Critics – wellness gurus, internet “influencers” and even politicians – are coming out of the woodwork to slam seed oils, even though many of them have been cooking with them for decades, but nutritionists and researchers say they “don’t know where it came from that seed oils are bad”.
In fact, in a study published last week, scientists recommended a switch to plant-based – or seed oils – after their large-scale longevity study suggested that more butter is bad.
Plant oils are better, they said, and substituting these for butter translated into better survival over several decades.
Until recently, reports Medpage Today, most Americans had never heard the term “seed oils”, the catchy term for common cooking oils like canola, soybean and corn oil, that have long been staples in many home kitchens.
But critics have begun referring to the top refined vegetable oils as “the hateful eight”, claiming they’re fuelling inflammation and high rates of chronic diseases like obesity and diabetes.
New Health Secretary Robert F Kennedy Jr has said Americans are being “unknowingly poisoned” by seed oils and has called for fast-food restaurants to return to using beef tallow, or rendered animal fat, in their fryers instead.
In response, some food makers have stripped seed oils from their products and some restaurants – and a salad chain – have removed them from their menus completely.
Many Americans say they now avoid seed oils, according to a recent survey by the International Food Information Council, an industry trade group.
The seed oil discussion has exasperated nutrition scientists, who say decades of research confirms the health benefits of consuming such oils, especially in place of alternatives like butter or lard.
“I don’t know where it came from that seed oils are bad,” said Martha Belury, PhD, RD, an Ohio State University food science professor.
In a Senate hearing last Thursday, Marty Makary, MD, nominated to lead the FDA, called for a closer review of the products.
“I think seed oils are a good example of where we could benefit from a consolidation of the scientific evidence,” he said.
What seed oils are
Simply put, they are oils extracted from plant seeds. They include eight commonly targeted by critics: canola, corn, cottonseed, grapeseed, soybean, sunflower, safflower and rice bran.
Seed oils are typically made by pressing or crushing the seeds and then processing them further with chemicals and heat to remove elements that can leave the oil cloudy or with an unpleasant taste or odour.
The result of such refining is a neutral-tasting oil that is inexpensive, shelf-stable, and able to be heated at a high temperature without smoking, said Eric Decker, PhD, a food science professor at the University of Massachusetts Amherst.
By contrast, olive oil and avocado oil are considered fruit oils. They're often cold-pressed, which retains many of the plant-based compounds that benefit health – but also makes the oils more expensive and prone to smoking at high heat.
Seed oils are composed mostly of unsaturated fatty acids, including monounsaturated and polyunsaturated fat. Most seed oils are high in one type of fatty acid, omega-6, and low in another type, omega-3. Those fatty acids are essential for human health, but our bodies don’t make them on their own, so we must get them from foods.
Claims about seed oils and health
Critics of seed oils make a range of claims many scientists say are not borne out by research.
Some contend that the way the oils are produced leaves behind toxic by-products of a chemical called hexane. Hexane is considered hazardous in a gas form, but Decker said the hexane used as a liquid solvent to extract the oil is evaporated off and that the remaining residue “is very low and would not present a risk”.
Another common claim is that the seed oils’ high omega-6 and low omega-3 composition causes an imbalance that may increase the risk of chronic conditions by boosting inflammation in the body.
Belury, who has studied fatty acids for three decades, said that claim was based on an over-simplification and misunderstanding of the science.
Studies have shown increased intake of linoleic acid, the most common omega-6, does not significantly affect concentrations of inflammatory markers in the blood, she said.
“Scientists who study omega-6 and omega-3 think we need both,” Belury said. “Seed oils do not increase acute or chronic inflammation markers.”
Additionally, research from the American Heart Association and others has consistently shown that plant-based oils reduce so-called bad cholesterol, lowering the risk of heart disease and stroke, especially compared with sources high in saturated fat.
That’s found in new research from Brigham and Women’s Hospital scientists as well. A study of more than 200 000 adults over more than 30 years released recently found that people who ate the highest amounts of butter had a 15% higher risk of dying than those who ate the least. People who ate the most plant-based oils – including seed oils – had a 16% lower risk than those who ate the least.
Daniel Wang, MD, ScD, who led the research, said new modelling data suggests that swopping less than a tablespoon a day of butter for equal calories of plant-based oils could lower premature deaths from cancer and overall mortality by 17%. Such a small daily change could result in “a substantial benefit”, he added.
Seed oil consumption has risen
Groups like the Seed Oil Free Alliance, which charges firms to certify their products are free of the oils, note that seed oil consumption in the US has soared in recent decades and that they provide empty calories that “displace other, more nutritious foods”.
Corey Nelson, co-founder of the group, said that just as consumers can buy low-sodium and low-sugar versions of foods, they should be able to choose products that contain no seed oils.
Food scientists agree consumption of seed oils has increased, but say that’s because they're widely used in fried and fast foods and ultra-processed foods, which comprises nearly three-quarters of the American food supply.
Those foods, which have been linked to a host of health problems, also include high levels of refined grains, added sugars, and sodium. There’s no evidence seed oils themselves are responsible for poor health outcomes.
Research shows olive oil is the healthiest choice, so people should use it “as their cooking style and budget allows”, he noted.
Both proponents and detractors of seed oils agree more nutrition research is needed to resolve long-simmering issues, while scientists said a return to beef tallow, with its high levels of saturated fat, isn’t the answer.
“There is no evidence to indicate that beef tallow is healthier than seed oils,” Decker said. “Remember, tallow is also processed to purify the fat.”
Butter not recommended for longevity
Choosing butter over seed oils is not a good alternative either, said the experts.
In the large study carried out by Wang et al, they found that after 33 years of follow-up, deaths were disproportionately more likely among people in the highest quartile versus the lowest quartile of butter consumption (HR 1.15, 95% CI 1.08-1.22).
The opposite was true for the relationship between mortality and plant-based oil consumption as the latter appeared more protective (HR 0.84, 95% CI 0.79-0.90), even when olive oil was excluded from this category (HR 0.92, 95% CI 0.86-0.98), they said.
Substituting 10g of butter intake per day with an equivalent amount of total plant-based oils was associated with a survival benefit (HR 0.83, 95% CI 0.79-0.86).
This applied to prevention of cancer deaths in particular (HR 0.83, 95% CI 0.76-0.90), whereas cardiovascular disease (CVD) mortality was not affected by the switch (HR 0.94, 95% CI 0.86-1.03), reported Wang and colleagues in JAMA Internal Medicine.
“The findings are closely aligned with the dietary recommendations of the American Heart Association and the Dietary Guidelines for Americans, which advocate for reducing saturated fat intake and replacing it with polyunsaturated and monounsaturated fats to lower the risk of chronic disease,” they wrote.
“The present results indicate that replacing three small pats of butter (about 15g) with one tablespoon of plant-based oil (15g) in the daily diet could contribute to lowering the risk of premature mortality.”
There has long been debate about the health effects of butter. Studies have traditionally linked it to adverse health outcomes, but the evidence has been inconsistent and separate from the plant oil literature.
In the present study, the investigators made it a point to explicitly compare butter with alternative oils.
Yong-Moon Mark Park, MD, PhD, and Yikyung Park, ScD, both of the University of Arkansas for Medical Sciences in Little Rock, emphasised that not all plant-based oils are equally healthy based on the latest results.
In the study, the risk of mortality was significantly reduced with every 5g increase in daily intake of canola oil (HR 0.85, 95% CI 0.78-0.92), soybean oil (HR 0.94, 95% CI 0.91-0.96), and olive oil (HR 0.92, 95% CI 0.91-0.94).
“While olive, soybean, and canola oils are associated with reduced mortality risk, this was not observed for corn and safflower oils,” Park and Park wrote in an invited commentary.
“However, this association may not be robust due to the relatively low consumption of these oils compared with other plant oils in the study population. Additionally, there are other plant-based oils, like palm oil and coconut oil, that were not examined but which have been linked to adverse health outcomes in other studies,” the duo cautioned.
Park and Park nevertheless cited various mechanisms that could explain the study’s general findings favouring plant-based oils.
“Butter’s high saturated fat and cholesterol content, alongside its lower levels of beneficial nutrients, make it less suitable for long-term health. In contrast, plant-based oils, enriched with unsaturated fatty acids (including omega-3 fatty acids) and bioactive compounds, provide anti-oxidative and anti-inflammatory properties that protect against chronic diseases and premature mortality.
“Therefore, substituting these oils for butter may promote longevity and reduce cancer-related deaths. Dietary guidelines should emphasise the use of plant-based oils while minimising saturated fats, like butter, for optimal health outcomes,” the editorialists concluded.
For their prospective population-based cohort study, Wang’s group included more than 220 000 adults who were free of cancer, CVD, diabetes, or neurodegenerative disease at baseline from three large datasets: the Nurses’ Health Study (all women, mean age of 56.1), the Nurses’ Health Study II (all women, mean age of 36.1), and the Health Professionals Follow-up Study (all men, mean age of 56.3).
Eating habits logged in these studies were linked to US mortality data taken from nationwide records and assigned causes based on death certificates and medical records. There were 50 932 deaths identified for the study, including 12 241 from cancer and 11 240 from CVD.
Study participants had answered semi-quantitative food frequency questionnaires every four years, reporting the frequency and quantity of specific foods, types of fats and oils, and the brands or types of oils used for cooking and added at the table over the preceding year.
People in the highest quartile by butter intake self-reported eating just under three small pats of butter per day. Those in the highest quartile by plant oil consumption ate about 1.5 tablespoons of it daily.
The study’s main findings were generally supported when survival analyses were conducted considering butter and oil consumption in incremental increases instead of quartiles.
Total mortality correlated with every 5g daily consumption of butter added to food or bread (HR 1.04 per 5g/d increase, 95% CI 1.02-1.05). Every 10g of more butter per day was associated with higher cancer mortality (HR 1.12, 95% CI 1.04-1.20), while every 10g daily increase in plant-based oils intake was associated with significantly less cancer mortality (HR 0.89, 95% CI 0.85-0.94) and less CVD mortality (HR 0.94, 95% CI 0.89-0.99).
The 10g incremental butter showed no association with CVD mortality. There was no total mortality signal from incremental increases in butter from baking or frying, either (HR 1.00 per 5g/d increase, 95% CI 0.94-1.07).
Wang and colleagues acknowledged the smaller quantities of butter used for baking and frying may have limited their ability to detect the health effects of this practice.
They urged further investigation of the molecular mechanisms underlying the metabolic effects of butter and plant-based oils.
“One notable limitation is a lack of consideration for the role of individual socio-economic status (SES). Although the cohort’s homogeneous SES may reduce SES-related confounding, variations within this study population could still affect dietary choices and health outcomes. Even among individuals with higher SES, food costs may influence dietary choices and contribute to health disparities,” noted Park and Park.
“In this context, it is important to consider the affordability of various healthy fats,” they stressed. “This suggests that more affordable options, such as canola and soybean oils, may serve as accessible alternatives to olive oil, which tends to be more expensive.”
Study details
Butter and Plant-Based Oils Intake and Mortality
Yu Zhang, Katia Chadaideh, Yanping Li et al.
Published in JAMA Network on 6 March 2025
Abstract
Importance
The relationship between butter and plant-based oil intakes and mortality remains unclear, with conflicting results from previous studies. Long-term dietary assessments are needed to clarify these associations.
Objective
To investigate associations of butter and plant-based oil intakes with risk of total and cause-specific mortality among US adults.
Design, Setting, and Participants
This prospective population-based cohort study used data from 3 large cohorts: the Nurses’ Health Study (1990-2023), the Nurses’ Health Study II (1991-2023), and the Health Professionals Follow-up Study (1990-2023). Women and men who were free of cancer, cardiovascular disease (CVD), diabetes, or neurodegenerative disease at baseline were included.
Exposures
Primary exposures included intakes of butter (butter added at the table and from cooking) and plant-based oil (safflower, soybean, corn, canola, and olive oil). Diet was assessed by validated semiquantitative food frequency questionnaires every 4 years.
Main Outcomes and Measures
Total mortality was the primary outcome, and mortality due to cancer and CVD were secondary outcomes. Deaths were identified through the National Death Index and other sources. A physician classified the cause of death based on death certificates and medical records.
Results
During up to 33 years of follow-up among 221 054 adults (mean [SD] age at baseline: 56.1 [7.1] years for Nurses’ Health Study, 36.1 [4.7] years for Nurses’ Health Study II, and 56.3 [9.3] years for Health Professionals Follow-up Study), 50 932 deaths were documented, with 12 241 due to cancer and 11 240 due to CVD. Participants were categorized into quartiles based on their butter or plant-based oil intake. After adjusting for potential confounders, the highest butter intake was associated with a 15% higher risk of total mortality compared to the lowest intake (hazard ratio [HR], 1.15; 95% CI, 1.08-1.22; P for trend < .001). In contrast, the highest intake of total plant-based oils compared to the lowest intake was associated with a 16% lower total mortality (HR, 0.84; 95% CI, 0.79-0.90; P for trend < .001). There was a statistically significant association between higher intakes of canola, soybean, and olive oils and lower total mortality, with HRs per 5-g/d increment of 0.85 (95% CI, 0.78-0.92), 0.94 (95% CI, 0.91-0.96), and 0.92 (95% CI, 0.91-0.94), respectively (all P for trend < .001). Every 10-g/d increment in plant-based oils intake was associated with an 11% lower risk of cancer mortality (HR, 0.89; 95% CI, 0.85-0.94; P for trend < .001) and a 6% lower risk of CVD mortality (HR, 0.94; 95% CI, 0.89-0.99; P for trend = .03), whereas a higher intake of butter was associated with higher cancer mortality (HR, 1.12; 95% CI, 1.04-1.20; P for trend < .001). Substituting 10-g/d intake of total butter with an equivalent amount of total plant-based oils was associated with an estimated 17% reduction in total mortality (HR, 0.83; 95% CI, 0.79-0.86; P < .001) and a 17% reduction in cancer mortality (HR, 0.83; 95% CI, 0.76-0.90; P < .001).
Conclusions and Relevance
In this cohort study, higher intake of butter was associated with increased mortality, while higher plant-based oils intake was associated with lower mortality. Substituting butter with plant-based oils may confer substantial benefits for preventing premature deaths.
Medpage Today article – Huge Diet Study: More Butter, More Problems (Open access)
JAMA Network article – Butter and Plant-Based Oils Intake and Mortality (Open access)
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