Seed oils like canola and sunflower oil have attracted controversial claims about harmful effects in recent times. Is there any truth to them?

You might have a bottle of sunflower oil or canola (rapeseed) oil stashed away in a kitchen cupboard somewhere. Whether you cook with them or drizzle them over salads, seed oils are popular across the world.

But these unassuming seed oils have become the centre of a heated debate online.

In recent years, seed oils have become the target of countless social media posts, with people claiming that they are "toxic", "poisonous" and, ultimately, are damaging our health. Critics have nicknamed some seed oils ‘the hateful eight' – referencing eight popular seed oils, canola, corn, cottonseed, grapeseed, soy, rice bran, sunflower and safflower – and blame them for causing heart disease and type 2 diabetes.

Are seed oils really the enemy, or is the antagonism towards them unjustified?

Much of the recent criticism of seed oils focuses on their high omega-6 fatty acid content.

Omega 6 fatty acids are essential fatty acids, which means we need them, but can't produce them ourselves. In recent years some scientists have argued that omega 6 can cause chronic inflammation (which can increase the risk of developing diseases including heart disease and cancer).

But controlled trials have found that omega-6 fatty acids do not increase inflammation, says Dariush Mozaffarian, professor and director of the Food is Medicine Institute at Tufts University in Massachusetts in the US.

"New research shows that omega-6 fatty acids give rise to unique natural molecules, like lipoxins, that have powerful anti-inflammatory effects in the body," says Mozaffarian.

Recent research studied the diet and health of over 200,000 people in the US for around 30 years. The researchers found that people who consumed more plant oils (including seed oils) were less likely to die from cardiovascular disease or cancer over the course of the study. On the other hand, those with a higher intake of butter were more likely to die during the same period.

There are numerous observational studies looking at how omega 6 effects our heart health – where scientists look at data on diet and health, and find associations between the two.

But some observational studies rely on people's own accounts of what they eat, says Matti Marklund, assistant professor of human nutrition at Johns Hopkins Bloomberg School of Public Health in the US. And this, he adds, can be problematic because people may misremember, or even be dishonest, about their dietary habits.

Another way to measure omega 6 intake is to measure the average amount in the individual components and ingredients in a person's diet. However, Marklund adds, it can be difficult to translate what people say they have eaten into certain quantities.

Numerous studies investigating the effects of omega 6 on our health focus on linoleic acid, an omega 6 fatty acid found in high amounts in seed oil, that has been found to lower the 'bad' LDL cholesterol in our blood.

In a 2019 study, Marklund instead focused on the levels of fatty acids in the blood of participants from around 30 observational studies – some which followed people for up to 30 years – and looked at how many developed cardiovascular disease and died from it. He found that those with the highest levels of linoleic acid in blood had the lowest risk of developing cardiovascular disease.

There is some confusion regarding omega 6 and heart health, says Christopher Gardner, director of nutrition studies at the Stanford Prevention Research Center in the US.

This partly stems from omega 6's role in the process of blood clotting, which Gardner says people mistakenly only associate with strokes and heart attacks. Omega 3, he says, tends to be more blood-thinning. "If you had a wound in your hand, you'd want it to clot," he says. "You need balance."

Meanwhile, scientists concluded in a 2019 analysis of 30 studies that people with higher amounts of linoleic acid in their blood were 7% less likely to develop heart disease.

"Linoleic [acid] might improve cholesterol to reduce the risk of........

© BBC