people with type 2 diabetes.
Soybean oil has also been linked to improved cholesterol levels. One study found a 6% lower risk of death from all causes for every 5 grams of soybean oil consumed daily.
"Seed oils provide a balanced mix of healthy fats," Mozaffarian said. "They're one of nature's most nourishing gifts."
Misplaced blame and ultra-processed foods
Some critics link seed oils to the rise in obesity and diabetes, but researchers say the real culprits are ultra-processed foods (UPFs), which often contain seed oils alongside excess sugar, salt, and artificial additives.
"The dangers come from starch, sugar, and salt, not the oils themselves," Mozaffarian said. "You can use seed oils healthily in salads or stir-fries."
Christopher Gardner, director of nutrition studies at Stanford, echoed this sentiment: "I'd hate to see people discard seed oils because of this misinformation."
Conclusion: what the evidence shows
While some scientists call for more rigorous trials, others argue the existing evidence is clear. Omega-6 fatty acids in seed oils have been linked to lower risks of heart disease, stroke, diabetes, and all-cause mortality.
"Omega-6 fatty acids are excellent for health," Marklund said. "They improve cholesterol, blood glucose, and insulin levels."
