The Controversy of Seed Oils

Written by: Martha Junestita

Seed oils have been a controversy in recent times thanks to social media. Some people claim that seed oils are toxic and dangerous which causes inflammatory effects in the human body, and is therefore linked to cancer, obesity, diabetes and numerous other diseases.

Why do people think seed oils are dangerous?
Many claims on social media highlight the use of seed oils in fast-food chains – but this fails to substantiate why, or how, seed oils are either toxic or dangerous – or even demonstrate whether seed oils are actually the problem. We know chemically that many fats – not limited to seeds oils only, undergo structural changes when continuously treated with heat, resulting in molecular changes that can cause oils to exhibit trans-fat properties with enough exposure. 

This does not automatically make seed oils problematic – but does show why some people might believe there is an association. However fast-food chains serve ultra-processed foods (UPFs), heavy in saturated fats, sodium and sugars – why are these factors always ignored, and the finger pointed at seed oils? Since fast food is nearly, or totally absent in its use of fresh and whole foods, including those rich in fibre it seems far more likely the food itself, not the oil, is the issue, and these factors have a much clearer link to CVDs and other diseases.

Professor of medicine at Stanford, Christopher Gardner stated “Until we do a controlled trial where we compare the effects of all this junk food with versus without seed oils, I would argue that any change in your health is due to the combination of all ingredients of the foods, not just the seed oils.”. This highlights the research gap into the causative effect of seed oils on health issues – no such gap exists in comparison to sodium, saturated fats or UPFs.

So, what is seed oil to be exact?
Seed oils are oil extracted from the seeds of various plants, such as sunflower, soybean, flaxseed, sesame seeds and canola. They are commonly used in cooking and food preparation for their flavour and cooking utility. As for their nutritional profile, seed oils are typically high in unsaturated fats, particularly polyunsaturated fatty acids (PUFA) like omega-6. Many seed oils also contain vitamin E, polyphenols and other antioxidants.

Now, what about the processing methods? Whole and unprocessed seeds such as flaxseed are rich in omega-3, an essential fatty acid alongside fibre and polyphenols. These benefits are retained in its whole form, whereas the extraction of oil typically leads to a loss of some nutrients. There are two primary methods for extracting oil in general which are cold press and industrial processes that involve heat and chemical solvents. 

Does it make flaxseed oil bad for you? Processing reduces nutritional quality, but how much we eat in the context of our whole diet is clearly more important. While flaxseed oil can still offer health benefits, whole flaxseed evidently provides a more comprehensive nutritional profile. We use oil for variety of reasons, so in terms of choosing oils, generally cold pressed seed (or vegetable) oils would be the better option as they preserve more of the nutritional content. When cooking with oils, minimise exposure to heat and avoid re-using oils, as repeated heating tends to produce more trans-fat. 

Omega-6 Fatty Acid and Cardiovascular Diseases
Omega-6 fatty acid is polyunsaturated fat, most common form of this is Linoleic Acid (LA). Linoleic Acid classified as “essential” fatty acid because the body cannot produce them on its own. Essential fatty acid must be acquired from dietary sources (food) and can be found in a variety of foods, including nuts, seeds, vegetable oils and seed oils.

Dyslipidaemia (unhealthy levels of fats in the blood) is a dominant risk factor for CVDs, commonly heart attacks and strokes; because the condition leads to inflammation and damaged blood vessels. The association between omega-6 fatty acid intake and dyslipidaemia is well studied – with a 2022 systematic review concluding “Omega-6 fatty acids have beneficial effects on cancers, blood lipoprotein profiles, diabetes, renal disease, muscle function, and glaucoma without inflammation response.” In addition to omega-6 fatty acid consumption, consuming sufficient omega-3 fatty acids like ALA, DHA, and EPA are also necessary to promote the anti-inflammatory effects in the human body.

Such strong evidence leads to dietary recommendations from leading bodies such as The American Heart Association: consuming omega-6 fatty acids (5–10% of total energy) can prevent cardiovascular disease by improving lipoprotein profiles. Additionally, another cohort study from 2010 to 2016 in overweight and obese individuals has shown there is strong evidence that omega-6 fatty acid promotes health by reducing LDL cholesterol and the risk of cardiovascular diseases. As a result, seed oils may play a positive role in supporting a variety of health outcomes.

Conclusion
Evidently, consumption of a well-balanced whole-food diet is more important than whether we have a tablespoon or two of seed oil per day. We should not obsess over single foods such as seed oils in the context of a healthy, balanced diet – one that includes wholegrains, nuts and seeds, vegetables, fruits and modest amounts of fish and white meat. Whether we include seed oils or not, it really depends on our overall dietary circumstances.

The Australian Dietary Guidelines recommend 2 servings of fruits and 5-6 servings of vegetables per day – but only a single serving (65 grams cooked) lean red meat. For unsaturated fats such as nuts, seeds, avocado, olive oil – or even seed oils, the recommendation is 28-40 grams for men and 14-20 grams for women. The evidence and guidelines clearly suggest limiting our intake of saturated fats, sugar, sodium and UPFs – but there’s no concern over small doses of seed oils, particularly given they contain important essential fatty acids.

So, if you’re going to hit the drive thru – don’t sweat the seed oils, just make sure your treat doesn’t turn into a daily dose!

References:

1. Fornari Laurindo L, Fornari Laurindo L, Dogani Rodrigues V, da Silva Camarinha Oliveira J, Leme Boaro B, Cressoni Araújo A, Landgraf Guiguer E, Rucco Penteado Detregiachi C, Maria Cavallari Strozze Catharin V, Federighi Baisi Chagas E, Cavallari Strozze Catharin V, Direito R, Barbalho SM. Evaluating the effects of seed oils on lipid profile, inflammatory and oxidative markers, and glycemic control of diabetic and dyslipidemic patients: a systematic review of clinical studies. Front Nutr. 2025 Feb 7;12:1502815. doi: 10.3389/fnut.2025.1502815. PMID: 39996006; PMCID: PMC11849496.
2. Telle-Hansen VH, Christensen JJ, Ulven SM, Holven KB. Does dietary fat affect inflammatory markers in overweight and obese individuals?-a review of randomized controlled trials from 2010 to 2016. Genes Nutr. 2017 Oct 4;12:26. doi: 10.1186/s12263-017-0580-4. PMID: 29043006; PMCID: PMC5628471.
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5. https://www.eatforhealth.gov.au/