A Close Look at Seed Oils: How Bad Are They Really?

Seed oils have become one of the most debated topics in nutrition today. Social media influencers call them “toxic,” while nutrition scientists defend their safety. The truth lies somewhere in between, and understanding both sides can help you make informed choices about these common cooking oils.

What Are Seed Oils?

Seed oils are vegetable oils extracted from the seeds of various plants. The most commonly discussed seed oils include soybean oil, corn oil, sunflower oil, safflower oil, canola oil, cottonseed oil, grapeseed oil, and rice bran oil. These oils are found in countless processed foods and used for cooking in restaurants and homes worldwide.

The process starts with cleaning and crushing the seeds, then pressing or using solvents to pull out the oil. After that, the crude oil gets refined to remove impurities, odors, and unwanted flavors—making it safe and stable for cooking.

How Seed Oils Are Processed

Understanding how seed oils are made helps explain both the benefits and concerns surrounding them. The processing involves several key steps that significantly alter the final product.

Extraction Methods

Two main methods dominate seed oil production—cold-pressed and refined—each with distinct advantages. Cold-pressed oils retain more nutrients and natural flavors since they’re extracted without heat or chemicals, while refined oils undergo processing for a neutral taste and longer shelf life.

Most commercial seed oils use solvent extraction with hexane, a chemical solvent. Hexane-based solvent extraction is the go-to method for pulling oil from seeds like soybeans and canola, and it’s no surprise why—it’s incredibly efficient, recovering over 99% of the oil. The hexane is later removed through distillation, leaving behind crude oil ready for refining.

The Refining Process

Edible oils purchased in stores are known as “RBD” oils. These are oils that have been Refined, Bleached, and Deodorized. This multi-step process transforms crude oil into the clear, neutral-tasting products found on grocery shelves.

Degumming and Refining: The first step removes phospholipids and other impurities from the crude oil. The objective of bleaching (or decolorizing) is to reduce the levels of colored pigments (carotenoids and chlorophylls). It also further removes residue traces of phosphatide, soap, phospholipid contaminants, lipid peroxidation products, and other impurities.

Bleaching: Despite the name, this process doesn’t use actual bleach. Color pigments, residual metals, and other impurities are removed using adsorbents like activated clay. (No actual bleach is involved). The oil is heated to around 200-220°F and mixed with bleaching clay that absorbs unwanted compounds.

Deodorization: The final step removes flavors and odors through steam distillation at very high temperatures. Deodorization is a steam-distillation process in which volatile and odoriferous compounds are stripped off with steam. The objective is to produce a bland and stable product. This process occurs at temperatures exceeding 400°F under vacuum conditions.

Processing Trade-offs

The refining process can have both a positive and negative impact on the oil. Some benefits include a longer shelf life, better taste and color, higher smoke point (allows for high heat cooking/frying) and the elimination of toxins that could make us sick.

However, there are downsides. Some drawbacks of the refining process include the loss of beneficial compounds like antioxidants, polyphenols and phospholipids. The processing also can result in the formation of unhealthy trans-fatty acids.

The Omega-6 Controversy

Much of the concern about seed oils centers on their high omega-6 fatty acid content, particularly linoleic acid. Understanding this controversy requires examining both the science and the misconceptions.

What the Research Shows

Recent studies have challenged many assumptions about omega-6 fatty acids. Research also shows that linoleic acid can improve glucose metabolism and even prevent a person from developing type 2 diabetes, which is a well-known risk factor for cardiovascular disease.

A new study is turning heads by challenging the popular belief that seed oils are harmful to health. Researchers analyzed blood markers from nearly 1,900 people and found that higher levels of linoleic acid — an omega-6 fat commonly found in seed oils — were linked to lower inflammation and better cardiometabolic health.

The Inflammation Question

Seed oils do not cause inflammation, according to nutrition scientists. The concern that they do is based on a misunderstanding of the omega-3 and omega-6 essential fatty acids they contain—essential meaning that humans cannot produce these on their own, so we have to get them from food.

From a relative perspective, omega-3 fats are more anti-inflammatory than omega-6 fats. But to flip that and suggest this means omega-6 fats are pro-inflammatory is wrong. Both omega-6 and omega-3 fats play a role in the inflammatory response of the body. What is optimal is a balance.

The Ratio Debate

Many health influencers focus on the omega-6 to omega-3 ratio in modern diets. Some influencers claim that our consumption of omega-6s far outweighs that of omega-3s and that we should aim for a 1:1 ratio.

However, nutrition experts offer a different perspective. If people want to get the ratio closer to 1:1, the recommendation should not be to reduce omega-6, but rather to increase your omega-3 intake. Eat more walnuts or fatty fish.

The Real Concerns About Seed Oils

While seed oils themselves may not be inherently harmful, there are legitimate concerns about how they’re used in the modern food system.

Ultra-Processed Foods

We know that ultraprocessed foods generally are not good for your health. They are usually high in sodium or salt, added sugars, unhealthy fats, and additives. That’s why it’s bad for you, not the inclusion of seed oils.

The real problem with seed oils is that they’re frequently found in highly processed and fast-food items. These foods are problematic for many reasons beyond just the type of oil used.

Oxidation and High-Heat Cooking

When seed oils are heated to very high temperatures or reused repeatedly, they can form harmful compounds. It’s important to note that these oils shouldn’t be cooked at high temperatures or reused.

The refining process can also create concerning compounds. Some research suggests that oxidized linoleic acid metabolites may contribute to inflammation and cardiovascular issues, though this remains an active area of scientific investigation.

Comparing Seed Oils to Alternatives

When evaluating seed oils, it’s helpful to compare them with other common cooking fats.

Saturated Fats vs. Seed Oils

Many arguments against consuming seed oils include the claim that it’s healthier to cook with lard, beef tallow, and butter, despite decades of evidence that saturated fats raise cholesterol and risk of heart disease and stroke.

People are cooking with these oils, not drinking them. In a situation where you need some kind of fat for cooking or food preparation, you can use plant oils or you can use butter or lard. Very consistently, all the data say butter and lard are bad for our hearts.

Less Processed Alternatives

For those seeking alternatives, several options provide different nutritional profiles:

• Extra virgin olive oil: Rich in monounsaturated fats and antioxidants, particularly suitable for medium-heat cooking and dressings
• Avocado oil: High smoke point with mostly monounsaturated fats, good for high-heat cooking
• Coconut oil: Primarily saturated fat with a unique fatty acid profile, though should be used in moderation
• Cold-pressed seed oils: Retain more nutrients than refined versions but have shorter shelf lives

Making Informed Choices

The seed oil debate highlights the importance of looking at the bigger picture of dietary health rather than focusing on single ingredients.

Focus on Overall Diet Quality

Rather than focusing on seed oils specifically, reduce your intake of processed foods more broadly and focus on eating fresh foods. So don’t stress out too much about using a bit of seed oils in your cooking if you are generally focused on all the right things.

If you cut back or eliminate your consumption of pre-packaged foods like cookies, potato chips, crackers, frozen meals and deep-fried foods, you will significantly reduce your seed oil intake. These foods are also typically high in added sugar, refined carbohydrates and sodium, so avoiding them will have additional health benefits.

Practical Guidelines

1. Cook at home more often: This gives you control over the types and amounts of oils used in your food.

2. Choose less processed oils when possible: Cold-pressed or expeller-pressed oils retain more nutrients than highly refined versions.

3. Use appropriate oils for different cooking methods: Save delicate oils for low-heat applications and dressings.

4. Don’t reuse cooking oil: This prevents the formation of harmful oxidation products.

5. Emphasize whole foods: A diet rich in fruits, vegetables, whole grains, lean proteins, and healthy fats from various sources supports overall health.

The Bottom Line

Seed oils can be part of both healthy and unhealthy diets. Instead of cutting out all foods containing seed oils, consider eating less ultraprocessed food and more whole foods, fruit, and vegetables.

Overall, if you’re worried about inflammation and disease there are probably more important things to worry about than seed oils. They may or may not play a role in inflammation (the research picture is mixed). What we do know, however, is that a high-quality diet rich in unprocessed whole foods (fruits, vegetables, nuts, seeds, grains and lean meats) is the number one thing you can to do reduce inflammation and your risk of developing diseases.

The current scientific evidence suggests that moderate consumption of seed oils as part of a balanced diet is unlikely to cause harm for most people. Rather than avoiding specific oils entirely, focus on reducing ultra-processed foods, increasing omega-3 intake from fish and other sources, and maintaining an overall healthy eating pattern.

For those who prefer to minimize seed oil consumption, there are plenty of alternatives available. The key is making informed choices based on reliable science rather than fear-based claims, and remembering that no single food or ingredient determines health outcomes—it’s the overall dietary pattern that matters most.

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Always consult with a healthcare provider or registered dietitian for personalized dietary advice, especially if you have specific health conditions or concerns.