Online forums, influencers, US health officials and now, Harvard researchers – people in every facet of life are weighing in on seed oils.

While some call them toxic and blame them for sky-high rates of obesity and other chronic diseases, some experts say there simply isn’t enough information to make that conclusion. 

Researchers know very little about the benefits and harms of consuming seed oils, which include canola, corn, sunflower, and other refined oils made from the seeds of certain plants.

Dr Jane Zhao, an expert in global and public health at Harvard University and a nutrition researcher focusing on seed oils, says evidence is multi-directional and inconclusive, marked by flawed studies and cherry-picked facts that fit a person’s own narrative, whether that’s for or against.

The oils are mainly composed of unsaturated fat, primarily omega-6 fatty acids, which detractors claim drive up rates of inflammation in the body.

‘The truth is,’ Dr Zhao said, ‘we don’t fully understand what the causal effect of omega-6 fatty acids is, which dominate most seed oils, on disease risk.’

The panic over seed oils has been fueled by a single meta-analysis published in 2016 that found seed oils to be harmful, increasing the risk of some measures of heart disease by 13 percent. This analysis, though, was an outlier.

Dozens of other meta-analyses on the topic show mixed results, ranging from a very mild benefit to heart health and stroke prevention to having no association with increased risk of either.

Seed oils are mainly composed of unsaturated fat, primarily one type, omega-6 fatty acids. Detractors claim that high amounts of omega-6s relative to heart-healthy omega-3s drive up rates of inflammation in the body

‘Taken together, these findings suggest that the case for seed oils as unequivocally “heart-healthy” is not closed,’ Dr Zhao wrote for Stat News.

This lack of conclusive evidence, however, has not stopped HHS Secretary Robert F Kennedy, Jr from shunning seed oils and telling people to do the same – replacing them with saturated fats such as beef tallow.  

Many traditional studies have suggested eating more omega-6 fats, like those found in seed oils, is linked to better heart health, Dr Zhao said.

But these types of studies have some problems: people who eat more of these oils might also be doing other healthy things, like exercising or eating more vegetables, that could influence the results.

When it comes to omega-6 fats, the evidence is mixed. Meta-analyses vacillate between the findings that fats in seed oils are associated with a decreased risk of heart disease or that there is no link at all.

Researchers behind the 2016 report that turbocharged the debate over seed oils analyzed data from the Minnesota Coronary Experiment in the 1970s and combined it with other studies. 

The Minnesota Coronary Survey from the late 1960s and early ’70s involved over 9,000 people and tested whether swapping saturated fat for vegetable oil high in linoleic acid (an omega-6 fat) would reduce heart disease risk.

Results showed they didn’t. 

Authors of the 2016 report then revealed swapping out a saturated fat like butter for a seed oil lowered cholesterol but failed to reduce heart disease — and may have actually increased deaths and heart attacks. 

Evidence is mixed and often shaped by flawed studies and cherry-picked facts to suit either side, says Dr. Jane Zhao, a Harvard global health expert and seed oil researcher

Consuming a diet full of seed oils has also been tied to prostate and colorectal cancer. Some research suggests they may increase inflammation, making it easier for cancer cells to replicate and spread. 

Chronic inflammation also suppresses the immune system’s ability to destroy those abnormal cells.  

Despite links described in those studies, there are plenty that come to the opposite conclusion.  

A 2019 meta-analysis found higher levels of omega-6 in the blood were associated with a lower heart disease and stroke risk, as did an analysis released in 2023.

Dr Jane Zhao is an expert in global and public health at Harvard University and a nutrition researcher focusing on seed oils

But a report in 2024 found no association between omega-6s and heart disease, while a 2018 systematic Cochrane review of trials found no relationship between this type of fat and heart health.

‘The scientific community should be honest about this uncertainty. It’s OK to say: “We don’t yet have all the answers,”’ Dr Zhao said.

‘Doing so doesn’t confuse the public; it respects them. It signals that we’re still asking questions and are open to new evidence, even if it challenges longstanding beliefs.’

Seed oils are everywhere in the Western diet, appearing in their unhealthiest form in ultra-processed foods, salad dressings, and fast food. 

Seed oils are also commonly used to cook healthy Mediterranean diet-style meals, however, which prioritize whole produce, minimally processed ingredients, omega-3 fatty acids in the form of salmon and nuts, and limited carbohydrates. 

Sautéing vegetables in canola oil or making a salad dressing with sunflower oil is far healthier than a fast food meal, and any risks from seed oils are greatly outweighed by the benefits of fiber- and nutrient-rich vegetables

Americans now get roughly eight percent of their daily calories from linoleic acid, the primary omega-6 in seed oils, up from about three percent at the turn of the 20th century.

While seed oil critics draw a link between the ingredient and rocketing rates of obesity, diabetes, and other chronic diseases, given their omnipresence in the US food supply, it’s impossible to blame one single ingredient in processed foods that contain hundreds. 

When all those ingredients, including seed oils, come together, they make ultra-palatable and calorie-dense snacks like chips, cookies, baked goods, and more, which, over time, degrade one’s health.

Sautéing vegetables in canola oil a few times per week or mixing a simple salad dressing with sunflower oil packs a different nutritional punch than a fast food meal, and, overall, any health risks linked to seed oils are outweighed by the benefits of eating fiber- and nutrient-rich vegetables.

‘Until we know more, we should be cautious about making sweeping recommendations,’ Dr Zhao said.

‘In the meantime, the best advice may be this: favor balance over extremes. That means not fearing every bite of croissant, but also not assuming that loading every meal with seed oils is a surefire path to good health.

‘The seed oil debate doesn’t need more hype — it needs more humility. And more science.’