What’s wrong with seed oils?

Is the bad rep they get on social media justified?

Well oil be. Credit: Canva / www.realbreadcampaign.org CC-BY-SA-4.0

When making plain Real Bread, it is not necessary to add fats or oils. Their use falls within the Campaign’s definition though, and some bakers use seed oils – particularly in vegan / plant-based versions of laminated (e.g. croissants) and enriched (e.g. Chelsea buns) breads. Alongside palm oil, they are also commonly found in UPF industrial dough products, such as some brands of the sliced white factory loaf.

The main two seed oils used in the UK are sunflower and rapeseed*, but the very broad category also includes corn, cotton seed, grape seed, rice bran and more. High in polyunsaturated (and, in some cases, monounsaturated) fats, low in saturates, versatile and without the same environmental and other ethical questions associated with animal fats, edible oils from seeds seem to have a lot going for them. Why, then, do they receive such negativity from some quarters?

Are seed oils ‘bad’ for you?
Based on masses of research and other credible evidence gathered over decades, the general consensus seems to be no, they are not. As Heart UK puts it: ‘There’s no human evidence that seed oils cause human harm!'

In fact, the weight of evidence suggests that, as part of a balanced diet, moderate amounts of seed oils can be good for you. The devil’s in the details, though, so please read on.

*Commonly grown in North America, canola (a contraction of Canada and ola) are varieties of rape (Brassica napus, B. rapa and B. juncea) developed in the 1960s to reduce the levels of erucic acid (see below).

Finding facts and dispelling myths

This is a summary of responses to some of the various reported health benefits and accusations, drawn from the articles and peer-reviewed research listed at the end.

Omega-6 fatty acids

Omega-6 is an essential polyunsaturated fat. The human body can’t produce this, so we need to get it from food. There is little evidence that omega-6 fatty acids increase inflammation. Some controlled trials have found that they do not, and that they can even give rise to substances that have anti-inflammatory effects.

Linoleic acid

This is an omega-6, which has been found to lower the LDL (aka 'bad' cholesterol) in blood. A 2019 study found people with the highest levels of linoleic acid in their blood had the lowest risk of developing cardiovascular disease. Other studies have concluded higher levels of linoleic acid have a positive effect on blood glucose and might reduce the risk of developing type-2 diabetes.

Omega-3 fatty acids

These occur in some seed oils (including rapeseed) and have been found to help reduce triglycerides in the blood and be anti-inflammatory. Seed oils have lower amounts of omega-3 than omega-6, but the latter does not ‘block’ the positive effects of the former. For people wanting / needing to adjust the balance, the advice seems to be increase intake of omega-3, rather than reduce their omega-6.

A 30-year study of more than 200,000 people in the US found that those who consumed more seed, and other, plant oils were less likely to die from cardiovascular disease or cancer over the course of the study.

Erucic acid

This fatty acid has been linked to heart disease in some animal studies, though not in human studies. EU and UK regulations require rapeseed oil to have less than 2% erucic acid, and many modern, ‘double-low’ cultivars have even lower levels. Research has suggested potentially beneficial effects in neurodegenerative diseases, with call for further investigation.

Hexane

This is a petrochemical solvent sometimes used as part of industrial extraction and refining of vegetable oils. It is then removed then during the process, leaving trace amounts that have to be below ‘safe’ levels. Whether or not you believe this assurance of safety is up to you, though it has been suggested that your expose to hexane is higher when you fill up at a petrol station. Note: hexane is not used in the production of oils that say cold-pressed on the bottle / container. It is also banned by organic food certification standards in the UK and EU.

Oxidation

Polyunsaturated fatty acids are more prone than some other fats / oils to oxidation, which releases free radicals, particularly through prolonged exposure to heat, light or air. An imbalance between free radicals and antioxidants in the body is known as oxidative stress, which may cause cell damage that can lead to non-communicable diseases.

The EUFIC notes, however, that ‘randomised controlled trials consistently show no significant differences in markers of oxidative stress from seed oils.’ As well as using oil well within its use by date, the organisation advises that ways to reduce oxidation are to store it in a tightly sealed container, in a cool, dark place, then avoid overheating when cooking.

Other points to note are that seed oils typically contain antioxidants, which help prevent oxidative stress, and that other sources of free radicals (with little to no health benefits) include smoking and drinking.

Aldehydes and trans fats

At very high temperatures, linoleic acids can break down into aldehydes, which may be harmful. This seems to be more likely when oil is repeatedly heated, cooled and reheated (e.g. reused in deep frying), rather than food that is cooked once.

A 2022 meta-analysis found that heating edible oils up to 200°C had a minimal effect on levels of trans fatty acids, though there was an increase at temperatures between 200°C and 240°C. The internal temperature of bread during baking does not exceed 100°C, though the crust will exceed 200°C during browning.

Correlation vs. causation

The internet echo chamber repeats many arguments that blame increased consumption of seed oils for increased incidence of a range of non-communicable illness and chronic conditions, such as cardiovascular disease, inflammation, obesity. Such claims fail to demonstrate a direct causal link between a particular issue and seed oils specifically.

Yes, as part of the rise of ultra-processed foods (UPFs) seed oil consumption has gone up, but so has intake of calories, other fats / oils, sugars, salt, a whole cocktail of additives (including artificial preservatives and emulsifiers), pesticide residues, microplastics and more. Exactly which of the factors (or combination of factors) contribute to each of the many negative health outcomes being laid at UPFs’ door is a massive, messy tangle that needs unpicking.

Summary

'There is not enough evidence behind claims that seed oils contribute to inflammation and increase the risk of cardiovascular disease.' British Heart Foundation

'When it comes to cancer, there’s no strong evidence to show that seed oils increase risk. What matters most is your overall diet.' World Cancer Research Fund

A diet high in any types of oil / fat is not a great idea. Most of the (credible) evidence to date, however, suggests that small amounts of seed oils are not bad for us, and can even have some health benefits.

After sifting through the evidence, the main question mark left in my mind hangs over a substance used in some industrial refining, though even that is related to the process and not the oils themselves, so doesn’t apply to cold-pressed oil.
Another one is agrochemical use in production of oil seed plants. This applies to pretty much everything on your plate, so if you’re concerned about this much wider bundle of problems, you might consider switching to organic or biodynamic food.