The Worst Cooking Oils for Your Health (and the Most Stable Oils to Use Instead)

Nearly everything cooked at home or in a restaurant starts with a fat, and the fat you choose changes what ends up on the plate. This guide ranks the worst cooking oils by how easily they break down under heat, then ranks the most stable oils for cooking so you have something to reach for instead. The ranking is built on one number: how much polyunsaturated fat, and specifically linoleic acid, each oil contains.

Key takeaways

• The worst cooking oils are the ones highest in polyunsaturated fats: grapeseed, safflower, sunflower, corn, soybean, and cottonseed. Canola oil sits in the same group at a lower percentage.
• The most stable oils for cooking are saturated fats: coconut oil, ghee, beef tallow, lamb tallow, and cocoa butter.
• Published frying studies show polyunsaturated oils generate toxic aldehydes at high temperatures, and those compounds migrate into the food.
• Olive oil and avocado oil are monounsaturated fats and belong in cold and low-heat uses, with attention to authenticity.
• Seed oils you already own can be used up around the house and workshop instead of thrown out.

This article is for education only and does not diagnose, treat, or replace medical care. I am a clinical herbalist, not a physician. Do not start or stop any medication based on what you read here. Dietary fat changes can affect lipid panels, gallbladder conditions, and some medications, and individual responses vary. Speak with a qualified provider before changing your diet if you are pregnant, managing a diagnosed condition, or taking prescription medication.

How seed oils became food

Evidence: documented. Primary sources are the 1961 American Heart Association report and a firsthand account from the plant breeder who developed canola.

Rapeseed oil was grown in Canada during the Second World War because it clung to wet metal better than any other lubricant, which made it essential for steam-powered ships and trains.² When the war ended, that demand collapsed and a large growing industry was left looking for a market. Keith Downey and Baldur Stefansson at the University of Manitoba spent the following decades breeding erucic acid out of the crop, and the first low-erucic, low-glucosinolate variety was released in 1974 under the name canola.²

The food market for polyunsaturated oils had been opened years earlier. In 1961 the American Heart Association's Central Committee published a report advising Americans to reduce saturated fat and replace it with polyunsaturated vegetable oils.¹ Ancel Keys and Jeremiah Stamler were among the authors. That one recommendation redirected the American diet toward corn, soybean, cottonseed, and later canola oil, and it remains the basis for most mainstream advice on heart health today.

Mainstream sources will tell you these oils are heart healthy, and most of them are not wrong about the fatty acid math in isolation. What gets left out is what happens to polyunsaturated fats during storage, refining, and cooking at high temperatures, and that is what the rest of this article covers.

What a PUFA is and why it breaks down under heat

Evidence: strong for the chemistry of oxidation and aldehyde formation. Human outcome data on long-term dietary intake is contested.

Fats are chains of carbon atoms. A saturated fat has no double bonds in the chain, a monounsaturated fat has one, and a polyunsaturated fat (PUFA) has two or more. Each double bond is a weak point where oxygen can attack. Linoleic acid, the main PUFA in seed oils, has two of them, and alpha-linolenic acid (ALA) in flax and canola has three. More double bonds means faster oxidation, both on the shelf and in the pan.

When linoleic acid oxidizes it forms peroxides, and those peroxides break apart into aldehydes such as 4-hydroxynonenal (HNE) and 4-hydroxyhexenal (HHE), along with a family of oxidized linoleic acid metabolites (OXLAMs). Researchers at De Montfort University measured these compounds directly in frying oils using proton NMR. Sunflower oil heated to 338°F (170°C) for repeated frying cycles produced aldehydes at concentrations they called toxicologically significant, and the same aldehydes were found in the potato chips cooked in it at concentrations in the low tens of parts per million.⁴ The same group published a review in 2021 arguing that polyunsaturated oils should no longer be recommended for high-temperature frying on these grounds.⁵

Ray Peat made the same argument for decades from a different direction. He wrote that unsaturated oils suppress thyroid function and that the animal studies he reviewed showed tallow and coconut oil did not produce the liver and heart damage seen with unsaturated oils.⁷ Whether or not you accept his full framework, the oxidation chemistry he pointed to is now measured routinely in food science labs.

How seed oils are made

Evidence: documented. Residue levels come from published analytical chemistry.

Most commercial seed oils are extracted with technical-grade hexane, a petroleum solvent that dissolves fat efficiently and evaporates at a low temperature. Hexane is neurotoxic, so the European Union set a maximum residue limit of 1 mg per kg of finished oil.⁶ Testing of retail oils has found residues in the parts-per-million range, most samples under that limit, and at least one sunflower oil sample over it.⁶ After extraction the crude oil is degummed, bleached, and deodorized at high temperatures, which strips flavor and color along with much of the vitamin E that would otherwise slow oxidation.

The money behind the advice deserves a mention. Sponsored social media posts from registered dietitians defending seed oils are common, and the sponsorship flag on those posts tells you who paid for the message. Refined seed oils cost a fraction of what tallow, ghee, or coconut oil cost, so the companies that make them have a direct interest in keeping demand high. When you read a claim about healthy oils, check who funded it before you weigh it.

Start here The Ray Peat Diet Guide Every food he mentioned, his hormone framework in order, and links to his interview audio transcripts, and articles. Open the guide →

The most stable oils for cooking, ranked

Evidence: strong for fatty acid composition (gas chromatography data). Composition of animal fats varies with feed.

These fats are ranked by polyunsaturated content, lowest first, with heat tolerance and sourcing notes. PUFA percentages are approximate and drawn from published composition data.^3,10 The top five are saturated fats and the safest choice for any cooking above a gentle sauté.

Rank	Fat or oil	Approx. PUFA	Best use and notes
1	Coconut oil, virgin	2%	About 90% saturated. Sautéing and frying. Cold-pressed or centrifuge-extracted virgin oil has had the least processing.
2	Ghee (clarified butter)	3%	Milk solids and water removed, so it tolerates higher heat than butter. Grass-fed where possible.
3	Beef tallow	4%	Shallow frying and roasting. Pasture-raised beef keeps PUFA at the low end.
4	Lamb tallow (suet)	4%	Very stable, savory flavor. Good for high-heat roasting.
5	Cocoa butter	3%	Highly saturated. Baking and light sauté with a mild chocolate aroma.
6	Palm kernel oil	2%	Saturated and stable. Choose a certified sustainable source.
7	Red palm oil	10%	Half saturated, rich in carotenoids and tocopherols. Medium heat, strong color and flavor.
8	Butter (my favorite)	3%	Low PUFA and the best flavor. Low to medium heat since milk solids brown; switch to ghee for higher temperatures.
9	Duck or goose fat	13%	Saturated and monounsaturated blend. Roasting. Avoid repeated deep-fry cycles.
10	Lard	12%	PUFA rises when pigs are fed soy and corn.10 Choose pastured, soy-free and corn-free lard and use medium heat.
11	Macadamia oil	3%	Monounsaturated and very low PUFA. Cold uses and medium heat. Check the harvest date.
12	Extra-virgin olive oil	10%	Monounsaturated with polyphenols. Cold use. Authenticity is a known problem, covered below.
13	Avocado oil	13%	Monounsaturated. Cold use. Buy lab-verified brands only, covered below.
14	High-oleic sunflower oil	5%	Bred for low linoleic acid. More stable than standard sunflower, still second to saturated fats for frying.
15	High-oleic safflower or canola	8%	Usable in a pinch. Favor coconut oil, ghee, or tallow for frying.

On olive oil and avocado oil authenticity

Monounsaturated fats such as olive and avocado oil are more stable than seed oils but less stable than saturated fats, and both markets have documented quality problems. The UC Davis Olive Center tested imported oils labeled extra virgin in 2010 and found 69% of imported samples failed international standards, compared with 10% of California oils.⁸ A 2020 study from the same university tested 22 avocado oils sold in the United States and found 82% were rancid before their expiration date or adulterated with other oils.⁹ Buy both from producers that publish harvest dates and third-party lab results, in dark glass, and use them cold.

The worst cooking oils, ranked

Evidence: strong for linoleic acid content and oxidation behavior under heat. Long-term human outcome data is contested.

These are the oils that break down fastest at high temperatures, ranked by linoleic acid content with the highest first. A high smoke point on the label does not protect against oxidation; the aldehyde measurements in the frying studies were taken well below the smoke point of the oils involved.⁴ Percentages are approximate.³

Rank	Oil	Approx. linoleic acid	Why it ranks low
1	Grapeseed oil	70%	Among the highest in linoleic acid. Forms peroxides and aldehydes under heat. Avoid for cooking.
2	Safflower oil (linoleic type)	75%	The highest linoleic content of any common oil. Oxidizes fast regardless of smoke point.
3	Sunflower oil (linoleic type)	63%	The oil used in the aldehyde frying studies. Choose high-oleic sunflower if sunflower is your only option.
4	Corn oil	55%	Unstable in fryers. Common in commercial kitchens.
5	Soybean oil	53%	Linoleic plus some ALA. The most common oil in packaged food in the United States.
6	Cottonseed oil	52%	High PUFA and a cotton crop treated as a fiber rather than a food during growing.
7	Wheat germ oil	55%	Goes rancid quickly, even at room temperature. Not a cooking oil.
8	Rice bran oil	33%	Marketed on smoke point. Still oxidizes at frying temperatures.
9	Canola oil	20% plus 9% ALA	About 30% PUFA in total. The ALA fraction oxidizes even faster than linoleic acid. Low heat only if you must use it.
10	Peanut oil	32%	The standard fryer oil in many restaurants, which means repeated heating cycles and accumulated aldehydes.
11	Sesame oil	41%	Carries some natural antioxidants but still oxidation-prone. A finishing drizzle, not a frying oil.
12	Pumpkin seed oil	50%	Flavorful cold drizzle only. Store away from light and heat.
13	Hempseed oil	55% plus ALA	Very high PUFA. Cold use only, refrigerated.
14	Walnut oil	55% plus ALA	Cold use in small amounts. Refrigerate to slow rancidity.
15	Flaxseed oil	15% plus 55% ALA	The most unstable oil on this list because of its ALA content. Never heat it.

The pattern across the table is simple. Every oil listed is polyunsaturated, and the more linoleic acid or ALA it contains, the faster it oxidizes and the more aldehydes it produces when heated. Rice bran, peanut, and canola oil rank a little better on percentage alone, but all three are frying oils in commercial use, and the frying studies showed aldehyde content in food climbing with each repeated heating cycle.⁴

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Smart ways to use up seed oils you already own

Evidence: practical household use. No health claims.

Switching to saturated fats does not mean pouring a full bottle down the drain. Seed oils have plenty of uses around the house and workshop where nothing touches food or skin.

Adhesive and label remover

Soak the sticker with oil, wait 7 minutes, scrape, then wash with hot soapy water or rubbing alcohol. Commercial goo removers are oil-based products, so this does the same job. Use on non-porous surfaces only: glass, metal, and finished wood.

Light rust inhibitor for garden tools

Clean and dry the metal, wipe on a very thin film of oil, and buff off the excess. This works for pruners, shovels, and hand saws. Skip it on precision chains and bearings, where vegetable oils gum up over time.

Squeaky hinges

A drop on the hinge pin and a few swings of the door will quiet it for a while. Vegetable oil thickens as it oxidizes, so switch to a proper machine oil when you get the chance.

Shop wax and release agent

Melt 4 parts oil to 1 part beeswax for a soft wax that eases wooden drawer runners and screw threads. A thin wipe on clamp faces or concrete forms keeps them from sticking. Shop use only.

Fire starters

Mix the oil with sawdust or soak cotton pads, then tuck them into a cardboard egg carton and light the cardboard edge. Ventilate, use small amounts, keep away from accelerants, and never use these in a charcoal chimney that specifies dry kindling.

Emergency oil lamp

Fill a stable glass jar with oil, float a cork or foil disk with a cotton wick through it, and light the wick tip. It burns dim and long. Never leave it unattended, and keep it away from curtains, children, and pets.

What not to do with old seed oil

Keep it off anything that touches food or your body. Cutting boards and cast iron seasoned with high-PUFA oil turn sticky and rancid, which is the same oxidation covered earlier in this article. Keep it out of bikes and sewing machines, where it gums the mechanism. Keep it out of the compost pile in any real quantity, since it attracts pests; if you must, add a small amount well mixed with a large volume of dry browns.

Frequently asked questions

Evidence: summarized from the sections above.

What are the worst cooking oils?

The worst cooking oils are the ones highest in polyunsaturated fats, especially linoleic acid: grapeseed, safflower, sunflower, corn, soybean, and cottonseed oil. Flaxseed, hempseed, and walnut oil rank even lower for heat because of their ALA content, though they are rarely used for cooking.

What is the most stable oil for cooking?

Virgin coconut oil is the most stable common cooking fat at about 90% saturated fat and 2% PUFA. Ghee, beef tallow, and lamb tallow are close behind and tolerate frying and roasting temperatures well.

Is canola oil bad for you?

Canola oil contains about 30% polyunsaturated fat, including ALA, which oxidizes faster than linoleic acid. It is less unstable than sunflower or safflower oil, but it still produces aldehydes when heated, and standard canola is solvent-extracted and refined. If you must use it, choose high-oleic canola and keep it at low heat.

Is olive oil safe at high temperatures?

Extra-virgin olive oil is a monounsaturated fat with about 10% PUFA, and its polyphenols slow oxidation. It handles low and medium heat better than seed oils, but for frying, saturated fats such as tallow and coconut oil produce fewer oxidation products. The bigger issue with olive oil is authenticity, since a large share of imported oils have failed extra-virgin standards in published testing.

Does a high smoke point mean an oil is safe for frying?

No. Smoke point describes when an oil starts to visibly smoke, not when it starts to oxidize. Researchers measured toxic aldehydes forming in sunflower oil at 338°F (170°C), well below its smoke point, and found the same aldehydes in the food fried in it.

Which fats are healthy for cooking?

Saturated fats such as coconut oil, ghee, butter, tallow, and lard from pastured animals are the most stable for cooking. Monounsaturated fats such as olive, avocado, and macadamia oil belong in cold uses. Polyunsaturated seed oils belong in the garage.

Sources

1. Central Committee for Medical and Community Program of the American Heart Association. Dietary fat and its relation to heart attacks and strokes. Circulation. 1961;23:133-136. doi:10.1161/01.CIR.23.1.133
2. Downey RK. Canola becomes a crop: a firsthand history of the wartime rapeseed lubricant program and the breeding of canola. Hosted by SaskCanola, a producer-funded industry organization; cited as the breeder's own account. saskcanola.com
3. Orsavova J, Misurcova L, Vavra Ambrozova J, Vicha R, Mlcek J. Fatty acids composition of vegetable oils and its contribution to dietary energy intake and dependence of cardiovascular mortality on dietary intake of fatty acids. Int J Mol Sci. 2015;16(6):12871-12890. doi:10.3390/ijms160612871
4. Moumtaz S, Percival BC, Parmar D, Grootveld KL, Jansson P, Grootveld M. Toxic aldehyde generation in and food uptake from culinary oils during frying practices: peroxidative resistance of a monounsaturate-rich algae oil. Sci Rep. 2019;9:4125. The algae oil tested was supplied by its manufacturer. doi:10.1038/s41598-019-39767-1
5. Grootveld M. Evidence-based challenges to the continued recommendation and use of peroxidatively-susceptible polyunsaturated fatty acid-rich culinary oils for high-temperature frying practises. Front Nutr. 2021;8:711640. doi:10.3389/fnut.2021.711640
6. Samsuri S, et al. Optimization of residual hexane in edible oils analysis using static headspace gas chromatography. Int J Anal Chem. 2021;2021:1941336. doi:10.1155/2021/1941336
7. Peat R. Unsaturated vegetable oils: toxic. raypeat.com
8. Frankel EN, Mailer RJ, Shoemaker CF, Wang SC, Flynn JD. Tests indicate that imported "extra virgin" olive oil often fails international and USDA standards. UC Davis Olive Center, July 2010. The Olive Center receives funding from California olive oil producers. Report PDF
9. Green HS, Wang SC. First report on quality and purity evaluations of avocado oil sold in the US. Food Control. 2020;116:107328. doi:10.1016/j.foodcont.2020.107328
10. Wood JD, Enser M, Fisher AV, et al. Fat deposition, fatty acid composition and meat quality: a review. Meat Sci. 2008;78(4):343-358. doi:10.1016/j.meatsci.2007.07.019