You’ve likely heard claims about the health risks of seed oils on social media and wondered whether they can fit into a healthful eating pattern. The truth is, they can, and many trusted health professionals say they’re safe and beneficial.
In this deep dive, we’ll define seed oils, explain the health claims, and review the evidence showing that these oils are safe and are recommended to include in your diet.
What are Seed Oils?
Seed oils are made from seeds, as opposed to fruits (like olive, avocado, coconut, or palm). The types of seed oils that have been criticized are sometimes classified as the “Hateful Eight,” which includes oils made from seeds of the following plants:
- Canola
- Corn
- Cottonseed
- Soybean
- Sunflower
- Safflower
- Grapeseed
- Rice bran
Some of these oils may not be recognized, but commonly used “vegetable oils” typically consist of soybean oil or a blend of others from this list. Additionally, many of these oils are used in packaged foods such as chips, baked goods, frozen meals, and fried foods.
Other types of oils made from seeds – such as flaxseed, sesame, and hemp – are generally not included in these criticisms.
How Seed Oils are Made
All seeds have natural fat in them – mostly unsaturated fats rather than saturated – which is why they’re often recommended as an addition to a healthful eating pattern. Two methods can be used to remove the oil from the seeds:
- Mechanical extraction – The seeds are pressed to squeeze out the oil. This method is often used for higher-end oils like flaxseed or toasted sesame oil
- Chemical extraction – A solvent such as hexane is used to remove as much oil as possible. After extraction, hexane is evaporated from the oil. Chemical extraction is used for oils that will then be refined.
After extraction comes refinement. Higher-end, mechanically extracted seed oils – sometimes called extra-virgin, cold pressed, or finishing oils – are not typically refined. These oils have more flavor, stronger aroma, a shorter shelf-life, and are best used in low-heat cooking or drizzled over finished dishes.
The typical refinement process has four steps:
- Stabilization – Phospholipids – a type of fat naturally present in seeds – are removed using citric acid, found naturally in citrus fruits, to stabilize the oil, removing cloudiness in appearance.
- Neutralization – Free fatty acids – pieces of fat that have broken away from the whole oil molecules – are removed using an alkaline agent called sodium hydroxide to neutralize acidity and extend shelf-life.
- Color pigment removal – A filtering agent is added that removes color pigments, primarily chlorophyll, further increasing shelf-life. The agent most commonly used is called bentonite clay, derived from volcanic ash.
- Deodorization – This final step removes any remaining undesirable odors by injecting steam and vacuuming it out along with any aromas.
The refining process gives these oils a pale color, mild flavor and aroma, and the ability to withstand high cooking temperatures. This transformation classifies them as neutral oils, meant to be able to be used for a variety of dishes and cooking techniques.
Now that we know the process, let’s review the primary criticisms of refined seed oils.
Seed Oil Criticisms: Inflammation
The seeds used in the oils under criticism contain omega-6 polyunsaturated fats. Omega-6 fats are considered essential, along with omega-3s, because our bodies need them and have to get them through food. These fats assist with essential bodily processes like regulating blood flow, blood pressure, heart rate, and inflammation.
The acute inflammatory process, which resolves within hours or days, is necessary to heal wounds, beat viral illnesses, and prevent and treat infections. There is also chronic inflammation, occurring if the acute system fails or the cause of inflammation persists. Chronic inflammation can cause tissue damage and is an underlying contributor to health conditions including atherosclerosis, inflammatory bowel disease, and cancer. Omega-6 and omega-3 fats both influence inflammation: omega-3 fats help reduce it, while omega-6 fats play a more complicated role.
The most common type of omega-6 fats is linoleic acid, which the body breaks down into different types of fatty acids, one of which being arachidonic acid. This type of fat assists in both pro-inflammatory processes when necessary as well as anti-inflammatory. Critics claim that arachidonic acid leads to chronic inflammation. However, only a fraction of the linoleic acid we consume is converted into arachidonic acid, and only a portion of that is used for pro-inflammatory processes. The vast majority of research has demonstrated that increased linoleic acid intake on its own does not increase inflammation.
One of the largest sources of omega-6 fats in the American diet is processed, packaged snack foods, baked goods and sweets, frozen meals, and more. These tend to be high in saturated fat, sodium, added sugar, trans fat, as well as many industrial ingredients.
Chronic inflammation has been observed in people that eat significant amounts of ultra-processed foods. While these foods may contain linoleic acid, this type of fat does not appear to be the cause of inflammation. However, omega-6 intake has been observed by countless randomized control trials to improve heart health, supporting the claim that the fats themselves are not harmful.
Seed Oil Criticisms: General Processing
Another criticism of seed oils is that they are refined. In general, nutrition advice typically involves limiting highly processed items, also known as ultra-processed foods (UPFs). But refined seed oils are not considered UPFs.
There is no universally accepted definition of UPFs, but the Nova food classification system has been used in discussions about UPFs worldwide. This scale categorizes foods as:
- Unprocessed or Minimally Processed, including whole foods and foods like pasta or plain unsweetened yogurt.
- Processed Culinary Ingredients, which have been mechanically processed but have no added ingredients other than vitamins and minerals, and are typically found in home kitchens, like refined seed oils.
- Processed Foods, which are items from the previous groups have had added ingredients in the process of being canned, cured, smoked, salted, etc.
- Ultra-Processed Foods, which include multiple ingredients you wouldn’t see in a home kitchen, like high fructose corn syrup, artificial colors and flavorings, and stabilizers.
The ingredients lists of refined oils in your kitchen cabinet will typically only list the seeds or fruits that they came from. Though the refining process uses other agents to assist the process, these substances do not remain in the oil in any significant amount, so the end product remains a single-ingredient food. Ultimately, Americans eat many foods in excess that are significantly more processed than seed oils.
Seed Oil Criticisms: Hexanes
Hexane is often used to chemically extract oil when mechanical extraction is not enough. During refinement, hexane evaporates from the oil, leaving only trace amounts. Cooking with these oils further evaporates any remaining hexane.
The concerns about hexane come from the fact that it is also used in industrial, non-food environments where workers regularly inhale excessive amounts of hexane. This consumption has been associated with neurological problems such as numbness or muscle weakness. While this is a concern, the level and duration of hexane exposure in industrial settings are significantly higher than those involved in oil refining – and even more so than anything experienced by home cooks. No evidence has proven that hexane in these oils causes health concerns for consumers.
The Food and Drug Administration unfortunately does not measure levels of hexane in oils after refinement, making the amount remaining difficult to exact. If hexane remains a concern for you, refined seed oils that are certified organic are not allowed to be made with this chemical.
Seed Oil Criticisms: Oxidation
Another criticism of refined seed oils is that they can become oxidized. Oxidation is a chemical reaction in which harmful “free radicals” are created, which can lead to damage, including inflammation and health concerns such as cancer, cardiovascular disease, or type 2 diabetes. Free radicals are created naturally within the body when all foods are broken down into energy.
Oil oxidation can occur in the following situations:
- Improper storage – All oils should be stored in a closed cabinet, especially unrefined oils that still contain chlorophyll which absorbs sunlight. Refined oils have chlorophyll removed, extending the time until they go bad, or become oxidized. All oils eventually go bad, even if stored properly.
- High temperature cooking – Polyunsaturated fats have been demonstrated to produce byproducts known as aldehydes that are suggested to be harmful, though reliable research regarding these health claims has not yet been conducted. Aldehyde production has specifically been observed after cooking at 338–356°F, equivalent to restaurant deep frying temperatures. Home cooking, especially using methods other than deep frying, does not tend to meet these temperatures. Even when a home oven is set to 325°F or higher, the oil does not reach these temperatures.
- Oil reuse – Some restaurants reuse deep fryer oil to cut costs, but this practice is not recommended. When oils are reused, the likelihood of oxidation exponentially increases. Additionally, regularly consuming fried foods has been associated with coronary heart disease, though studies have been unable to determine whether aldehydes are the cause.
Antioxidants are compounds that protect our bodies from the potential damages caused by oxidation. Oil refinement unfortunately removes some naturally occurring antioxidants, but tocopherols – also known as vitamin E – remain or are often added back in. Tocopherols provide antioxidation assistance both to extend oil shelf-life and to eliminate free radicals in the body.
Though other antioxidants removed during refinement may have provided some protection, the primary health benefit of these oils is their polyunsaturated fat content, which always remains. The strength and amount of research demonstrating the health benefits of polyunsaturated fats far outweigh that of the research suggesting they cause oxidative harm.
Seed Oils Supporting Health
Now that we’ve reviewed the primary concerns, let’s talk about the health benefits we get from seed oils.
Eating polyunsaturated fats, including omega-3s and omega-6s, in place of saturated fat and refined carbohydrates has consistently been associated with reducing risk of coronary heart disease and cardiovascular disease. While omega-6 fats play a smaller role in reducing CVD than omega-3 fats, they’re still shown to reduce risk overall.
Consuming high levels of linoleic acid has been significantly associated with lower risks of total cardiovascular disease, cardiovascular mortality, and ischemic stroke. When compared to butter intake which has been associated with increased total and cancer mortality, higher intake of plant-based oils was strongly associated with lower total, cancer, and cardiovascular disease mortality. In trials that examined the amount of arachidonic acid in the body for those consuming very low amounts versus very high amounts of linoleic acid, the levels did not change, suggesting the body only converts a fraction of linoleic acid to arachidonic acid regardless of intake.
As mentioned, omega-6 oils are used in processed foods that Americans tend to eat in excess. We don’t need to make a conscious effort to eat these types of fats because we can find them so easily, so it’s best to focus your efforts on omega-3s and monounsaturated fats, and the omega-6s will come with little effort.
What ultimately matters most is that we follow the general guidelines of eating more plants – vegetables, fruits, whole grains, legumes, nuts, and seeds – as well as fatty fish, lean proteins, and limited ultra-processed foods and red and processed meats. If refined seed oils help us roast or stir fry our vegetables, sear the perfect salmon fillet, and make delicious sauces to flavor our plant foods, then they’ve done their job helping us eat more of the foods recommended for overall health. Only about 12% of Americans eat the recommended 3 servings of vegetables and 2 servings of fruit per day, so we can use all the help we can get!
Cooking with Seed Oils
Both seed oils and fruit oils have a place in a healthful eating pattern. While unrefined options like extra virgin extra virgin olive oil are preferred when their flavors are intended to contribute to the overall dish, neutral oils are used for the opposite. They act as a quiet contributor, assisting in the cooking process while letting the flavors of the aromatics, herbs, and spices shine.
Refined seed oils have higher smoke points than unrefined oils, giving them more culinary applications. Smoke point refers to the temperature to which the oil can be heated before it becomes smoky. It’s best to use refined oils for higher temperature cooking, such as roasting and stir frying. One exception is avocado oil: a fruit oil that has a neutral flavor and can also withstand high temperatures. Another point for refined seed oils is that they’re typically much less expensive than fruit oils or unrefined options. For recommendations on the best oils to stock in your kitchen, see our pantry staples guide. Hint: we recommend canola, avocado, and extra virgin olive oil for go-to oils. Hint: we recommend canola, avocado, and extra virgin olive oil for go-to oils.
To safely store oils, keep them in a cabinet away from natural light. It’s recommended to use unrefined oils within 3-6 months and refined oils within 12-18 months after opening. If the oil begins to smell similar to paint, it’s become oxidized and should be discarded. If concerned about finishing oil before it turns rancid, freezing it stops the oxidation process. Try freezing oil in ice cube trays so you can just take what you need.
Decide for Yourself
It’s important to do your own research to determine your health goals. Social media influencers make health claims all the time, but it’s important to know your sources, look for reliable credentials, and be wary of sweeping statements claiming that a specific food will absolutely cause a specific health issue.
For a reliable source, you can always count on To Taste to provide evidence-based nutrition education. Check out our growing library of educational articles, videos, and resources.
