Seriously, Are Seed Oils Bad For You? A Dietitian Weighs In
Seed oils are one of the recent additions to the growing list of foods you can absolutely never eat again… according to the internet. You’ve heard the claims, read the social media posts, and are probably wondering: Are seed oils bad for you?! Let’s take a look at what the evidence has to say to dispel some of the common myths around sees oils (and hopefully put your mind at ease).
Seed oils are the latest food controversy to take social media by storm. At the time of writing this article, the hashtag #seedoils has nearly 50 million views on TikTok—with many of the top videos under this hashtag featuring claims that these oils are toxic, inflammatory, and at the root of obesity and other chronic diseases. It’s tricky to decipher between fact and fiction when attempting to answer the question: Are seed oils bad for you? So, we’ve done the deciphering for you.
What are seed oils?!
A seed oil is any vegetable oil extracted from a plant’s seed. Most of the vegetable oils you’re probably familiar with are seed oils (except for olive oil—it comes from the fruit). More recently dubbed the “hateful eight”, the most common seed oils include canola, corn, cottonseed, grapeseed, rice bran, safflower, soy, and sunflower oils.
Seed oil nutrition facts
All seed oils have a slightly different nutritional makeup, but one common denominator is that they are high in omega-6 fatty acids. Omega-6s are one of the two major classes of polyunsaturated fats—the other being the beloved omega-3 fatty acids.
However, despite what you may have been led to believe, no seed oil is 100% omega-6s. All oils contain a blend of fatty acids; for seed oils, the fatty acid profile typically goes like this (in order of highest to lowest percentage): omega-6s (mostly linoleic acid), monounsaturated, saturated, and finally, small amounts of omega-3s.
What is linoleic acid? There are numerous fatty acids in the omega-6 family, but the most predominant omega-6 fatty acid (and, well, fatty acid, period) in seed oils is called linoleic acid. Linoleic acid constitutes approximately 40 to 80% of the total fatty acid content of most seed oils, with the exception of canola oil, which is approximately 20% linoleic acid.
Why are seed oils “bad”?
Well, they’re not. But to answer our question—seriously, are seed oils bad for you?— it’s important to first understand how they got such a bad rap in the first place. Seed oil antagonists often like to back their arguments with a graphic or statement showing that seed oil consumption increased drastically in the last century, mirroring the exponential rise in chronic diseases like obesity and heart disease. (We’re talking to you, shirtless man in the grocery store screaming at the seed oils.)
It is partly true—over the course of the twentieth century, there was a dramatic increase in the intake of seed oils, particularly soybean oil, as a result of numerous factors, including recommendations to consume more polyunsaturated oils instead of animal fats (for health reasons) and efficiency of processing techniques for seed oils, such as solvent extraction and partial hydrogenation.
This rise helped markedly increase the dietary intake of linoleic acid—remember, the main omega-6 in seed oils—to the current estimated level of about 7% of total dietary energy. In the United States, this level falls in line with with the recommendations for linoleic acid intake from the American Heart Association (5 to 10% of total energy) and the World Health Organization (2.5 to 9% of total energy). Yet, the high content of omega-6s, particularly linoleic acid, of seed oils remains one of the main points of contention regarding their potential role in inflammation and chronic disease.
And yes, there’s no denying that chronic disease rates have been on the rise. But you know what they say, correlation does not necessarily mean causation. As you’re about to find out…
So, are seed oils inflammatory?
The (very convincing) arguments you’ll hear for reducing seed oil consumption are often framed around one or more of the following claims:
- In the body, the linoleic acid in seed oils is converted to arachidonic acid, which is a precursor for inflammatory compounds
- Skewing the omega 6 to 3 ratio too far in favor of omega-6s perpetuates chronic low-grade inflammation
- Seed oil processing methods result in toxic byproducts
- Seed oils are highly unstable and oxidize easily when heated
There’s often a kernel of truth in every good myth, so let’s break down each one and see what the evidence has to say.
Claim: linoleic acid in seed oils is converted to inflammatory compounds
Here’s the argument: In the body, the linoleic acid in seed oils is converted to arachidonic acid—another omega-6 fatty acid that is used as a building block for other bioactive compounds, some of which are proinflammatory.
Although it is true that linoleic acid (from any source) can be converted to arachidonic acid in the body, arachidonic acid is converted into many different bioactive compounds, including ones with anti-inflammatory roles, too.
Even so, tracer studies have found that only a small amount of dietary linoleic acid—just ~0.2%—is converted to arachidonic acid. And a systematic review of studies measuring changes in dietary linoleic acid intake and the subsequent changes in tissue levels of arachidonic acid found that dietary intakes of linoleic acid greater than what is required to prevent essential fatty acid deficiency do not appreciably increase arachidonic acid levels in tissues. Arachidonic acid levels are kept at a relatively constant level in the body, even with large increases in dietary linoleic acid.
Claim: seed oils throw the omega 6 to 3 ratio out of whack
You’ve seen it before—health influencers up in arms over how “out of whack” our current omega-6 to omega-3 ratio is. They express concerns that a high omega-6 intake can increase the risk of chronic disease by skewing the omega-6 to 3 ratio too far in favor of omega-6s, promoting a proinflammatory state in the body. (If you’ll recall, omega-3s are the polyunsaturated fat with the sunny reputation.)
Some have compared today’s estimated omega-6 to omega-3 ratio of about 10:1 with a presumed ancestral ratio of 1:1 to 2:1 and have concluded that this increase is largely to blame for the mirroring rise in chronic diseases, such as cardiovascular disease and obesity. But are omega-6-rich seed oils the driving factor here, or is it more likely that the Western Diet as a whole, featuring increased consumption of ultra-processed foods and little fruits, vegetables and fiber, drives this association?
Think about it—the top sources of omega-6 fats in the typical Western diet are seed oils, but Americans aren’t getting their seed oil intake from cooking broccoli and chicken breast in a little sunflower oil for dinner, they’re getting it from the drive-thru lane and the snack aisle. Seed oils are often found in foods, like fried and ultra-processed packaged foods, that are high in calories, total fat (including saturated fat), and/or sugar, and devoid of fiber and other health-promoting and satiating nutrients. So, yes, overconsumption of these foods drives the omega-6 to 3 ratio up, but it also significantly increases calorie intake.
And here’s another thought: Are omega-6s too high, or are omega-3s too low? Why is the focus always on decreasing omega-6s? Why not increasing omega-3s?Are omega-6s too high, or are omega-3s too low? Why is the focus always on decreasing omega-6s? Why not increasing omega-3s?
Regardless, use of the omega-6 to -3 ratio as a meaningful measure to assess the inflammatory potential of the diet has been questioned repeatedly by experts, and for good reason. It’s incredibly difficult to accurately define and measure this ratio and its use is based on invalid assumptions, including the one discussed and rebutted above.
Another assumption being that high linoleic acid intake will inhibit the conversion of the omega-3 alpha-linolenic acid to the important, anti-inflammatory omega-3s eicosapentaenoic acid (EPA) and (docosahexaenoic acid) DHA in the body.
While it is true that linoleic acid and alpha-linolenic acid compete with each other for the enzymes involved in the production of arachidonic acid and EPA/DHA, the extent to which linoleic acid may inhibit this conversion is still unclear, although likely more so for EPA than DHA. In any case, conversion of alpha-linoleic acid to EPA/DHA is already quite low, and limiting linoleic acid intake would not lead to the significant increases in EPA and DHA that can be obtained from moderate consumption of dietary sources of EPA and DHA, such as fatty fish.
The takeaway: Having adequate omega-3 intake in conjunction with adequate omega-6 intake is essential. That said, don’t subscribe to this distracting and misguided “ratio-thinking”. Instead, focus on increasing intake of dietary sources of omega-3s, which are traditionally less emphasized in the diet.
Claim: seed oil processing results in toxic byproducts
Most seed oils on the market are refined—meaning they use solvents and heat to extract the oil from the seeds. This creates a final product with a neutral flavor and light color.
There are concerns that trace amounts of the chemical solvent hexane used to extract the oil can remain in the final product. However, any amounts left are microscopic (estimated at less than 1 ppm), and there is no evidence of negative health effects from consuming these trace amounts. Plus, most of the hexane you take in on a daily basis doesn’t come from foods—you breathe it in (vehicle fumes being the main contributor).
There are also concerns that when the oil is subjected to very high heat during the final processing step, a very small amount of unsaturated fats are transformed into trans fats. This can occur in the processing of all vegetable oils, but manufacturers have come a long way with this process over time to minimize this. Analysis of common cooking oils indicates that this may be more common in corn oil, whereas other oils (such as soybean, rapeseed and sesame) do not contain detectable levels of trans fats.
The refining process does decrease the amounts of good-for-you nutrients, such as omega-3s, vitamin E and other antioxidants. However, this isn’t of great concern since we obtain much higher amounts of these nutrients from whole food sources. It’s of even less concern if you use a variety of oils in your recipes. For example, extra-virgin olive oil is a great choice for salad dressings and marinades where its flavor can shine, whereas canola oil might be a better choice for baking when you want a neutral flavor.
Unrefined versions of seed oils that don’t use hexane or high heat do exist, and they are typically labeled “cold-pressed”. They have a more distinct flavor and darker color and retain more of healthful nutrients, but can be difficult to find and are often significantly more expensive.
Claim: seed oils are highly unstable and oxidize easily when heated
Heating seed oils at high temperatures and/or over long periods of time can generate high levels of inflammatory polar compounds. This is fact—and one that is often repeated out of context and used to vilify seed oils. Let’s put it into some context.
In a study investigating the oxidative stability of common cooking oils, canola oil was found to have a total polar compound level of 27.5%—above the permissible limit for human consumption or 25%—when heated to a temperature of ~464°F over 20 minutes. However, canola oil is unique amongst seed oils in that it has a lower omega-6 content and higher omega-3 content, the latter of which is the most vulnerable to oxidation, followed by the former. The other seed oils tested did not exceed permissible limits after 20 minutes.
In the same study, canola oil heated to ~356°F—in the range of medium to medium-high heat stove cooking—had a total polar compound level of 7% at 20 minutes. And after 6 hours at this temperature, the total polar compound level was still below permissible limits at 17%. (For comparison, coconut oil and extra virgin olive oil had a total polar compound level of approximately 10% at 6 hours.)
And it’s worth mentioning that these tests were carried out without food being cooked, which the authors noted may have allowed for a greater impact on oil oxidation.
The takeaways here are simple: Using seed oils for general at-home pan-frying/sauteing is unlikely to be of concern as lower temperatures are typically used. Try not to make fried foods (especially from restaurants) a regular part of your diet and don’t reuse your cooking oils.
Storing your oils properly in a cool, dark place away from sunlight and any heat sources and purchasing oil in quantities you can use within the next 3 to 5 months or so (once opened) can also help decrease the formation of inflammatory compounds.
Bottom line: are seed oils inflammatory?
Ultimately, even if these mechanisms and theories held weight at a mechanistic level, they still must be linked to actual human outcomes, and human data does not support the notion that these omega-6-rich seed oils are inflammatory.
A meta-analysis of 15 randomized controlled trials (RCTs) investigating the effects of linoleic acid on inflammatory biomarkers concluded that there was virtually no data from RCTs among healthy adults to show that the addition of linoleic acid to the diet increases markers of inflammation. Similarly, a separate meta-analysis of RCTs investigating the long-term effects of polyunsaturated fatty acids on inflammatory bowel disease and inflammatory markers concluded that the available data suggests no effect of omega-6 PUFAs on inflammatory markers in patients with IBD.
Seed oils and heart disease
It has been established that linoleic acid reduces major risk factors for coronary heart disease (CHD)—notably, total and low-density lipoprotein (LDL) cholesterol—when replacing saturated fat in the diet. For this reason, the American Heart Association has long supported the intake of 5% to 10% of energy from omega-6s—particularly linoleic acid, and, in the context of other lifestyle and dietary recommendations—to reduce the risk of heart disease.
However, seed oil antagonists have raised concerns that such advice actually increases heart disease risk. The logic follows that because heart disease has an inflammatory component, reducing linoleic acid intakes will reduce the inflammatory potential of the diet, therefore lowering risk. And, given that seed oils are a significant contributor of linoleic acid in the diet, it has been advised to avoid these oils for cardiovascular benefit and disease risk reduction.
This advice is often backed by the mechanistic speculation on seed oils and inflammation discussed previously and the results from controversial meta-analyses of randomized-controlled trials (RCTs) demonstrating no effect on or possible increased risk of coronary outcomes from higher omega-6 intakes. However, these studies should be interpreted with caution due to the inclusion of RCTs—notably the Sydney Diet Heart Study and the and Minnesota Coronary Experiment—with numerous design flaws, including short durations, intermittent treatment administration, large percentages of withdrawals, and probable confounding of the LA interventions by trans fats.*
*When these trials were conducted in the 60’s and 70’s, partial hydrogenation of vegetable oils, which results in the production of trans fats, was common practice. Trans fat intake is a well-known risk factor for heart disease; and, so, the results may have been confounded by trans fat in the special margarines used for the diet inventions.
A well-performed meta-analysis (and presidential advisory from the American Heart Association) that excluded these specific RCTs found that increasing linoleic acid intake, primarily through polyunsaturated vegetable oils, to replace saturated fats in the diet significantly reduced cardiovascular disease events.
Considering the practical challenges of conducting and interpreting large-scale intervention trials, such as those included in the aforementioned meta-analysis, observational studies using biomarkers of linoleic acid intake are also of high importance when examining the cardiovascular effects of linoleic acid.
In a recent meta-analysis of 30 prospective cohort studies, higher circulating and adipose tissue linoleic acid were associated with lower risks of major cardiovascular events. And in a separate large meta-analysis reporting on 44 prospective cohorts, higher linoleic acid intake, assessed by biomarkers or diet surveys, was associated with a modestly lower risk of CVD mortality.
Bottom line: are seed oils bad for you?
Recommendations to eliminate seed oils from the diet to reduce inflammation and cardiovascular disease risk are based on minimal direct evidence. On the contrary, there is data that shows either neutral or beneficial effects of dietary intakes or blood levels of linoleic acid—the main fatty acid in most seed oils— on markers of inflammation and chronic disease risk. Yet, despite this, misinformation surrounding seed oils continues to circulate on social media.
So, what should you do? Given that the Western diet is already rich in omega-6s and that oils, in general, are calorically dense, offer fewer health-promoting nutrients than whole fat sources, and should be consumed in moderation, there’s probably no need to go out of your way to increase seed oil intake—unless your current diet includes high amounts of saturated cooking fats, in which case replacing some with omega-6-rich oils may significantly lower your risk of cardiovascular disease.
Ultimately, the current evidence suggests there is absolutely no reason to stress over seed oils being featured on the ingredients list of your favorite food products or using them in recipes and meals where a neutral-flavored oil is preferred.
And since having adequate omega-3 intake in conjunction with adequate omega-6 intake is essential, a much more worthwhile place to focus your efforts is likely on increasing intake of omega-3s, which are traditionally less emphasized in the diet.