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Are seed oils bad?

This is an excerpt from Plant-Based Sports Nutrition-2nd Edition by D. Enette Larson-Meyer,Matt Ruscigno.

Concerning “seed oils,” very little research is in support of the hype that seed oils are responsible for all things that are of poor health.50 As a plant-based athlete, it is important to understand the misconceptions and evidence-based conclusions around these oils and their dietary sources.

At the time our last edition went to press, neither of us had heard of the term seed oils as a pejorative. We were certainly familiar with industrial food oils—sunflower, safflower, corn, cottonseed, and canola (rapeseed) oils—and well aware of discussions regarding the balance of omega-6 to omega-3 fatty acid ratio for inflammatory control. We were also aware of concerns surrounding the unknowns of genetically modified foods and, in fact, discreetly recommended that athletes focus on incorporating monounsaturated fats and omega-3s at the expense of omega-6s because of their abundance in the vegetarian diet.51 Fast forward to 2026, and it is hard to talk nutrition without someone asking about seed oils. Many—athletes included—are jumping on the anti–seed oil bandwagon or wondering if they should. Seed oils, as it seems, are the cause of all of our problems—which sources purport are due to their high omega-6 or LA content. Fears of seed oils have even driven anti-seeders back to eating butter, bacon, and beef tallow.

So what exactly is a seed oil? A seed oil refers to oils derived from plant seeds and includes canola, corn, cottonseed, sunflower, safflower, grapeseed, and rice bran oil. Before we consider the evidence against seed oils, however, it is important to point out that these oils vary considerably in their nutrition profile, and that many are not even high in LA. Additionally, if we are being precise, several commonly consumed oils, such as flaxseed, sesame, butternut, and pumpkin oils are also seed oils, but they have escaped scrutiny (maybe for obvious reasons), highlighting that not all seed oils are “evil.” And in case you are wondering, olive, avocado, and hazelnut oils are fruit (not seed) oils because they are extracted from the flesh of the fruit and not the seed itself. Apparently peanuts and soybeans are also seeds, but hazelnuts and walnuts are nuts and not seeds. Yes, this makes our head spin too!

So what is it about seed oils? Is it their LA content, their “seedy” status, or their association with agrobusiness and food manufacturing? To be honest, we are not fully sure. Upon googling and asking artificial intelligence tools why seed oils are “bad,” what came up was that they (1) have a high omega-6 fatty acid content and imbalanced profile of omega-6 to omega-3 fatty acid profile; (2) tend to be processed and refined and form trans fats during (ultra)-processing and cooking; (3) are unstable and prone to oxidation when exposed to heat; and (4) have a negative environmental impact. A link to macular degeneration and infertility also appear in popular discussions, but intake of saturated and trans fats is typically the concern for fertility (particularly sperm vitality), and the macular degeneration link from a single study was likely driven by processed foods containing LA (as nuts that are high in LA were protective).52 While there is likely an element of truth and misconception to the others, let’s take a look at the evidence.

• Omega-6 fatty acids content and imbalanced omega-6 to omega-3 ratio: The targeting of LA may be a little unfair to this polyunsaturated fatty acid that is also considered one of the two essential fatty acids—meaning we need to consume LA in our diets because it cannot be synthesized in our bodies. The adequate intake (AI) for LA is 17 and 12 g/day for male and female adults 19 to 50 years, respectively. The other essential fatty acid is the omega-3 fatty acid ALA that has an AI of 1.6 and 1.1 mg/day for men and women, respectively, in this same age group.2 Note the almost 10 fold difference in requirements. Interestingly, strong evidence from epidemiological and clinical trials has consistently observed the health benefits of polyunsaturated oils in the prevention and treatment of cardiovascular disease. Numerous clinical trials have found that replacing saturated fat in the diet with polyunsaturated fat, mainly from LA, reduces circulating concentrations of “atherogenic” lipids that can cause cardiovascular disease and stroke.53 These lipids include bad LDL cholesterol, total cholesterol, and apolipoprotein B. This should not be surprising because LA downregulates LDL production by the liver and enhances its removal from blood.54 Furthermore, the total amount of available LA determines the lipid-elevating (or atherogenic) effects of other dietary fat components including saturated and trans fatty acids; thus, the more the LA, the greater its lipid-lowering effect. Interestingly, the sources of polyunsaturated fats used in these studies have included quite a variety and include corn,55,56 cottonseed,57,58 sunflower,59 canola,60,61 rice bran,62 and high-oleic soybean oil48—all of which improve atherogenic lipids. And according to a large-scale meta-analysis that looked at 13 oils and solid fats—corn, safflower, sunflower, canola, hempseed, flaxseed, olive, soybean, palm, and coconut oils as well as butter, beef fat, and lard—replacement of 10 percent of Calories from butter with an equivalent amount of safflower, sunflower, canola, olive, flaxseed, corn, or soybean oil all lowered LDL by clinically significant amounts of 10 to 16 mg/dL.63 Clinical trials have also found that when LA is increased, markers of inflammation in the blood—specifically C-reactive protein and interleukin 1b—are not64,65—disputing the false assumption that LA intake causes inflammation. Likely this misconception stemmed from a poor understanding that LA can be elongated to arachidonic acid (AA)—another fatty acid that has some inflammatory effects—but this conversion is regulated and increasing plant-based LA intake does not seem to increase AA. Interestingly, AA is found in animal products with rich sources including butter, lard, and beef tallow. To this end, a recent prospective cohort study in 221,054 U.S. adults found that higher intake of plant-based oils (including canola, soybean, and olive) was associated with a lower total and cancer-related mortality, whereas higher intake of butter was associated with higher mortality, including cancer mortality.66
  
  The ratio of omega-6 to omega-3 fatty acids is certainly something to consider, but this perspective is currently considered outdated as an evidence-based optimal ratio of omega-6 to omega-3 fatty acids has never been identified. Previously, an omega-6 to omega-3 ratio of 6 to 1 was proposed.54 One expert review concluded that such a ratio was a mathematical exercise in dividing good by good.67 Additionally, if you look at the seed oils, canola, flaxseed, and walnuts actually meet this criteria, but olive oil (our star oil) would not. Thus, emphasis may best be placed on consuming sufficient amounts of both omega-3s and omega-6s, knowing that the former may take some effort but the latter will be easily accomplished on a plant-based diet.
  
  
• Processed, refined, and trans fat formation: Seed oils are indeed key ingredients used in many processed foods, and before 2021 would have been the source for partial hydrogenation. If you are a reader of food labels, consider how many times you may have seen “partially hydrogenated corn, cotton seed, sunflower, or safflower oil.” This, however, is guilt by association rather than identity. As processed oils, nevertheless, they are often highly refined and some may be extracted using high heat and chemical solvents such as hexane. This process can create harmful by-products that include oxidized lipids and trans fats68,69 (and may be a source of the trans fats still lurking in our ultra-processed food supply). According to food experts, however, there is movement by the food industry to improve refining techniques and produce oils of greater purity.68 There is also an emergence of minimally processed oils, trait-enhanced oleic oils, and enzyme use in oil processing, with some manufacturers shifting to mechanical (cold-press) extraction methods to preserve more nutrient and reduce harmful by-products.68 Currently flax, sesame, and peanut are processed by cold or expeller techniques. Perhaps keep an eye out for such oils.
  
  
• Instability: By the nature of their double (or unsaturated bonds), polyunsaturated fats are less stable than fully saturated fatty acids and are prone to oxidation, especially when exposed to high temperatures during cooking and frying. While stability of these oils can be enhanced with use of natural antioxidants68 (check the food label), proper cooking techniques and storage of oils are important when preparing food at home. However, in commercial food settings, the handling, heating, and reuse of oils are often less transparent, making it more difficult to assess their degree of oxidation or overall quality. For example, high-heat cooking or frying with low-smoke-point oils may increase the formation of harmful by-products, but repeated use of frying oil (8-10 frying cycles) at high cooking temperatures and with intermittent heating and cooling (as happens when food is prepared for the masses) could be a major concern.69 Vegetable oils should be stored in a cool dark place (cupboard or pantry), preferably in opaque or dark glass bottles or tin-plated steel cans, away from heat and excess light, with the cap tightened after use to avoid oxygen exposure. Less refined oils may also last longer if refrigerated. Oxidized or rancid oils, however, have a distinct off-flavor that you should learn to recognize if you don’t know already. Rancid oils or products should never be consumed. For best quality, plant oils should be used within 6 to 12 months of purchase, and within three to five months of opening, if properly stored.69
  
  
• Environmental impact: The production of seed oils, like many monocultured crops, certainly has the potential to have negative environmental impacts that include deforestation and pesticide use. While there are reports that many seed oil producers are adopting regenerative agriculture techniques, such as crop rotation and reduced pesticide use, it may be important to stay abreast of how your food is produced, including the oils you purchase. That said, however, the movement by some “anti-seeders” to use butter, beef tallow, and lard is not strongly supported by current evidence. Hum, we fail to see how these fats could be better than seed oils. In fact, I see this as an argument for more sustainably grown plant oils—thus hats off to the anti-seed movement for starting these discussions.

More Excerpts From Plant-Based Sports Nutrition-2nd Edition

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