What is the Deal with Seed Oils?

A Look at the Science and the Skepticism

In the masterpiece that was Mean Girls, Regina George once said “why are you so obsessed with me?” In my mind, that is what seed oils must be thinking too.

In recent years, lowly seed oils have found themselves at the center of a heated health debate. Critics argue that they contribute to inflammation and chronic disease, and should be replaced by animal fats. These claims have left health experts and the public scratching their heads as it contradicts long-standing guidance to limit saturated fats and replace them with unsaturated fats like those found in seed oils.

While most folks consider seed oils healthy, about one in four Americans have never even heard of them. So, what exactly are seed oils, why are they under scrutiny and what does the science really say? Let’s break it down.

TLDR: The preponderance of research shows seed oils can benefit our health when they are consumed as a part of a healthy diet. Most concerns lack important context.

What are seed oils?

Seed oils are the oil that is extracted from the seeds of plants – typically canola, soybean, corn, sunflower, cottonseed, grapeseed, safflower and rice bran. Other common oils, like olive, coconut, palm and avocado, are taken from the fruit of the plant.

The oil can be extracted via a mechanical or solvent process. Mechanical extraction physically presses the oil out while solvent extraction uses a chemical like hexane to dissolve the oil. Oils can then be refined to remove impurities – a process known as RBD (Refined, Bleached, and Deodorized). The processing method affects the nutrition, taste profile, stability and smoke point of the oil, which are important considerations for culinary use.

Bottom line – Seed oils are extracted from plant seeds through a mechanical or solvent process which affects its nutritional, physical and cooking properties.

Are seed oils healthy?

Seed oils consist primarily of polyunsaturated fatty acids (PUFAs), which include omega-3 and omega-6 types. These are considered “essential” fatty acids because our bodies can’t make them on their own. PUFAs support our cell membranes and nervous system, and play an important role in regulating fat metabolism, immune function and inflammation. 

There is a strong and consistent body of evidence that shows diets high in PUFAs are linked to lower risk of heart disease, particularly when they replace saturated fat. This protective effect is driven mostly by reducing LDL (“bad”) cholesterol and triglyceride levels.

• The American Heart Association (AHA) Presidential Advisory “strongly concludes” that reducing saturated fat and replacing it with PUFAs will reduce rates of heart disease. In fact, their meta-analysis found swapping saturated fat with vegetable oil PUFAs reduced heart disease risk by around 30%, similar to the effect of statins.
• Last year, a cohort study of over 400,000 adults found eating more plant-based fats lowered the risk of death from all causes and heart disease. Specifically, those with the highest intakes of vegetable oils were 15% less likely to die of heart disease than those with the lowest intakes, and replacing saturated fat with unsaturated fat lowered the risk of heart disease death by up to 30%.
• Recently, a 33-year cohort study of over 220,000 adults found that eating more plant-based oils significantly reduced the risk of premature death whereas higher butter intakes were linked to increased risk. While the negative effects of butter were not seen in folks with overall healthy diets (a nod to the oh-so-valuable concept of balance), the researchers did estimate that replacing about one tablespoon of butter with plant-based oil cut the risk of death from all-causes and cancer by 17%.

Bottom line – Seed oils offer essential PUFAs that have recognized benefits for heart health, especially when they replace saturated fats.

Why are seed oils scrutinized?

Critics of seed oils claim they are “poisonous,” “toxic” and contribute to the rising rates of chronic disease. It’s important to understand where these concerns come from, and many warrant nuanced discussion, so let’s address four common points of contention.

1. Correlation with Poor Health

Let’s start with the concern of correlation as this implication drives home an important message. A common argument against seed oils is that increased consumption has been positively associated with the increase in chronic disease. Provocative graphs are typically shared that show both vegetable oil intake as well as obesity and/or diabetes prevalence increasing over time. So, what could be going on here?

• It’s true, U.S. consumption of vegetable oils has increased dramatically since the early 20^th century along with the prevalence of both obesity and diabetes.
• Vegetable oil, particularly soybean oil, is commonly found in processed foods for its favorable properties (solubility, gelation, shelf-stability, and emulsification).
• U.S. consumption of ultra-processed foods (UPF) has also significantly increased over time and currently makes up more than half of our calories.
• Interestingly, heart disease mortality and high LDL cholesterol have decreased since the 1970s – a nod to improvements in both lifestyle prevention and medicine. 

This poses the question – is it the seed oils or the UPFs that contain them that are really harming metabolic health? While increased consumption of UPFs, that are typically calorically dense and nutrient-poor, are likely contributing to poor health, we must remember that chronic disease is influenced by a variety of complex factors at both the individual and systemic level. The correlation with seed oil and UPF intake is worth noting, but there are many other drivers of health at play too. Simple graphs make for easy (and attractive) messaging, but as we know health is rarely that straightforward.

2. Omega-6 Fatty Acids

Seed oils contain a variety of fatty acids but are mostly made up of the omega-6 fatty acid, linoleic acid. Critics claim seed oils cause inflammation because linoleic acid is converted to arachidonic acid (which can be pro-inflammatory). However, eating linoleic acid doesn’t strongly affect our arachidonic acid levels and research doesn’t link linoleic acid to most inflammatory markers or long-term inflammation. Some evidence actually points to reduced inflammation. Here’s the kicker … we get most of our arachidonic acid from animal-based foods – primarily chicken, eggs and beef, not seed oils.

Another concern is that seed oils have increased our omega-6 to omega-3 ratio to levels much higher than historically consumed. This topic is worth an entire article on its own, but I’ll aim to be brief. It’s true, our intake of omega-6s and its ratio to omega-3s significantly increased in the 1900s and the early 2000s thanks to dramatic changes in how we eat (see the section above on UPF). Indeed, this change deserves attention and further research. However, the clinical significance of this ratio remains up for debate. Nevertheless, most experts agree Americans are not meeting omega-3 recommendations and can certainly benefit from eating more omega-3 foods. 

Lastly, while there are demonstrated benefits of replacing saturated fat with omega-6 PUFAs (with even more favorable results than monounsaturated fats), the effects of increasing omega-6s alone are more controversial where studies show both neutral and potentially harmful effects on heart health, calling for more specific research. 

3. Solvent Processing 

Many seed oils are processed using hexane, so there are concerns around residual byproducts that could be toxic. Hexane is a very effective solvent that can extract oil from seed material and evaporate quickly. While there can be trace amounts leftover (~0.8 ppm), this is fairly trivial compared to our daily exposure from inhaling pollutants (<2%) which is actively monitored by the EPA.  

Solvent extraction also requires oils to be further refined, bleached and deodorized. This process removes impurities, but also removes nutrients such as fatty acids, antioxidants, vitamins and proteins. This creates a more stable oil with a higher smoke point. When oils are heated too high and too long, they burn and create harmful acrylamides. These compounds are more of a concern in foods like potato chips and French fries, rather than home cooking. However, even for everyday household use, it’s important to know common smoke points (e.g., don’t use extra virgin olive oil to stir fry at high heat).  

At the end of the day, if you are concerned about the processing method you can opt for oils that are mechanically extracted, such as cold-pressed oils. These products preserve more of the oil’s natural nutrients but come at a higher price with a shorter shelf-life and lower smoke point.

4. Conflicting Evidence

Critics often share “gotcha” studies that question the healthfulness of seed oils and omega-6 fats, as well as the validity of dietary recommendations to replace saturated fat with them. Challenging knowledge and being open to revision are a good thing and underscore the spirit of scientific progress, but not all evidence is created equal. Let’s discuss a few common scientific inquiries. 

• The Minnesota Coronary Experiment was a clinical trial in the late 1960s that explored the effects of replacing saturated fat with omega-6 linoleic acid from vegetable oils. Interestingly subsequent analyses found that while vegetable oil helped lower LDL cholesterol, it had no benefit on heart disease outcomes and lower cholesterol was actually linked to an increased risk of death (!). However, this report had serious methodical limitations related to conflicting reporting, short duration, high attrition rates and poor generalizability. Therefore, experts say we shouldn’t draw conclusions from this single (and weak) study and its results shouldn’t impact current dietary recommendations (which are based on the overall body of evidence).
• A study on Seed Oil and Colon Cancer also made for provocative headlines late last year. The researchers compared colon tissue from cancer patients and healthy people to explore the effects of fat content and inflammation. They found greater levels of arachidonic acid (a derivative of omega-6 linoleic acid) and inflammation in tumor cells. These results are important and highlight potential effects of lipid metabolism on tumor development, but the study did not collect any data on diets or seed oil intake and therefore cannot imply a causal relationship. It was also conducted in patients with an average age of 64 and at only one point in time, which makes conclusions difficult as cancer develops over the course of years. Check out this Science Feedback article and Dr. Andrea Love’s Immunologic article to learn more about the study and the issues with misleading headlines.
• In Defense of Saturated Fat. Critics of seed oils often point to conflicting evidence on saturated fat and heart health. There is certainly growing evidence that calls for re-evaluating the relationship between saturated fat and heart disease, particularly when it comes to whole foods such as dairy foods and unprocessed meat (thanks to potential “food matrix” effects). However, these arguments create a false dichotomy between seed oils and saturated fat. Two things can be true: replacing saturated fat with unsaturated fat can have favorable effects on heart health outcomes, and nutritious foods that contain saturated fat can be part of a healthy diet. Phew, we don’t have to pick a side and different fats can peacefully exist together healthy diets.

Further research on the interaction between dietary fat, food type and health are indeed welcome. However, established health recommendations don’t come from single studies, but from critical reviews of the entire body of evidence. Questioning science can be a good thing, but undermining expertise and eroding trust in public health recommendations based on limited or weak data is not helpful and potentially harmful.

Bottom line – There are concerns over seed oils due to correlation with poor health, omega-6 content, processing techniques and contradictory studies.  However, most evidence does not link seed oils, specifically, or their processing method to adverse health effects. 

Key takeaways and final thoughts

1. Seed oils can be a beneficial part of a healthy diet, particularly when they replace saturated fat. They are useful for cooking, baking, dressings and marinades.
2. Americans are eating significantly more seed oil (and omega-6s) today due to the increased availability and consumption of UPFs. Eating more UPFs likely contributes to seed oil’s correlation with chronic disease prevalence, but there’s always more to the story of health.
3. Research suggests the omega-6 content of seed oils alone doesn’t affect inflammation and can even have favorable effects on health. However, trends of increasing omega-6 intake and a higher omega-6 to omega-3 ratio warrant more research. Nonetheless, overall diet quality is a much better predictor of health outcomes.
4. Solvent processing in refined oils can leave trace amounts of hexane, but this exposure hasn’t been linked to adverse health effects and it’s a fraction of what we inhale daily from other sources. Cold-pressed oils are available for those who are concerned. 
5. Emerging research on saturated fat and heart health is noteworthy but shouldn’t discourage seed oil consumption.

From a public health perspective, we have a lot of room for improvement in our diets, but removing seed oil is far from the top of the list. The vilification of seed oils is not only misleading, it’s potentially harmful, undermines scientific consensus and distracts from more meaningful drivers of health. Our attention, time and resources would be better spent investing in programs and policies that effectively improve overall diet quality and health rather than attacking a single, and nutritious, food.

Cheers,

~Megan

p.s., I will continue to write about topics I am passionate about on this blog, but I recently teamed up with Your Local Epidemiologist to support their Substack newsletter that covers a variety of public health topics. Be sure to check it out and subscribe!