Toxic or Harmless? What Seed Oils Are & Why You Don’t Need to Be Afraid

• What Are Seed Oils?|
• The Seed Oil Debate|
• Seed Oils and Heart Health|
• How to Cook With Seed Oils|

Seed oils have created some controversy in the functional health community. Are they bad for you? Are they heart-healthy? What even are seed oils, and does it matter how you use them?

These are all questions I’ve heard from people trying to assess the right type of diet for specific health conditions, weight loss goals, and cardiovascular health.

Over the years, the questions around cooking oils have persisted—is saturated fat always bad? Should I just cook everything in olive oil? Should I skip the oils entirely? It can get confusing and overwhelming pretty quickly.

Let’s focus on what the science says about seed oils—what they are, how they’re made, and whether or not they really are bad for you.

What Are Seed Oils?

Seed oils are cooking oils extracted from seeds. This does not include cooking oils made from whole fruits like avocados or olives. The most common seed oils you’ll see at your grocery store are 1:

• Canola oil (aka rapeseed oil)
• Corn oil
• Cottonseed oil
• Grapeseed oil
• Soybean oil
• Sunflower oil
• Safflower oil
• Rice bran oil
• Peanut oil

These oils are quite stable (less likely to go rancid) and have a relatively high smoke point. That’s largely why seed oils are used in deep fryers, which cook food at high temperatures, and in products that will sit on the shelf for a long time. Some examples of foods that include seed oils are 1 2:

• Ultra-processed foods
• Salad dressings
• Other condiments
• Baked goods 
• Fried foods 
• Home or restaurant pantries

Fatty Acids in Seed Oils

Seed oils are lower in saturated fat than coconut oil, palm oil, or animal fats 1 2.

Instead, seed oils are high in polyunsaturated fatty acids (PUFAs), especially omega-6 fatty acids, which include 1 2:

• Linoleic acid (LA) (the main PUFA in the Western diet)
• Gamma-linoleic acid (GLA)
• Arachidonic acid (AA)

Seed oils are also lower in omega-3 fatty acids (also a polyunsaturated fat), like alpha-linolenic acid (ALA, the omega-3 found in plants), than olive oil and avocado oil 1 2.

Omega-6s and omega-3s are essential fatty acids necessary for human health, especially brain function and normal growth and development. The word essential in this context means we have to get them in food because the body cannot make them. Our bodies are at their healthiest if we get a balance of each from a healthy diet 2 3.

According to a large meta-analysis (high-quality study of studies), the ideal ratio of these dietary fats is 6 grams of omega-6 fatty acid for every 1 gram of omega-3 fatty acid 4 5.

Unfortunately, the essential fatty acid ratio in the standard American diet is closer to 20 grams of omega-6 for every 1 gram of omega-3. That means many of us tend to have too many omega-6 fatty acids in our diet 6, mostly from eating lots of foods containing seed oils. 

Many of the seed oils we eat come in ultra-processed foods that may also contain trans fat, which is the worst type of fat for our health 7. 

As I dive into the pros and cons of seed oils, I’ll also explain why trans fats are truly bad for us.

The Seed Oil Debate

There are two schools of thought around seed oils and their health effects 1 2. 

The anti-seed oil team claims that seed oils are an independent driver of heart disease, obesity, and other chronic diseases because they’re high in omega-6 fats that promote inflammation in the body. 

The pro-seed oil team claims that the unsaturated fats in seed oils have a neutral or possibly even beneficial effect on cardiovascular disease when they replace saturated fats (like animal lard) in the diet.

What Does Science Say?

Scientific research has shown that the omega-6 fatty acids present in seed oils are not inherently pro-inflammatory. In fact, omega-6 fats are generally healthy 8. However, when we overeat seed oils, we undereat omega-3 fatty acids, which is not healthy. 

Omega-3s are anti-inflammatory and their absence gives way to the inflammation associated with heart disease and other metabolic diseases. 

In other words, as one researcher wrote, “The problem is not the presence of the omega-6 fatty acids 8.

Therefore, it makes more sense to try to eat more omega-3 fatty acids than to eliminate seed oils from your diet.

What About Hexane?

Unlike certain fruits like avocados and olives, seeds have lower oil content and are hard to physically press to extract their oils 9. But a chemical solvent called hexane can easily extract common seed oils and other specialty oils (like sesame, wheat germ, primrose, or essential oils).

Many anti-seed oil proponents claim that such refined seed oils are toxic because hexane is part of the extraction process, and some of its toxic byproducts remain in the bottle of oil.

It’s true that hexane can be toxic to humans and our environment. It’s a petroleum product that can cause nervous system problems when inhaled. It contains benzene, which is a carcinogen (meaning it can cause cancer) and a mutagen (mutates genetic material). Hexane can also disrupt hormone levels and fertility in females and males, but how it affects us through the digestive system is not clear 9.

The good news is that food-grade hexane has had most of the benzene removed, and refined seed oils have almost no hexane in them. 

In fact, when food products like canola oil, corn oil, and sunflower oil were tested for hexane, the highest amount found was 23 times below the level Europe designates as concerning to human health (1 mg of hexane per kg of oil) 10.

So, yes, hexane can be toxic, but the dose makes the poison. There’s no evidence that hexane is present anywhere near toxic amounts in commercial seed oils 9 10.

What About Trans Fat?

Trans fats are partially hydrogenated oils made mainly in the process of turning liquid oils into solid fats, like when turning vegetable oil into margarine or shortening 7. Small amounts of trans fats also occur naturally in meat and dairy from ruminant animals like cows and sheep 11. 

Food manufacturing companies have notoriously used trans fats to increase the shelf life of certain products and make some ingredients smoother for a better “mouth feel” 7.

Despite these benefits, trans fats are the worst type of dietary fat we can consume. They can increase LDL cholesterol (the “bad” kind) and reduce HDL cholesterol (the “good” kind). Higher LDL and lower HDL increase the risk for cardiovascular disease 7. 

Trans fats also increase the risk of obesity and related health issues like Type 2 diabetes 7.

Since 2018, the USDA has banned food manufacturers from adding trans fats to foods 12. The 2020–2025 Dietary Guidelines for Americans recommend avoiding them 7. Research even shows that replacing trans fats (like margarine or vegetable shortening) with seed oils in your diet should be good for your health 13.

Many people mistakenly believe that the heat from hexane extraction turns seed oils into trans fats. But science says differently: Refined seed oils contain low enough levels of trans fats that they essentially have none 14.

Are Seed Oils Good or Bad for Your Heart?

Cases of cardiovascular disease increased after seed oils became part of the American diet in the early 1900s, but that doesn’t mean seed oils are to blame. There’s no proof that seed oils cause heart disease, but there is evidence that these other factors can play significant roles 15:

• Tobacco use
• Trans fat intake
• Increased pollution
• Urbanization leading to sedentary lifestyle

In fact, since the mid-20th century, coronary heart disease deaths have decreased even as people have continued to eat more seed oils 15.

The decrease in deaths was likely due to:

• Removal of trans fats from the food supply
• Reduced tobacco use
• Improved medical care

Seed Oils Are Neutral or Beneficial

Two thorough meta-analyses provide a complete picture of how seed oils affect heart health. 

The first study concluded that seed oils were neither harmful nor beneficial for cardiovascular health 16. The second one found that replacing saturated fats with seed oils may lead to a 21% reduction in cardiovascular events 17. Each of these meta-analyses examined a different set of studies, which is likely why they came to different conclusions. 

Ultimately, the results indicate that non-hydrogenated seed oils do not contribute to heart disease. However, seed oils processed into trans fats like vegetable shortening or margarine may 7, and so may eating more than about 10% of daily calories as saturated fat 17.

Replacing some saturated fats (like animal lard or butter) with seed oils can improve heart health by lowering cholesterol 13, triglycerides 18, and other markers of heart disease risk 19.

Plus, eating more linoleic acid (high in seed oils) is linked to a lower risk of:

• Coronary heart disease 20
• Type 2 diabetes 21
• Death in general 22

In summary, non-hydrogenated seed oils are not bad for your heart and may even be beneficial when they replace trans fats and some daily saturated fat.

How to Cook With Seed Oils

Now that we’ve established that seed oils, in moderation, are fine and healthy to include in your diet, let’s talk about when and how to use them in your kitchen.

Your options at the grocery store will be either refined or unrefined varieties of common seed oils (canola, corn, sunflower, peanut, etc.): 

• Refined seed oils are typically extracted with hexane, with virtually no hexane left in the finished product.
• Unrefined seed oils are typically extracted using mechanical pressure. These may be labeled as “expeller pressed” or “cold pressed.”

However, this difference may not matter much, as two studies showed no difference in inflammatory effects between refined and unrefined seed oils 23 24. 

In general, seed oils are best for low-to-medium-heat cooking (less than 400 ℉), such as sauteing and light stir-frying. If you see smoke while cooking, the seed oil is too hot. Do your best to avoid reheating seed oils or allowing them to burn in the cooking process.

Balancing Dietary Fats is Key

It’s important to have a healthy balance of fats in your daily diet. Olive oil and avocado oil are great alternatives to seed oils for day-to-day recipes. Both of these oils are high in heart-healthy monounsaturated fat and free radical-busting antioxidants. 

• For medium-heat, low-heat cooking, or no-heat cooking (like for salad dressings), use extra virgin olive oil. It has a similar smoke point (376 ℉) to seed oils.
• For higher heat, use avocado oil. It has a smoke point around 482 ℉.

To ensure that you’re getting enough omega-3 fatty acids, shoot for at least two servings of fish per week, ideally as salmon, sardines, oysters, mackerel, or herring 25. If fish isn’t on the menu, you can also take 1,000 to 1,500 mg of a daily fish oil supplement 26.

If you’re vegan or vegetarian or would just rather take flaxseed oil than fish oil, you can take it as a supplement or include it in a salad dressing or snack that isn’t heated. Flaxseed oil is not very stable and needs to stay cool.

The Much-Maligned Seed Oil

It turns out that the anti-seed oil fervor is a bit overblown. Although some of the concerns around hexane and trans fats seem valid, the evidence doesn’t support the claims that these harmful byproducts are present in commercial seed oils at the grocery store.

At worst, the omega-6 PUFAs found in seed oils have no positive or negative effect on your health. At best, they’re beneficial, especially when they’re replacing trans fats or too many saturated fats. When eaten in moderation and in a diet with plenty of omega-3 fats, seed oils can be part of a healthy diet.

If you’re looking for guidance on how to improve your cardiovascular health, lose weight, or reduce chronic inflammation, we’d love to help. Reach out to our clinic. 

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