Which oil should not be used for frying? Understanding the Risks

November 9, 2025 •

4 min read

According to a study in rats, long-term consumption of reheated frying oil may be linked to increased neurodegeneration. Choosing the right oil is critical for both flavor and health, so understanding which oil should not be used for frying is essential to avoid harmful compounds and preserve nutritional value.

Quick Summary

Several oils are unsuitable for frying due to low smoke points, unstable polyunsaturated fats, or health concerns associated with processing. Reusing oil also generates toxins. Proper selection based on fatty acid composition and heat stability is key to safer and healthier cooking.

Key Points

Avoid oils high in polyunsaturated fats: Oils like soybean, corn, and sunflower are unstable when heated and release harmful compounds.
Check for a high smoke point: Oils with low smoke points, such as unrefined extra virgin olive oil or flaxseed oil, should be avoided for high-heat frying to prevent oxidation and a burnt flavor.
Refrain from reusing oil repeatedly: Each time oil is reheated, it breaks down further, accumulating harmful toxins like aldehydes and trans fats.
Use clarified butter or ghee instead of regular butter: The milk solids in butter burn at high temperatures, which can be avoided by using clarified versions.
Be cautious with generic "vegetable oils": These are often heavily processed blends of unstable oils and are not a safe or healthy choice for high-heat cooking.
Recognize signs of degraded oil: If your frying oil is dark, thick, smells rancid, or smokes excessively, it is time to discard it for safety.

Understanding the Science of Frying Oils

When it comes to frying, all cooking oils are not created equal. The key to making a healthy choice lies in understanding an oil's composition and how it reacts to heat. The two most critical factors are the oil's smoke point and its oxidative stability.

The Importance of Smoke Point

The smoke point is the temperature at which an oil begins to break down and produce smoke. Once an oil reaches this point, its nutritional properties diminish, and it can release harmful compounds like free radicals and aldehydes. For frying, especially deep frying, you need an oil with a high smoke point to ensure it remains stable and safe throughout the cooking process. Unrefined oils, like extra virgin olive oil, often have lower smoke points due to impurities and are not ideal for high-heat methods.

The Danger of Polyunsaturated Fats

Many common vegetable oils, including sunflower, corn, and soybean oils, are rich in polyunsaturated fats (PUFAs). While healthy for consumption when uncooked, these oils are chemically unstable and highly reactive when exposed to high heat during frying. This instability causes them to break down and oxidize, generating harmful compounds like HNE (4-hydroxy-2-nonenal) and other toxic byproducts linked to health problems. This is why oils high in PUFAs are a poor choice for frying, despite some having high smoke points when refined.

Which Oils to Avoid for High-Heat Frying

Oils with low smoke points

Some oils are simply not built for high-heat cooking and should be reserved for salad dressings or finishing dishes. Heating these delicate oils past their smoke point will destroy their flavor and potentially release harmful compounds.

Flaxseed Oil: With a very low smoke point around 225°F (107°C), flaxseed oil should never be heated.
Extra Virgin Olive Oil (EVOO): EVOO has a moderately low smoke point, typically 325–375°F (163–190°C), making it great for sautéing but unsuitable for deep frying.
Unrefined Coconut Oil: While refined coconut oil has a higher smoke point, the unrefined version burns easily at around 350°F (177°C).
Walnut Oil: Similar to flaxseed, walnut oil has a low smoke point and is best used cold.

Vegetable oils high in polyunsaturated fats

Despite being common and often inexpensive, these oils are chemically unstable when heated and should be avoided for high-heat frying.

Soybean Oil: High in unstable omega-6 fatty acids, it should be limited, especially for frying.
Corn Oil: Like soybean oil, corn oil is rich in omega-6s and creates harmful compounds when heated.
Refined Sunflower Oil: Standard refined sunflower oil is high in PUFAs and generates damaging aldehydes upon heating.
Canola Oil: While a common frying oil, it contains a significant amount of unstable polyunsaturated fats, making it a poor choice for prolonged high-heat cooking.

Solid fats and blended oils

Butter: Regular butter contains milk solids that burn easily at high temperatures (around 302°F or 150°C), producing a harsh flavor and smoke. Clarified butter or ghee is a better alternative for high heat.
Generic "Vegetable Oil": Often a mix of various refined seed oils (soybean, corn, canola), these blends are high in unstable polyunsaturated fats and heavily processed with chemicals.

Comparison of Unsuitable vs. Suitable Frying Oils


Feature	Unsuitable Frying Oils	Suitable Frying Oils
Fatty Acid Profile	High in unstable polyunsaturated fats (PUFAs).	Higher in stable monounsaturated or saturated fats.
Smoke Point	Often low (unrefined oils) or misleadingly high (refined PUFA oils).	High and reliable for the intended cooking temperature.
Oxidative Stability	Poor; breaks down readily, especially when reused.	Excellent; resists oxidation and harmful compound formation.
Flavor Impact	Can become acrid and burnt once past smoke point.	Imparts a pleasant, complementary flavor or remains neutral.
Health Implications	Releases harmful compounds and free radicals with repeated heating.	Maintains stability and preserves nutritional integrity during cooking.

The Risks of Reusing Frying Oil

Just as important as choosing the right oil is knowing when to discard it. Reheating cooking oil, especially repeatedly, accelerates the degradation process. Each time oil is heated, it breaks down and forms more harmful substances like aldehydes and trans fats. These toxic compounds accumulate in the oil and can increase the risk of various chronic diseases. Signs that oil is past its prime include darkening, thickening, a rancid smell, and smoking excessively at normal frying temperatures. Filtering can extend the life of an oil slightly, but it will inevitably lose its stability and should be replaced.

Conclusion

For safer and healthier cooking, avoid using oils that are high in unstable polyunsaturated fats or have a low smoke point. Instead, opt for oils rich in heat-stable monounsaturated or saturated fats like refined avocado or olive oil for high-heat frying. Additionally, never reuse frying oil more than a few times to minimize the accumulation of harmful toxins. By making informed choices, you can enhance the flavor of your food and protect your health. For more on dietary fats, consult resources like the Heart Foundation's guide to healthy eating, found here: https://www.heartfoundation.org.nz/about-us/news/blogs/which-oils-are-safe-to-cook-with.

Frequently Asked Questions

Yes, you can use refined or 'light' olive oil for deep frying, as it has a higher smoke point than extra virgin olive oil (EVOO). EVOO, with its lower smoke point, is better suited for sautéing or finishing dishes rather than high-heat deep frying.

Oils with high levels of polyunsaturated fats, like corn and sunflower oil, are chemically unstable when heated. They break down and oxidize, creating toxic compounds such as HNE (4-hydroxy-2-nonenal) and harmful free radicals.

Frying with an oil past its smoke point causes it to degrade, lose nutritional value, and release harmful compounds like acrolein. This can also give your food an unpleasant, bitter, or burnt taste.

Reusing frying oil is not recommended, as repeated heating accelerates the formation of toxic compounds like trans fats and aldehydes. For safety and the best flavor, oil should be replaced after a few uses, especially if it appears dark, thick, or smells rancid.

Regular butter contains milk solids that burn easily at high temperatures and give food a bitter taste. For high-heat cooking, it is better to use clarified butter or ghee, which have had the milk solids removed, giving them a higher smoke point.

Allow the oil to cool completely, then pour it into a sealed, non-breakable container. You can then dispose of it with your regular household trash. Never pour oil down the drain, as it can cause serious plumbing issues.

Not all, but many common 'vegetable oils' are blends of unstable seed oils like soybean and corn. These are often highly processed and rich in polyunsaturated fats that are easily damaged by heat. Better vegetable oil options exist, but it is important to check their specific fatty acid profile.