Are Seed Oils Bad for You?

In the court of public opinion, seed oils often get a bad rap. The debates range from their nutritional value to their potential health risks—including causing cancer. With strong opinions on both sides, it can be hard to understand how seed oils fit into your own diet in a way that supports your lifestyle, health, and fertility goals. Here’s what science has to say about seed oils, including whether they’re safe for consumption, how they can affect your fertility, and the bottom line on whether they cause cancer. 

• What are seed oils?
• Are seed oils bad?
• What does the WHO say about seed oils?
  
• How do seed oils affect fertility?
• Do seed oils cause inflammation?
• Do seed oils cause cancer?
• Should I eat seed oils?

What are seed oils?

Seed oils are a subset of vegetable oils extracted from the seeds of plants, often using mechanical or chemical processes. They’re a versatile ingredient and widely used in cooking due to their high smoke points and neutral flavors. You’ll find them in everything from baked goods to salad dressings, margarine, and deep-fried foods. But they aren’t just in your packaged foods—you’ll also find them in supplements, and skin and hair care products. They’re a staple in many areas of our diet and care.

Common seed oils include:

• Soybean oil

• Canola (rapeseed) oil

• Cottonseed oil

• Sunflower oil

• Safflower oil

• Corn oil

• Grapeseed oil

• Peanut oil

Are seed oils actually bad?

The main concerns about seed oil are its omega-6 content, how it’s processed, and the potential for toxins when heated. 

Seed oil and omega-6

Seed oils are rich in omega-6 Polyunsaturated fatty acids, which are essential for health. These fats support normal cellular function and need to be obtained through the diet because the body can’t produce them on its own. Consuming omega-6s isn’t bad—in fact, it’s necessary. The problem lies in the balance (or imbalance) of omega-6 to omega-3 fatty acids (another essential nutrient) in modern diets.

The World Health Organization (WHO) found:

• Higher omega-6 intake was linked to a 9% reduced risk of death from any cause (but also a 31% increased risk of postmenopausal breast cancer).

• A higher omega-6 to omega-3 ratio was associated with greater risks of cognitive decline and inflammatory bowel conditions like ulcerative colitis.

• Conversely, a higher omega-3 to omega-6 ratio was linked to a 26% reduced risk of depression.

The takeaway? Omega-6 fatty acids aren’t inherently bad, but we’re eating too much of them, throwing off the important balance between omega-6 and omega-3. The optimal ratio based on research is no more than 4:1 omega-6 to omega-3 (meaning for every 4 grams of omega-6 you consume, you should aim for 1 gram of omega-3). Many of us in North America consume a 15:1 (or higher) omega-6 to omega-3 ratio. 

Seed oil and high-heat processing

Seed oils often undergo high-heat processing during extraction, which can degrade their nutritional quality. Additives and preservatives may also be introduced, which have their own health implications. Additionally, repeated heating (like in restaurant fryers) can produce harmful compounds, including:

• Polar compounds, a critical measure of the quality and safety of edible oils, particularly those used in frying 

• Free radicals and lipid peroxidases, which cause cellular damage.

• Trans fats, which are strongly linked to cardiovascular disease and increased risk of all-cause mortality.

How do seed oils impact health? 

Although seed oils may not be as bad as they’re made out to be, they do still impact your health.

Do seed oils affect female fertility?

The connection between seed oils and female fertility is largely around the omega-6 and omega-3 balance. Studies have shown that women with a higher intake of Alpha-Linoleic Acid (an omega-6 fatty acid found in seed oils like safflower, sunflower, and grapeseed) had a higher risk of primary infertility. The same study showed that in women aged 20 to 34, a higher omega-6 to omega-3 ratio was significantly associated with an increased risk of infertility.

While these findings suggest a potential impact, the broader context of an individual’s diet and lifestyle is likely more important than avoiding seed oils altogether. If you’re concerned, focus on increasing your omega-3 intake through supplementation, fish, flaxseeds, and walnuts to help maintain a healthy balance. 

Your prenatal supplement likely has seed oils in its formula, but consuming small amounts of sunflower (or other seeds oils) in antioxidant-rich prenatal and fertility supplements is not worrisome, especially if you’re also getting enough omega-3. 

Do seed oils lower testosterone?

Animal studies show that omega-6 fatty acids (found in grapeseed and soybean oil) may impair testosterone production, but this hasn’t been conclusively demonstrated in humans. What we do know is that trans fats (formed during the heating of seed oils) are associated with lower testosterone levels in otherwise healthy men. For most people, moderate consumption of seed oils is unlikely to affect testosterone levels, especially if they are not heated often.

Do seed oils cause inflammation?

Omega-6 fatty acids, particularly Arachidonic Acid (ARA), are precursors to pro-inflammatory molecules. However, the relationship isn’t straightforward. ARA is involved in inflammation, but it’s also critical for normal immune function, cell signaling, brain development, and growth in infants. Studies in healthy adults have found that increasing omega-6 intake does not consistently raise inflammatory markers. In other words, we can’t simplify the equation to: seed oils cause inflammation. It’s that important 4:1 ratio that will determine whether you’re consuming seed oils in a healthy way.

Do seed oils cause cancer?

This is the big one. First, when it comes to diet, there are only two things that are proven to cause cancer: alcohol and processed meat (sorry wine and charcuterie lovers!). But, we do know that for a few different types of cancer, having that good balance of omega-3 to omega-6 fatty acids is important. For breast cancer, a higher olive oil and omega-3 intake may be protective against future risk, and when it comes to colon cancer, large-scale studies have found no overall association between colorectal cancer risk and fish, omega-3, or omega-6 PUFA intake.

It is important to note that repeatedly heated cooking oils (like those used in deep fryers) when consumed in high amounts, are associated with several health concerns, including lung, colorectal, breast, and prostate cancers. 

Do seed oils cause heart disease?

In cardiovascular health, the evidence is reassuring. Moderate consumption of fried foods cooked in seed oils isn’t linked to a higher risk of cardiovascular disease. In fact, Alpha-Linolenic Acid, found in plant oils (such as rapeseed or canola oil) and walnuts may be slightly protective for some types of heart disease. 

Should I avoid seed oils in my diet?

Seed oils aren’t inherently harmful, especially when being consumed in a balanced, whole-food diet. But here are a few specific things to keep in mind:

• Balance your fats: Focus on increasing your omega-3 intake while moderating omega-6-rich foods like seed oils. Omega-3 fatty acids are found primarily in cold water fatty fish and supplements containing DHA and EPA. 

• Limit high-heat cooking: Remember that heating oils, especially repeated heating of oils, creates harmful byproducts that we definitely want to avoid for our fertility and general health. 

• Choose whole foods: A diet rich in minimally processed, nutrient-dense foods is key to long-term well-being. You don’t need to completely avoid seed oils, especially if you’re consuming a diet rich in vegetables, whole grains, and lean proteins. But if you’re looking for an alternative, consider switching those oils out for extra virgin olive oil for added health benefits.  

Small amounts of seed oils found in supplements that contain other beneficial nutrients like antioxidants and omega-3 fatty acids, such as DHA, are not a cause for concern. 

When consumed in moderation and as part of a balanced diet, seed oils are unlikely to pose significant health risks. The goal isn’t to eliminate them but to prioritize healthier choices and maintain a well-rounded approach to nutrition.