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While seed oils are not specifically mentioned in the updated guidelines, Kennedy has previously claimed seed oils were “poisoning Americans”, and the new guidelines point to other types of oil and fats for cooking.
So what are seed oils and are they really the dietary villains that some people make them out to be? And what do the new guidelines say about saturated fat?
What are seed oils?
Seed oils (often sold as vegetable oils), as the name implies, are oils derived from seeds.
There are eight types that have been collectively referred to by critics as the “hateful eight”; canola, corn, cottonseed, grapeseed, rice bran, safflower, soy and sunflower. (Olive oil and coconut oil are classified as oils from fruits.)
These seed oils are readily available at supermarkets, typically affordable and often used in cooking thanks to some of them having a higher smoke point (meaning they can be heated to higher temperatures than other oils without smoking).
Canola oil is classifed as a seed oil as it is made from the seeds of the canola plant.
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Seed oils contain monounsaturated and polyunsaturated fats, which include omega-3 and omega-6 fatty acids.
Critics of seed oils claim they cause inflammation due to their high omega-6 content.
But Emma Beckett, a senior lecturer of nutrition and food science at Australian Catholic University, says a lot of claims against seed oils come from “nuggets of truth that are misunderstood”.
Understanding omegas
Unsaturated fats are essential for life, but they can’t be made by your body so you have to get them through your diet.
There are monounsaturated fats, and polyunsaturated fats, which branch into two types; omega-3s and omega-6s.
Omega-3 fatty acids are found in foods such as fatty fish and plant sources like chia seeds and walnuts, while omega-6 fatty acids are more commonly found in vegetable oils, seeds and nuts.
Traditionally it was assumed omega-3s had an anti-inflammatory effect, while omega-6 were thought to be pro-inflammatory.
But Beckett says this concept is outdated and over simplified.
“That comes from some old data that showed that you need omega-6s to create inflammatory mediators in the body.”
A recent study led by Daisy Crick from the University of Queensland’s Institute of Molecular Bioscience shows the relationship between omega-3 and omega-6 is much more complex than previously thought.
“Surprisingly, although we expected to see an anti-inflammatory effect of omega-3 fatty acids, our results suggest that they also increased levels of some biomarkers of inflammation,” Crick says.
Her study also found that higher levels of omega-6 to omega-3s “could result in a chronic, albeit low-grade, inflammatory state.”
But Beckett says worrying about inflammation markers is “reductionist” and it is better to focus on the quality of foods eaten.
“More important is the inflammatory impact of foods high and low in omega-3s, and the inflammatory status of people with high and low omega-3 intakes; because foods with naturally occurring omega-3s will also be high in bioactive compounds that are anti-inflammatory,” she says.
“This is why we can’t assume supplements – specific and concentrated isolates, or highly processed foods with omega-3s as an ingredient, will improve health in the same way whole foods containing omega-3s will.”
Clare Collins, a laureate professor in nutrition and dietetics at University of Newcastle, agrees.
“As a normal person, you have no chance figuring out what your ratio is, you’re better off looking at what types of fats you use at home,” Professor Collins says.
‘Correlation is not causation’
Arguments favoured by critics are that consumption of seed oils has risen at similar rates to chronic disease incidents.
“Oh, that’s a bit of a tired one, isn’t it?” Professor Collins says.
“Car ownership has gone up as well – air pollution has gone up.
“Correlation is not causation.”
She point out a 2022 metanalysis of randomised control trials found no link between effects of omega-6 fatty acids and the risk of cardiovascular disease.
“There was no difference in the risk of either heart disease or death,” Professor Collins says.
Likewise, the World Cancer Research Fund “found no evidence that eating vegetable and seed oils in moderation increases the risk of cancer. In fact a small amount of fat is essential and helps the body absorb vitamins A, D and E.”
Instead, Professor Collins says, the biggest shift we have seen is not due to seed oils – it is the rise in high body weight and obesity.
“When you eat more ultra-processed foods, you take in more kilojoules and fewer micronutrients. That has a far bigger impact on chronic disease risk than which oil you choose.”
Guilty by association
Children and teenagers are being exposed to the widespread marketing of junk food on social media and it’s having an impact on their diet.
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Ultra-processed foods , also referred to as discretionary or industrialised foods are typically lower in micronutrients and higher in calories.
These foods are usually higher in both saturated fats and trans fats.
Trans fats are linked to cardiovascular disease, diabetes and cancer, and it is recommended that no more than one per cent of daily calories come from trans fats.
Small amounts of trans fats naturally occur in meat and dairy products, but they are largely consumed in processed foods where unsaturated fats have been converted to solids.
Major sources of trans fats include packaged foods like biscuits, crackers, animal products, margarine, potato chips and popcorn.
Seed oils are often used as ingredients in ultra-processed foods thanks to their low cost.
When seed oils are hydrogenated, as they sometimes are for processed foods like pastries, pies, chips and crackers, they can become trans fats.
But seed oils aren’t the only types of oil that can be converted to trans fats, Beckett says.
“Hydrogenating oils makes them act more like saturated fats, but this is true for all hydrogenated oils. And hydrogenated oils are definitely not the major way seed oils are consumed,” she says.
Rather than focus on seed oils, Beckett says it is more important to consider the bigger picture of a diet.
“You find these oils in typically less healthful foods, so it becomes food that is high in seed oils and those foods aren’t healthy, which becomes ‘seed oils aren’t healthy’.
“And because olive oil is typically used on vegetable-based dishes that gives the idea that olive oil is going to be more health-promoting,” she says.
“Demonising seed oil is not the solution.”
“We need to talk more about the whole value of the diet because if we just say seed oils are bad, people just hear ‘We’ll take out the seed oils and make the same unhealthy foods with a different oil that will be better for you’ and that’s not true at all.”
So, what do guidelines recommend?
Health bodies including the World Health Organization, Australian Dietary Guidelines and Dietitians Australia recommend fats of any type should be limited to roughly 30 per cent of daily energy intake.
Of that figure, polyunsaturated and monounsaturated fats should be the primary source, with no more than 10 per cent of total energy intake from saturated fats.
And, while the new US dietary guidelines suggest using olive oil for cooking, as well as using butter and beef tallow that are higher in saturated fats, the guidelines still recommend keeping saturated fat intake to less than 10 per cent of daily calories.
Saturated fats have traditionally been thought of as “unhealthy” fats that are typically solid at room temperature, like butter.
Beckett says while these kinds of fats were OK in small doses, it is the amount consumed that can cause problems.
“Saturated fats are not inherently bad, but too much, or a high intake, is linked to raising levels of blood cholesterol,” she said.
“A higher intake is linked to raising levels of LDL cholesterol – low-density lipoprotein, which is the one that if too high causes increased risk for cardiovascular diseases.”
All guidance on trans fats is aligned from health bodies, recommending no more than one per cent of daily calories come from trans fats.
According to the World Health Organization, a higher intake of trans fats increases the risk of death by 34 per cent and “has no known health benefits”.