Healthful Oils: Myths and Facts About Seed Oils
By Carrie Dennett, MPH, RDN
Today’s Dietitian
Vol. 25 No. 6 P. 18

Social media, podcasts, and the internet can be a mixed bag of nutrition and health information, and one of the latest questionable claims making the rounds is that seed oils cause everything from heart disease and diabetes to leaky gut and brain fog. Sometimes referred to as “the hateful eight,” seed oils are cooking and salad oils pressed from seeds: canola, corn, cottonseed, grapeseed, rice bran, sunflower, safflower, and soy.

The demonization of seed oils has a tangled origin story, combining elements from the Paleo movement, the clean eating trend, and conspiracy theories. But far from being toxic destroyers of health, seed oils are a victim of guilt by association and misunderstanding of the research. And the resulting misinformation can produce its own harms.

“Anti-oil sentiment is nothing new for those who have been in the plant-based community, although now the wider wellness community is zeroing in on seed oils,” says Taylor Wolfram, MS, RDN, LDN, a Chicago-based antidiet, weight-inclusive vegan dietitian. “I’ve seen countless clients whose disordered eating was triggered or worsened by anti-oil claims and fearmongering made by wellness influencers and celebrity doctors. Even folks who are well into their recovery may still carry fear and anxiety about oils and processed foods due to these kinds of messages.”

Dietary Recommendations
Recommendations from the 2020–2025 Dietary Guidelines for Americans, and other health and nutrition experts, to replace most of the saturated fats in our diet with heart-healthy unsaturated fats go back decades. Unsaturated fats include the following:

• monounsaturated fats—found in olive, avocado, canola, peanut, safflower, and sesame oil as well as in avocados, peanut butter, and many nuts and seeds; and

• polyunsaturated fats—found in soybean, corn, and sunflower oil, as well as walnuts, salmon, flaxseeds, sunflower seeds, tofu, and soybeans.

All fats and oils are a mix of saturated, monounsaturated, and polyunsaturated fatty acids (PUFAs). For example, canola oil is about 7% saturated fat, 63% monounsaturated fat, and 30% polyunsaturated fat. Other seed oils tend to be higher in polyunsaturated fats. It’s well known and widely accepted that omega-3 polyunsaturated fats—found in fatty fish, walnuts, flaxseeds, and canola—are good for us. Yet the healthfulness of omega-6 polyunsaturated fats has been a subject of debate.

One criticism of seed oils is that they’re too high in omega-6 fatty acids—especially linoleic acid—and that this can cause inflammation in the body. The reasoning is that linoleic acid turns into a different fatty acid, arachidonic acid, in the body, and arachidonic acid is a building block for compounds that cause inflammation. However, only a small percentage of linoleic acid is converted to arachidonic acid.

“There’s an ongoing debate that omega-6 PUFAs cause inflammation because they’re converted to arachidonic acid, which is a precursor of inflammatory markers. However, a lot of research doesn’t show a relationship with omega-6 fatty acid intake and inflammation,” says Penny Kris-Etherton, PhD, RD, FAHA, FNLA, FASN, CLS, a professor of nutritional sciences at Penn State University and a cardiovascular nutrition researcher. “People need to know that a high intake of linoleic acid does not increase tissue levels of arachidonic acid.” She cites a 2011 systematic review that shows tissue arachidonic acid levels remain constant despite different quantities of linoleic acid intake.1

What antiseed oil influencers miss is that arachidonic acid also is a building block for compounds that fight inflammation. The inflammatory claims mostly are based on research in rodents, but mice and rats don’t respond to linoleic acid the same way humans do. In fact, human research has found that linoleic acid isn’t inflammatory overall.2

A 2017 review and meta-analysis of 30 randomized controlled trials involving 1,377 people found no connection between a diet high in linoleic acid and markers of inflammation.3 A 2019 review of 30 observational studies from 13 countries involving 68,659 people found that high levels of linoleic acid were associated with reduced risk of cardiovascular events.4 A 2020 systematic review of prospective cohort studies that assessed linoleic acid intake through dietary surveys or biomarker testing found that higher linoleic acid intake was associated with a modestly lower risk of mortality from all causes, including cardiovascular and cancer, supporting the potential long-term benefits of PUFA intake.5

“I see the recent increase in hate on seed oils as unfounded and harmful,” Wolfram says. “The literature doesn’t support the claims, and to the contrary there’s plenty of research showing that diets that contain plant oils, including seed oils such as canola oil, can have health advantages.”

Another criticism is that seed oil manufacturers use heat and solvents, such as hexane, to extract oil from seeds, creating unhealthful trans fats and chemical contaminants. But seed oils contain little residual hexane, and heating during processing is brief, so any trans fats created are minimal.6 When it comes to heat, the real problem is when seed oils are reheated, as they are in commercial deep fryers. This oxidizes the oils, and oxidized oils are inflammatory.

Counseling Recommendations
So how should dietitians counsel clients and patients who ask questions about seed oils—or already believe some of the rampant misinformation about them? It’s not hard to find people who say they felt better after they stopped consuming seed oils. But how did they stop consuming them? Many seed oils are used in commercially fried foods and ultraprocessed foods. These foods typically are high in added sugars, sodium, and refined flour, while lacking fiber and nutrients—so if someone stops eating them and feels better, it’s likely because of the foods themselves, not just the oils they contain. In other words, guilt by association.

“Liquid plant oils have been recommended by the Dietary Guidelines for Americans and many scientific societies (such as the American Heart Association and the American Diabetes Association) because of their health benefits,” Kris-Etherton says. “The key is to incorporate them wisely into a healthful dietary pattern and don’t overdo it (because of their relatively high-calorie density).” She says some healthful ways to consume liquid plant oils include lightly tossing salads in an oil-based dressing, preparing plant foods such as lightly seasoned oven-roasted vegetables, or sautéing tofu, chicken, or fish.

Because the four enemies of any cooking or salad oil are heat, light, oxygen, and time, dietitians can advise clients to store all oils in a cool, dark cabinet (especially if they’re in clear bottles) or in the refrigerator if recommended on the label, and to screw caps back on tightly once opened. If clients don’t use much of a particular oil, they shouldn’t buy a big bottle of it. And if an oil smells or tastes rancid or “off,” the best advice is to toss it, no matter how much it cost.

Wolfram says she gets more questions about oil generally—not just seed oils—since she specializes in vegan nutrition. “Many folks in the vegan community have been hearing anti-oil messages for decades due to talking points from popular plant-based doctors,” she says. “This is a myth that I wish would die once and for all. It’s one in particular that I’ve seen cause so much damage to so many peoples’ relationships with food. The tactics that the antioil crusaders use seem to be exceptional at eliciting intense fear responses in people that can play a significant role in their disordered eating or eating disorders.”

Wolfram says when people are interested in learning about oil and health, she shares what the research says and explains how oils can be a healthful part of a balanced eating pattern. “What we usually spend most of our time on is exploring how the antioil messages have impacted them, including their thoughts, feelings, and behaviors around food,” she says. “Many folks come to realize how anti-oil messages drove them to restrictive, disordered eating and feeling very obsessive and anxious about food. They often choose to incorporate oils in spite of any lingering fears, as they know it will aid in their recovery and ultimately have a positive impact on their mental and physical well-being.”

— Carrie Dennett, MPH, RDN, is the nutrition columnist for The Seattle Times, owner of Nutrition By Carrie, and author of Healthy for Your Life: A Non-Diet Approach to Optimal Well-Being.

References
1. Rett BS, Whelan J. Increasing dietary linoleic acid does not increase tissue arachidonic acid content in adults consuming Western-type diets: a systematic review. Nutr Metab (Lond). 2011;8:36.

2. Harris WS, Mozaffarian D, Rimm E, et al. Omega-6 fatty acids and risk for cardiovascular disease: a science advisory from the American Heart Association Nutrition Subcommittee of the Council on Nutrition, Physical Activity, and Metabolism; Council on Cardiovascular Nursing; and Council on Epidemiology and Prevention. Circulation. 2009;119(6):902-907.

3. Su H, Liu R, Chang M, Huang J, Wang X. Dietary linoleic acid intake and blood inflammatory markers: a systematic review and meta-analysis of randomized controlled trials. Food Funct. 2017;8(9):3091-3103.

4. Marklund M, Wu JHY, Imamura F, et al. Biomarkers of dietary omega-6 fatty acids and incident cardiovascular disease and mortality. Circulation. 2019;139(21):2422-2436.

5. Li J, Guasch-Ferré M, Li Y, Hu FB. Dietary intake and biomarkers of linoleic acid and mortality: systematic review and meta-analysis of prospective cohort studies. Am J Clin Nutr. 2020;112(1):150-167.

6. Wakelyn PJ, Wan PJ. Edible oil extraction solvents: FDA regulatory considerations. Inform. 2004;15(1):22-23.