Are Seed Oils Really Toxic? 

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This article was written by Associate Registered Nutritionist (ANutr) Sophie Gastman, and reviewed by Registered Dietitian Maeve Hanan.


Yet another questionable nutrition claim circulating the internet is that seed oils are toxic and should be avoided at all costs. They’ve become such pariahs that some have named them as ‘the hateful eight’ (rapeseed, corn, cottonseed, soy, sunflower, safflower, grapeseed and rice bran oils). People are claiming they can cause anything from headaches to heart disease, but are they really that bad for us?

This article will dig into the science behind seed oils and whether they’re safe to include in our diets. 

What Are Seed Oils? 

Seed oils are a term for the oils extracted from plants, aka vegetable oils. The oils are normally extracted from the seed by a combination of heating and chemical extraction. They’re often found in ultra processed foods, but are also commonly used in cooking and baking. 

Here are some common examples of seed oils:

  • Rapeseed oil 
  • Soybean oil 
  • Sunflower oil 
  • Corn oil 

Are They Actually Unhealthy? 

As is the case with most nutrition related topics, it’s not black and white.

The demonisation of seed oils on the internet is yet another case of the research being misunderstood and spiralling out of control. 

Omega-6 Fatty Acids

The main issue that often arises is linked to the omega-6 content of seed oils. Omega-6 fatty acids are a type of fat called polyunsaturated fat, which are essential in the diet as our bodies cannot make them on their own. 

Vegetable oils contain quite high levels of omega-6, particularly one called linoleic acid. This particular fatty acid plays an important role in promoting skin health and maintaining cell membranes. Linoleic acid itself doesn’t trigger inflammation. Instead, it is converted in the body to another fat known as arachidonic acid. This fatty acid serves as a building block for various compounds, some of which do play key roles in regulating inflammation. 

This is where the concern comes in. If seed oils increase the risk of inflammation, and therefore other diseases that are linked to inflammation like cardiovascular disease and cancer, then surely they must be bad for us? 

Whilst this appears to be a logical conclusion, the science doesn’t back it up. Firstly, only a small percentage of linoleic acid is converted to arachidonic acid in the body. A systematic review on linoleic acid consumption in adults found that high intakes of linoleic did not increase levels of arachidonic acid in the body (1). 

Secondly, research is yet to show a relationship between omega-6 fatty acid intake and inflammation.

In fact, some studies have even suggested that linoleic acid intake may be linked to reduced inflammation (1).

Another study found that linoleic acid intake and arachidonic acid levels didn’t raise the risk of heart disease, but those with higher amounts in the bloodstream were 7% less likely to develop it (2). 

What a lot of health gurus on the internet forget to mention is that arachidonic acid is involved in a lot of other beneficial processes in the body, such as wound healing and blood clotting. It’s also important to remember that inflammation is a welcome response to injury and infection. 

Finally, much of the research against consuming omega-6s is based on rodent studies. Unsurprisingly, humans do not respond to linoleic acid in the same way mice and rats do, simply because we are not rodents.

The Extraction Process

The second concern surrounding seed oils revolves around the extraction process, with sceptics claiming the refined oils end up packed full of chemicals and trans-fats. 

Heat is used to extract oil from the seeds, which can create trans-fats. However, the heating process is brief, and industrially produced oils in the U.K. and the U.S. only contain trace amounts. In fact, there are regulations set out that limit how much trans-fats are allowed in our food chain. 

The only true concern is when seed oils are repeatedly reheated and reused, for example, for deep frying.

This oxidises the oil, which will lead to a buildup of toxic compounds. However, this primarily affects restaurants rather than the average home cook. Fortunately, regulations are in place to safeguard consumers from these risks. 

In terms of other chemicals used to extract the oil, only trace amounts remain in the final product, and again, the oil produced in the U.K. undergoes rigorous quality checks. For example, hexane is used as a solvent in the extraction process, yet the majority of it is effectively removed during processing. Essentially, the regulations and industry practices put in place ensure the overall quality and safety of seed oils is maintained.  

Nutritional Benefits of Seed Oils

Beyond being a fairly cheap and neutral oil to cook with, seed oils do also have a host of nutritional benefits. Rapeseed oil, for example, contains both omega-6 and omega-3 fats, which are vital for our brain and heart health. It has also been shown that the poly- and monounsaturated fats in seed oils can lower LDL cholesterol, and therefore the risk of heart attacks or other cardiovascular issues. (3). One large study in China even found that higher intakes of plant-sourced cooking oils, such as rapeseed and soybean oil in comparison to butter, were associated with lower mortality (4). 

Vegetable oils are also full of Vitamin E – another essential fat-soluble nutrient that the body cannot make itself. In fact, for lots of populations vegetable oils are the main dietary source of Vitamin E intake (5). Soybean and rapeseed oils are high in Vitamin K too; a fat-soluble nutrient necessary for blood clotting and helping wounds to heal. Just 1 tablespoon of rapeseed oil contains 10 micrograms of vitamin K (the daily recommended amount is about 1 microgram per kilo of body weight). 

Should You Avoid Seed Oils at All Costs? 

The short answer is no. The current evidence does not suggest that seed oils are detrimental to our health. 

Despite what the internet says, it’s important to separate fact from fiction and to always approach nutrition with a balanced perspective. Remember, consuming a diverse range of fat sources can offer a wide variety of health benefits from lowering cholesterol to providing essential vitamins. No one single fat source needs to be avoided entirely. If you’d like to read more about why consuming a range of fats is important, check out this article on ‘Why do We Need Fats in the Diet?’. 

References

  1. Rett, B.S. and Whelan, J. (2011) ‘Increasing dietary linoleic acid does not increase tissue arachidonic acid content in adults consuming western-type diets: A systematic review’, Nutrition & Metabolism, 8(1), p. 36. doi:10.1186/1743-7075-8-36. 
  2. Marklund, M. et al. (2019) ‘Biomarkers of dietary omega-6 fatty acids and incident cardiovascular disease and mortality’, Circulation, 139(21), pp. 2422–2436. doi:10.1161/circulationaha.118.038908. 
  3. Schwingshackl, L. et al. (2018) ‘Effects of oils and solid fats on blood lipids: A systematic review and network meta-analysis’, Journal of Lipid Research, 59(9), pp. 1771–1782. doi:10.1194/jlr.p085522. 
  4. Wu, F. et al. (2020) ‘Plant-sourced cooking oil consumption is associated with lower total mortality in a longitudinal nationwide cohort study’, Clinical Nutrition, 39(12), pp. 3703–3710. doi:10.1016/j.clnu.2020.03.031. 
  5. Zhang, Y. et al. (2022) ‘Contribution of tocopherols in commonly consumed foods to estimated tocopherol intake in the Chinese diet’, Frontiers in Nutrition, 9. doi:10.3389/fnut.2022.829091. 


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