Deep Dive into Seed Oils and Their Health Impacts

by Dr. Bojana Jankovic Weatherly, Melissa Reade MS, CDN, CNS, NB-HWC | June 23, 2025 | Articles, Cardiometabolic Health, Nutrition and Food Prep, Optimal Health

Seed oils have sparked heated debate in recent years, with some hailing them as heart-healthy staples and others warning of hidden health risks. This divide makes it all the more important to unpack the science behind their composition, metabolic effects, and long-term health implications.¹ To truly understand and evaluate the impact of omega-6 fatty acids– predominantly found in seed oils– it is essential to examine the biochemical composition, physiological effects, and their broader scientific landscape. This includes assessing the dietary balance between omega-6 and omega-3 fatty acids, the methods used to refine and process seed oils, and their influence on key health outcomes such as cardiovascular disease, metabolic disorders, and autoimmune conditions. This review synthesizes the latest evidence to offer a nuanced, research-based perspective on the role of seed oils in human health and nutrition.

What Are Seed Oils?

Seed oils are vegetable oils that are extracted from the seeds of certain plants.

Common examples include:

Canola oil (from rapeseed)
Soybean oil
Sunflower oil
Safflower oil
Corn oil
Cottonseed oil
Grapeseed oil

These oils are widely used in commercial cooking, baking, and processed foods because they’re neutral in flavor, inexpensive, shelf-stable, and have a generally high smoke point.

Why Are Seed Oils Controversial?

The controversy largely centers on three claims:

Inflammation: Seed oils are rich in omega-6 polyunsaturated fatty acids (PUFAs), and especially linoleic acid. While omega-6s are essential for health, some argue that modern, Western diets contain too much omega-6 compared to omega-3s, potentially promoting inflammation. The omega 6/3 ratio is something that can be quantified on a lab test, and in our medical practice, we are able to assess if someone is consuming excess omega 6’s relative to omega 3’s.
Processing: Most seed oils are refined using heat and solvents like hexane, which may cause oxidation. Oxidized fats can produce harmful compounds, especially when oils are repeatedly heated such as ultra processed foods (UPFs).
Association with Chronic Disease: Some claim seed oils are directly linked to heart disease, obesity, and metabolic issues, citing recent research to bolster positions in favor of minimizing PUFA intake in the setting of low omega-3 diets.

These points are further discussed and examined below.

The Omega Balance: A Nutritional Tightrope

In the context of modern dietary patterns, many populations continue to consume high amounts of saturated fats, primarily from animal products and processed foods, which are consistently associated with increased risk of cardiovascular disease. Leading health organizations, including the American Heart Association, recommend replacing saturated fats with unsaturated fats, particularly PUFAs from sources like seed oils, nuts, and fatty fish, to improve lipid profiles and reduce the risk of heart disease. ²³ Despite controversy, large scale epidemiological and clinical studies continue to support moderate intake of seed oils as part of a heart healthy diet.⁴

Omega-6 fatty acids, a type of PUFA found abundantly in seed oils, are essential for human health. However, it is estimated that most Americans and Westernized populations consume them in excess– often up to 12-15% or more of daily calories– due to widespread use of these oils in processed and packaged foods. ³ ⁵ ⁶⁷However, these recommendations have been challenged recently, with researchers calling for a more distinct definition of PUFAs as well as their unique properties and health impacts.¹

About 90% of PUFA intake is from omega-6, primarily linoleic acid from seed oils.⁸ The American Heart Association currently recommends that omega-6 PUFAs make up 5-10% of total daily energy, primarily to replace saturated fats.⁹ This guideline is based on the evidence suggesting that this level of intake can reduce the risk of coronary heart disease when part of a balanced diet that limits saturated fats.

For reference, this translates to approximately 11-22 grams of omega-6 fats per day on a 2,000-calorie diet. To put that in perspective: one tablespoon of safflower oil provides about 9 grams of omega-6s; one ounce of sunflower seeds also provides 9 grams; and one ounce of walnuts contains about 11 grams. ⁴While this intake can be beneficial within the right dietary context, some experts have called for a more nuanced evaluation of PUFAs, emphasizing the need to differentiate between various sources and types of omega-6s, and their unique health effects.

Adding to the complexity is the omega-6 to omega-3 fatty acid ratio. Historically, human diets maintained a near 1:1 balance between these two types of essential fats.² ¹⁰ In contrast, the modern western diet often features a ratio ranging from 10:1 to 20:1– largely due to increased omega-6 consumption coupled by inadequate omega-3 intake.¹¹ This imbalance may promote chronic inflammation, contributing to a range of health issues including cardiovascular, autoimmune, and metabolic diseases. One potential mechanism of action involves competition for shared enzymes– such as delta-6-desaturase, cyclooxygenase (COX), and lipoxygenase (LOX)– which metabolize both omega-6 and omega-3 fatty acids. When omega-6 intake dominates, these enzymes favor the production of pro-inflammatory eicosanoids from arachidonic acid, while anti-inflammatory compounds derived from omega-3s (like EPA and DHA) are underproduced. Balancing the omega-6 to omega-3 ratio closer to 1:1 to 4:1 may help regulate inflammation and reduce the risk of chronic disease.¹² Research suggests that a more favorable ratio– ideally between 1:1 and 4:1– may help regulate inflammation, as omega-3s offer a vital anti-inflammatory counterbalance to omega-6s . ¹³ ¹⁴

However, it is important to recognize that there is no universal consensus on the “ideal” omega-6 to omega-3 ratio; reputable sources cite no higher than 10:1 ratio and 1:1 up to 5:1 as an acceptable upper limit. ¹⁰ Ultimately, the focus should not be on eliminating omega-6 fats altogether, but on restoring dietary balance by increasing intake of omega-3-rich foods.

The best way to boost omega-3 status is by consuming low-mercury fatty fish (such as salmon, sardines, or mackerel) at least twice per week, as these are rich sources of eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA)—the two most bioactive forms of omega-3s. For individuals who do not regularly consume fish, supplementation with high-quality omega-3 oils is recommended. Look for products that specify EPA and DHA content, as these long-chain fatty acids are most directly associated with benefits for cardiovascular, neurological, and inflammatory health.

Plant-based sources of omega-3s—like flax seeds, chia seeds, and walnuts (which also contain a higher amount of omega 6 fatty acids: 2.5g of omega 3’s and 10g of omega 6’s per ounce of walnuts)—provide alpha-linolenic acid (ALA), a short-chain omega-3 fatty acid. However, the body converts ALA to EPA and DHA inefficiently: typically less than 10% of ALA is converted to EPA, and only 2–5% is converted to DHA under optimal conditions¹⁵^. Therefore, while plant sources offer supportive nutrition, they are not sufficient alone to meet therapeutic needs for EPA and DHA.

Although there is no universally agreed-upon Recommended Daily Allowance (RDA) for EPA and DHA, most expert panels suggest a minimum intake of 250–500 mg/day of combined EPA and DHA for general health. Higher, therapeutic doses (often in the range of 1–2 g/day) have been shown to reduce triglycerides, improve symptoms of depression and anxiety, and support inflammatory skin conditions like eczema¹⁶¹⁷

The Composition of Seed Oils

Seed oils are widely used in processed foods, salad dressings, and cooking oils. They are primarily composed of polyunsaturated fatty acids (PUFAs), especially omega-6 linoleic acid.¹⁸ These oils also contain smaller amounts of omega-3 alpha-linolenic acid (in some, like canola), monounsaturated fats (like oleic acid), and very little saturated fat. Their high PUFA content makes them prone to oxidation during processing and cooking, which may influence their health effects.

Omega-6 Fatty Acids: Essential Yet Controversial

Omega-6 fatty acids are essential fats, meaning they must be consumed in the diet as the body cannot synthesize them. However, the Western diet often leads to an imbalanced omega-6 to omega-3 ratio, with omega-6 fats disproportionately dominating the intake. This imbalance has been linked to increased systemic inflammation, which may contribute to chronic diseases such as heart disease, diabetes, and obesity.¹⁹Scientific studies suggest that excessive omega-6 intake, particularly from processed seed oils, can promote inflammatory pathways, altering the balance of prostaglandins and cytokines).¹⁹As inflammation plays a pivotal role in conditions like atherosclerosis and metabolic syndrome, the overconsumption of omega-6-rich seed oils is one leading factor that may contribute to these health issues and should therefore be modified.

One mechanism often cited is the metabolism of linoleic acid to arachidonic acid, a precursor to proinflammatory eicosanoids like prostaglandin E2 and leukotriene B4. Researchers argue that high linoleic acid intake leads to an accumulation of oxidized linoleic acid to metabolites (OXLAMSs). Which have been implicated in mitochondrial dysfunction and oxidative stress, particularly in the endothelium and adipose tissues.²⁰

Yet omega-6s are not intrinsically harmful. Omega-6 fatty acids have essential roles in maintaining skin health, cellular function, and immune system regulation. The key lies in achieving a proper balance with omega-3s, which have anti-inflammatory effects that counteract the negative impact of omega-6.

Omega-3 Fatty Acids: The Anti-Inflammatory Counterpart

Omega-3 fatty acids, found in oils like flaxseed, chia and fatty fish, offer a host of health benefits, including reducing inflammation, improving cardiovascular health, and supporting brain function. ²¹The high intake of omega-6s, especially from processed seed oils, hampers the body’s ability to harness omega-3’s benefits, leading to increased risk for conditions like cardiovascular disease, insulin resistance, and certain cancers.

Seed Oils and Chronic Disease: What Does Evidence Say?

Several epidemiological studies link high omega-6 consumption with increased inflammatory markers, insulin resistance, and autoimmune diseases. Research highlights the role of high omega-6/omega-3 ratio in rheumatoid arthritis, cardiovascular disease, obesity, IBD, skin related disorders, and other immune conditions. ¹⁴²⁰^,²²^, ²³²⁴

However, other reviews found that replacing saturated fats with linoleic acid-rich oils significantly lowered LDL cholesterol and reduced cardiovascular events.²⁵This nuance highlights the complexity of dietary fats. For example, canola oil is often promoted for its LDL-lowering effects and its modest omega-3 profile compared to soybean or corn oil. Yet when compared directly to extra virgin olive oil (EVOO), the benefits are less definitive. Trials such as the PREDIMED highlight EVOO’s superior effects on endothelial function, oxidative stress, and inflammatory markers, whereas some research suggests that canola oil may yield comparable outcomes when used to replace saturated fats. The health impact of canola oil– or any seed oil– may therefore be better understood not as a binary “healthy” or “unhealthy” designation, but in the context of what it replaces. Rather than a paradox, this illustrates a spectrum of relative risk: excess saturated fats likely confer the highest cardiovascular risk, followed by omega-6 rich oils which may still carry risk, albeit to a lesser extent and particularly when consumed in excess or outside the context of a balanced diet. This gradient of harm highlights the importance of the type of fat, quantity and its dietary context.

Omega 6’s Omega 3’s

Table 1: ²⁶²⁷^,²⁸^,²⁹

The Refinement Process: What Happens to Oils Before They Hit Your Plate

Most commercially available seed oils undergo multiple stages of processing—including degumming, neutralization, bleaching, deodorization, and solvent extraction (often with hexane)—to enhance shelf life, create a neutral flavor, and produce a clear appearance.³⁰ While these steps enhance consumer appeal and storage life, they may come at a nutritional cost. One of the drawbacks is the loss of important micronutrients, particularly tocopherols (vitamin E compounds), which serve as natural antioxidants. Tocopherols not only protect oils from oxidative damage but also support cardiovascular health by reducing lipid peroxidation and helping LDL cholesterol levels. .³¹However, the use of hexane and other high-heat refining techniques can deplete these compounds. Comparative research shows that enzyme-extracted oils retain much higher levels of vitamin E and squalene than their hexane-extracted counterparts, indicating that the method of extraction plays a critical role in preserving a seed oil’s nutritional quality.

The extent of nutrient loss varies depending on the specific oil and the type of refining process used, making it difficult to draw definitive conclusions about the health impacts of specific seed oils. This variability suggests that less refined, minimally processed oils may offer more health benefits due to their higher antioxidant content, especially in the context of cardiovascular protection.³²

Studies show that residual hexane levels in edible oils are generally below 1 part per million (ppm), which is within the safety limits established by the U.S. FDA and European Food Safety Authority (EFSA), and no clinical evidence currently links trace levels in oils to human health risks. ³³³⁴That said, the presence of solvents is a marker of the heavy processing behind these oils. Even if trace levels are not overtly harmful, their presence highlights the highly refined and potentially oxidation-prone nature of seed oils. This reinforces recommendations to moderate intake of omega-6-rich oils and instead prioritize less processed, more stable fat sources. ³⁵

Trans Fats and Processing

Trans fats are primarily produced through the industrial process of hydrogenating vegetable oils, creating partially hydrogenated oils commonly used in processed foods. These artificial trans fats have been shown to raise LDL (“bad”) cholesterol and lower HDL (“good”) cholesterol, increasing the risk of heart disease. Additionally, trans fat consumption is associated with impaired endothelial function and may contribute to insulin resistance and type 2 diabetes. While cold-pressed oils typically contain lower levels of trans fats compared to partially hydrogenated oils, they are not entirely free from trans fats due to processing methods like deodorization. Therefore, selecting minimally processed oils and consuming them in moderation is advisable to reduce trans fat intake.³⁶

The stability of seed oils during cooking is an increasingly relevant concern, particularly due to the potential formation of harmful byproducts during high-heat food preparation and should be considered in the conversation of omega-6 fatty acid impacts on health. PUFAs are prone to oxidation when exposed to high temperatures, such as during frying. ³⁷ This process can lead to the formation of lipid peroxides and aldehydes—compounds that have been associated with oxidative stress and inflammation in the body. Some studies specifically highlight that reheating oils exacerbates this degradation, further increasing the levels of toxic byproducts like aldehydes^.³⁸³⁹

Despite these concerns, there is currently a lack of standardized guidance around the safe culinary use of PUFA-rich oils, especially in home cooking and food service settings. While seed oils remain widely used due to their affordability and neutral taste, it may be advisable to limit their use in high-temperature cooking where oxidation is most likely to occur. Instead, oils with higher oxidative stability and smoke points—such as extra virgin olive oil (EVOO) and avocado oil—are often recommended as more suitable alternatives for sautéing, roasting, or pan-frying. These oils contain more monounsaturated fats, which are less susceptible to oxidation, and also retain beneficial antioxidants that may help counteract lipid peroxidation during heating. ⁴⁰

In addition to concerns around oil processing and cooking stability, the widespread consumption of ultra processed foods (UPFs) plays a significant role in the overconsumption of omega-6 fatty acids in modern diets. These industrially manufactured foods- often high in added sugars, sodium, and seed oils- are a primary source of linoleic acid (omega-6 PUFA), especially in western eating patterns. It has been posited that excessive intake of omega-6-rich seed oils via ultra processed foods may contribute to metabolic dysfunction and chronic inflammation, particularly when these fats are consumed in the absence of sufficient omega-3s. Given that UPFs now compromise over 50% of the average American daily caloric intake, this dietary shift underscores the need for greater emphasis on whole-food- based sources of fat- like nuts, seeds avocados, and fatty fish– which offer essential fatty acids alongside fiber, micronutrients, and antioxidants. Reducing reliance on ultra processed foods not only helps balance fat intake but also supports broader metabolic and cardiovascular health outcomes^.³⁷⁴¹

Health Impacts of Seed Oil Consumption

a. Cardiovascular Health

Seed oils have been a cornerstone of the American Heart Association’s dietary recommendations for replacing saturated fats to reduce cholesterol and heart disease risk. Indeed, some studies show that omega-6-rich oils can lower LDL cholesterol. However, emerging research questions whether lowering cholesterol alone is an adequate marker for cardiovascular health. Some evidence suggests that excessive omega-6 intake may lead to oxidation of LDL particles, promoting the development of atherosclerotic plaques.⁴² Additionally, the overproduction of omega-6-derived eicosanoids—when not countered by omega-3s—may exacerbate inflammation and endothelial dysfunction, both key factors in heart disease. ¹⁴⁴³⁴⁵

b. Metabolic Health and Insulin Resistance

A growing body of research links excessive seed oil consumption to insulin resistance, especially when paired with diets high in refined carbohydrates and low in anti-inflammatory fats.²⁷ Overconsumption of omega-6-rich oils has been associated with abdominal fat accumulation and impaired insulin signaling. In contrast, omega-3s may improve insulin sensitivity by modulating inflammation and promoting fat oxidation in liver and muscle tissue.

c. Gastrointestinal Health

Emerging research has linked high dietary intake of linoleic acid to adverse gut health outcomes. One notable study from the Gut journal, conducted within the large-scale EPIC cohort, found that individuals with the highest intake of linoleic acid had a 2.5-fold increased risk of developing ulcerative colitis compared to those with the lowest intake. The authors estimated that nearly 30% of ulcerative colitis cases could be attributed to high linoleic acid consumption. These findings suggest that excess omega-6 intake may play a pro-inflammatory role in the gut, potentially exacerbating or contributing to the development of inflammatory bowel diseases (IBDs).⁴⁵While more research is needed to fully understand the mechanisms, this evidence raises concerns about the widespread use of omega-6-rich seed oils and supports a more balanced dietary approach that prioritizes anti-inflammatory fats for gut health. ⁴⁶

d. Clinical Outcomes and Industry Bias

In clinical practice, we often consider the elimination of seed oils—such as canola, soybean, sunflower, and safflower oils—as part of a personalized strategy to address chronic inflammation and autoimmune symptoms. For some individuals, reducing or removing these oils may lead to noticeable improvements in joint pain, fatigue, digestive discomfort, and skin concerns. Following a targeted elimination phase, we typically support a gradual, mindful reintroduction to evaluate individual tolerance and inform a sustainable, long-term approach.

It’s important to note that much of the current literature on seed oils is influenced by industry sponsorship, particularly from canola oil and soybean oil sectors. While some studies report neutral or even beneficial cardiovascular effects,²⁶ these findings must be weighed against concerns about oxidation, processing methods, and individual inflammatory responses^.⁴⁷ ⁴⁸ We make a nuanced, evidence-informed approach– acknowledging both the potential benefits and drawbacks of seed oils, while centering the unique needs of each patient.

Rethinking Seed Oils Through a Balanced Lens

Despite extensive research, key questions remain about the health impacts of seed oils. Individual variability—driven by genetics, metabolic health, and inflammation—likely plays a role in whether omega-6-rich oils are beneficial or harmful. Additionally, more clarity is needed on the long-term effects of refined versus unrefined oils. While cold-pressed oils appear to retain more nutrients and produce fewer harmful compounds, longitudinal data comparing their influence on chronic disease risk is still limited. Most studies also treat seed oils as a uniform category, overlooking the unique effects of specific oils. As the field evolves, future research should explore these distinctions and assess real-world outcomes like cardiovascular events, insulin resistance, and autoimmune conditions.

One of the most debated aspects is the omega-6 to omega-3 ratio, which is commonly skewed in Western diets. Though there’s no universally agreed-upon “ideal” ratio, many experts recommend increasing omega-3 intake to counter inflammation. This includes consuming fatty fish like salmon, sardines, or mackerel two to three times weekly, or using high-quality fish oil or algae-based supplements. Plant-based sources such as flaxseeds, chia seeds, and hemp seeds offer ALA, which can support balance, though the conversion to EPA and DHA is limited. Complementary choices include using flaxseed, walnut, or perilla oils cold, while moderating intake of high omega-6 oils like soybean and canola, especially when refined.

Ultimately, the question isn’t whether seed oils are inherently “good” or “bad,” but how they fit within a broader dietary pattern. When used mindfully—favoring unrefined oils and pairing them with omega-3-rich, anti-inflammatory whole foods—seed oils can be part of a balanced, health-supportive diet. Prioritizing minimally processed foods, fiber-rich vegetables, and lean proteins while reducing ultra-processed and refined foods helps restore a more favorable omega ratio. The focus should shift from fear-based messaging to education—empowering individuals to make informed, context-specific choices that support metabolic, cardiovascular, and long-term health.

In our practice, we take a comprehensive approach to nutritional assessment, tailoring recommendations to our patients’ personal goals, health status, and risk factors. We also measure and monitor omega-6 and omega-3 fatty acids and their ratio as part of our broader strategy to evaluate inflammation and cardiovascular and metabolic health risks. This allows us to make targeted, evidence-based dietary modifications in a way that supports long-term wellness and aligns with our patients’ lifestyles and needs.

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