Seed Oils: What’s the Controversy?

You may have seen an uptick on social media about how seed oils are toxic. But what are they, and why are they so controversial? For years, we’ve been told to replace fats like butter with polyunsaturated fats (like canola and corn oil) because this would lower our LDL, making us heart-healthy. Personally, I never understood this reasoning for a few reasons. Let’s first look at seed oils and the science behind them.

What are seed oils?

Seed oils are plant-based oils such as corn, canola, cottonseed, soybean, grapeseed, sunflower, safflower, and rice bran oil.    They are relatively cheap and are great to cook with because they have a high smoke point and produce great results in cooking and baking.  They may even be a source rich in Vitamin E; some, like canola, may have some polyphenols in their native form.  Polyphenols are plant chemicals that fight for your health—they provide health benefits beyond nutrients.

The Science Behind Seed Oils

Seed oils are rich in omega-6 fat, which is considered an “inflammatory” fat, compared to omega-3 (like fish oil), which is considered an anti-inflammatory fat [1].
Our bodies actually need some omega-6 fat to help us fight infections and heal wounds.  The problem occurs with just how much omega-6 fats we eat.  Research shows that the ideal ratio of omega-6 to omega-3 is anywhere between 1:1 and 2:1 [2].  On average, people in this country consume around 20:1 or higher omega-6 to omega-3 [3]. This puts us in a state of chronic inflammation because there’s significantly more inflammatory fat than anti-inflammatory fat, and this type of ratio carries risks for chronic diseases [3].

In addition, many people think that taking fish oil and/or eating more fish to improve their omega-3 status is healthy.  However, omega-6 and omega-3 fats compete for the same pathway to enter our cells [4].   Therefore, even if someone is eating more omega-3 fat, it is harder for it to get into the cell because omega-6 will outcompete it because there’s more of it.  So, the key to raising one’s omega-3 status is not only to increase omega-3 fats but also to decrease omega-6 fats.  While eating a handful of nuts has repeatedly been shown to incur health benefits, seed oils can easily increase the ratio due to the amount people eat because they’re found predominantly in ultra-processed foods (like chips and snacks).

The problem also resides with processing and how we consume these oils.  Most of the seed oils are not cold-pressed organic oils.  By the time they hit the store shelves, they have been heavily processed, bleached, and deodorized, destroying all the nutrients and benefits of polyphenols.  Canola, corn, cottonseed, and soybean oil are also the top 4 oils to be genetically modified.  This is an issue because GMO crops are made to be “roundup ready,” meaning they’re also heavily contaminated with pesticides.  These chemicals are carcinogenic, disruptive to the gut microbiome, and therefore carry health risks [5], [6].

Omega-6 fat is highly unstable.  It will degrade easily when exposed to light, heat, and air.  Therefore, ANY type of cooking will oxidize this fat and form inflammation.  Even if LDL lowers, it would be prudent to see if it is oxidized.  You can ask your doctor for an NMR test (an advanced lipid test showing oxidation of lipids like LDL).  You can also experiment with this in your kitchen.  Compare your stovetop after repeatedly cooking with oils like canola oil to butter, coconut, or olive oil.  You will likely find that, after a while, your stovetop becomes sticky and hard to clean after cooking with seed oils but will only be greasy after cooking with olive, coconut, or butter.  That stickiness is oxidation. 

Several studies show that seed oils may disrupt the endothelial layer, which is risky for heart disease and may even cause neuroinflammation and dysregulated gene expression [7], [8], [9]. It is important to note that there are conflicting findings and many studies conducted in animals, but until we know more, some caution should be warranted.

Finally, and probably most importantly, seed oils are notoriously used in restaurants and ultra-processed food.  Worse, they are often reused and reheated continuously, which further oxidizes them.  In addition, these types of foods also have other problematic issues, furthering health risks.  These include the chemicals introduced to these foods to make them palatable and stable, which promote dysbiosis and inflammation.  Cutting down on these types of foods will also decrease the omega-6 to omega-3 ratio most dramatically. 

What to Do

It is best to consume oils such as olive oil, avocado oil, and even virgin coconut oil in moderation. These oils are very rich in polyphenols compared to seed oils (as long as you select cold-pressed organic oils). If you absolutely have to consume seed oil, look for organic, cold-pressed oils and don’t cook with them. Use them cold to flavor your salad dressing.

1.        Tortosa-Caparrós E, Navas-Carrillo D, Marín F, Orenes-Piñero E. Anti-inflammatory effects of omega-3 and omega-6 polyunsaturated fatty acids in cardiovascular disease and metabolic syndrome. Crit Rev Food Sci Nutr. 2017;57(16):3421-3429. doi:10.1080/10408398.2015.1126549

2.        Simopoulos A.  The omega 6/omega 3 fatty acid ratio: health implications.  Nutrition-Sante. 2010;17(5):267-275. Doi: 10.1051/ocl.2010.0325  

3.        Simopoulos AP. An Increase in the Omega-6/Omega-3 Fatty Acid Ratio Increases the Risk for Obesity. Nutrients. 2016 Mar 2;8(3):128. doi: 10.3390/nu8030128. PMID: 26950145; PMCID: PMC4808858.

4.        Surette ME. The science behind dietary omega-3 fatty acids. CMAJ. 2008 Jan 15;178(2):177-80. doi: 10.1503/cmaj.071356. PMID: 18195292; PMCID: PMC2174995.

5.        Pesticide Action Network.  Pesticides and Cancer.  Last Accessed 8/29/2024. https://www.panna.org/resources/pesticides-and-cancer/

6.        Walsh L, Hill C, Ross RP. Impact of glyphosate (RoundupTM) on the composition and functionality of the gut microbiome. Gut Microbes. 2023;15(2):2263935. doi:10.1080/19490976.2023.2263935

7.        Siqueira J, Smiley D, Newton C, Le NA, Gosmanov AR, Spiegelman R, Peng L, Osteen SJ, Jones DP, Quyyumi AA, Ziegler TR, Umpierrez GE. Substitution of standard soybean oil with olive oil-based lipid emulsion in parenteral nutrition: comparison of vascular, metabolic, and inflammatory effects. J Clin Endocrinol Metab. 2011; 96(10):3207-16. doi: 10.1210/jc.2011-0480. PMID: 21832112; PMCID: PMC3200250.

8.        Liu X, Tao R, Guo F, et al.  Neuroinflammation is considered the principal pathogenic mechanism underlying neurodegenerative diseases.   JTCM. 2024.  14(5): 522-533. 

9.        Deol P, Kozlova E, Valdez M, Ho C, Yang EW, Richardson H, Gonzalez G, Truong E, Reid J, Valdez J, Deans JR, Martinez-Lomeli J, Evans JR, Jiang T, Sladek FM, Curras-Collazo MC. Dysregulation of Hypothalamic Gene Expression and the Oxytocinergic System by Soybean Oil Diets in Male Mice. Endocrinology. 2020 Feb 1;161(2):bqz044. doi: 10.1210/endocr/bqz044. PMID: 31912136; PMCID: PMC7041656.