Are there oils to avoid when cooking? The truth about heat and stability

December 9, 2025 •

4 min read

According to the World Health Organization, heating certain vegetable oils can release high concentrations of chemicals called aldehydes, which have been linked to illnesses including cancer, heart disease, and dementia. This fact highlights the critical importance of selecting the right cooking oil for the right application.

Quick Summary

The stability of cooking oil is crucial for health and depends on its fatty acid composition, not just smoke point. Oils high in unstable polyunsaturated fats, especially when refined and repeatedly heated, can produce harmful compounds. Healthier options for high-heat cooking include oils rich in stable monounsaturated fats.

Key Points

Unstable Oils to Avoid: Stay away from cooking with industrial seed oils like corn, soybean, and standard sunflower oil, especially at high temperatures, due to their instability and risk of forming toxic compounds.
Prioritize Oxidative Stability over Smoke Point: An oil's ability to resist oxidation and heat damage is a more important health metric than its smoke point alone.
Choose High-MUFA Oils for High Heat: Opt for oils rich in monounsaturated fats (MUFAs) for high-heat cooking, as they are more stable. Good examples include avocado oil and refined olive oil.
Reserve Delicate Oils for Low Heat: Oils with low smoke points, such as flaxseed or extra virgin olive oil, are best for dressings and light sautéing to preserve their nutrients.
Avoid Reusing Oil: Never reuse cooking oil, particularly after high-heat applications, as repeated heating accelerates the formation of harmful, oxidized compounds.

Understanding Oil Stability: Beyond the Smoke Point

For many years, the smoke point of an oil—the temperature at which it visibly smokes—was considered the primary factor for choosing a cooking oil. While smoke point is still a relevant consideration, modern nutritional science points toward a more critical factor: oxidative stability. Oxidative stability measures an oil's resistance to breaking down when exposed to heat, air, and light. When an oil oxidizes, it can produce harmful compounds such as aldehydes and free radicals. These compounds can cause cellular damage and have been linked to various chronic diseases.

The Role of Fatty Acid Composition

The fatty acid makeup of an oil is the strongest predictor of its stability. Fats can be classified into three main types based on their chemical structure:

Saturated Fats (SFAs): These contain no double bonds, making them highly stable and resistant to heat. They are solid at room temperature and are found in coconut oil, ghee, and animal fats. While once widely vilified, their stability means they are less prone to oxidation under high heat, though their effect on cholesterol levels remains a topic of debate.
Monounsaturated Fats (MUFAs): These have one double bond and are also relatively stable. Oils like olive and avocado are rich in MUFAs and are considered heart-healthy. Their natural antioxidants also help protect against oxidative damage.
Polyunsaturated Fats (PUFAs): Containing two or more double bonds, these are the most unstable fats and are highly susceptible to oxidation when heated. Many common vegetable and seed oils fall into this category, including sunflower, soybean, and corn oil.

Oils to Avoid, Especially for High-Heat Cooking

When you apply high, prolonged heat to unstable oils, you risk generating toxic byproducts. Here are some of the oils you should be cautious about, particularly for frying and searing:

Industrial Seed Oils (Soybean, Corn, Sunflower, Safflower, Canola): These oils are typically high in omega-6 PUFAs and are often highly refined. This combination of instability and heavy processing makes them prone to oxidation when heated, producing harmful chemicals like aldehydes.
Partially Hydrogenated Oils (Trans Fats): These artificial fats are created through a process that adds hydrogen to liquid vegetable oils to make them more solid. They are considered very harmful to heart health and have been banned from food products in many countries. Check ingredient lists for "partially hydrogenated vegetable oil."
Flaxseed and Walnut Oil: These are excellent sources of omega-3s but have very low smoke points and poor thermal stability. They are best used unheated for salad dressings or drizzled over finished dishes to preserve their delicate nutrients.
Repeatedly Used Oils: Reusing oil, especially for deep frying, significantly increases the concentration of toxic byproducts. The oil's smoke point decreases with each use, and it becomes progressively more harmful.

Comparing Oils for High-Heat Cooking


Feature	Avocado Oil	Refined Olive Oil	Coconut Oil	Sunflower Oil (Standard)
Fatty Acid Profile	High MUFA, low PUFA	High MUFA, low PUFA	High SFA	High PUFA (Omega-6)
Oxidative Stability	Excellent	Very Good (refined)	Very Good	Poor
Smoke Point	Very High (up to 520°F)	High (up to 465°F)	Medium (350°F)	High (450°F)
Recommended Use	Frying, searing, roasting	Frying, roasting, sautéing	High-heat cooking, baking	NOT Recommended for high heat
Health Considerations	Rich in antioxidants, heart-healthy	Rich in antioxidants, heart-healthy	Controversial due to saturated fat	Promotes inflammation, linked to chronic disease

How to Choose the Right Oil for Your Cooking Method

To make the healthiest choices in your kitchen, consider the cooking method and temperature. The goal is to match the oil's stability and smoke point to the heat level you'll be using.

For High-Heat Cooking (Frying, Searing, Roasting): Prioritize oils with high oxidative stability. Excellent choices include avocado oil, ghee, and refined olive oil. Refined oils often have higher smoke points due to processing that removes impurities.
For Medium-Heat Cooking (Sautéing, Baking): Oils like extra virgin olive oil are great for these applications. While EVOO has a moderate smoke point, its high antioxidant content provides good thermal stability for typical sautéing temperatures.
For No-Heat Applications (Dressings, Drizzling): Reserve delicate oils with low smoke points, like flaxseed, walnut, or extra virgin olive oil, for dressings, marinades, or finishing dishes. This preserves their nutrients and delicate flavors.

The Takeaway for Smarter Cooking

Ultimately, being a smarter cook means thinking beyond the simple smoke point and considering how different oils behave under heat. By avoiding unstable polyunsaturated oils for high-temperature cooking and opting for stable, nutrient-rich fats instead, you can significantly reduce the potential for harmful compounds to form in your food. Always store oils in a cool, dark place to prevent premature oxidation and never reuse oil that has been heated past its smoke point.

Conclusion: Prioritize Stability for Healthier Meals

The question of whether there are oils to avoid when cooking has a clear answer: yes. Unstable, heavily processed vegetable oils are not suitable for high-temperature cooking due to their high polyunsaturated fat content, which can degrade into toxic byproducts. To protect your health, favor oils with greater oxidative stability, such as avocado or refined olive oil, for high-heat methods. For low-heat or raw uses, choose nutrient-rich options like extra virgin olive oil. By understanding the science of oil stability, you can make informed decisions that contribute to safer, more nutritious, and more flavorful cooking.

Optional Outbound Link

For more in-depth information on oil stability and its impact on health, you can read more from the Olive Wellness Institute.

Frequently Asked Questions

When an oil is heated past its smoke point, it starts to break down and oxidize, releasing free radicals and toxic compounds like aldehydes. This not only imparts a burnt or bitter flavor to food but can also be detrimental to your health upon consumption or inhalation of the fumes.

Industrial seed oils like corn, soybean, and standard sunflower oil are typically high in unstable polyunsaturated fats (omega-6s) and are heavily processed. When exposed to heat, these fats oxidize easily, contributing to inflammation and potentially creating harmful chemical byproducts.

Yes, extra virgin olive oil is safe for most cooking methods, including sautéing and roasting. While it has a moderate smoke point compared to refined oils, its high monounsaturated fat content and rich antioxidants provide good thermal stability.

It is not recommended to reuse cooking oil, especially oil that has been heated to high temperatures. Each time oil is reheated, its stability degrades further, increasing the concentration of toxic byproducts and reducing its smoke point.

Refined oils are processed to remove impurities, giving them a neutral flavor and a higher smoke point. Unrefined oils are minimally processed, retaining more nutrients and natural flavor but generally having lower smoke points. The refining process can also strip away beneficial antioxidants.

For deep frying, oils with very high smoke points and strong oxidative stability are best. Refined avocado oil and high-oleic safflower oil are excellent choices. Other options include ghee or tallow.

To prevent premature oxidation and rancidity, store all cooking oils in a cool, dark place, away from heat and light. Ensure the cap is sealed tightly after each use.