The oils most clearly harmful to your health are those containing industrial trans fats, oils that have gone rancid, and oils that have been reheated multiple times for frying. Beyond those, some common cooking oils carry risks depending on how they’re used, how they’re stored, and how much of them you consume. The full picture is more nuanced than a simple good-or-bad list.
Partially Hydrogenated Oils: The Worst Offenders
Partially hydrogenated oils are the most definitively harmful oils you can consume. These are liquid vegetable oils that have been chemically processed with hydrogen gas to make them solid at room temperature, creating artificial trans fats in the process. The hydrogenation process produces 14 different altered fatty acid molecules that your body can use for energy but cannot use for essential biological functions like regulating blood flow and building cell membranes.
These trans fats cause inflammation and calcification of artery walls, raise LDL (“bad”) cholesterol and triglycerides, and interfere with an enzyme your body needs to regulate blood flow. The World Health Organization considers any trans fat intake above 1% of total daily calories a risk factor for coronary heart disease, and has been pushing for global elimination of industrial trans fats since 2018.
Most countries have banned or restricted partially hydrogenated oils, but they haven’t disappeared entirely. In the U.S., FDA labeling rules allow manufacturers to print “0 g trans fat” on a label as long as a serving contains less than 0.5 grams. That means a product can contain partially hydrogenated oil and still claim zero trans fat. If you eat multiple servings, the amount adds up. Check ingredient lists for the words “partially hydrogenated” to catch what the nutrition label might not show.
Oils Reheated for Frying
Fresh oil used once for frying is one thing. Oil reused over and over, which is standard practice in many restaurants and fast-food operations, is a different substance entirely. Each round of heating breaks the oil down further through oxidation, hydrolysis, and polymerization. The result is a buildup of free radicals, peroxides, and aldehydes that get absorbed into whatever food is being fried.
Testing of repeatedly heated oils has found peroxide levels far exceeding the upper safety limit set by the American Oil Chemists’ Society (10 meq/kg). These breakdown products are the same types of compounds linked to chronic inflammation, oxidative stress, and cardiovascular damage. You can’t always tell whether a restaurant reuses its oil excessively, but a telltale sign is oil that looks dark or gives food a stale, off flavor.
Rancid Oils and Why Storage Matters
Any oil can become harmful if it goes rancid. Rancidity is oxidation: the fat molecules react with oxygen and break down into compounds your body treats as toxic. Consuming rancid oils destroys vitamins A and E in your food, and animal studies have linked the breakdown products to organ damage, accelerated atherosclerosis, reduced immune function, and increased insulin resistance. Long-term exposure to lipid oxidation products also appears to worsen inflammation and disrupt normal fat metabolism.
One North African study found that rancid animal fats were associated with a significantly increased risk of nasopharyngeal cancer. And the problem isn’t limited to cooking oils on your shelf. A 2018 Australian study found that 38% of fish oil supplements tested had already exceeded acceptable oxidation levels.
Oils high in polyunsaturated fats are the most vulnerable to rancidity because their chemical structure has multiple points where oxidative damage occurs. This includes oils like flaxseed, walnut, and fish oil. Store these in dark bottles, keep them refrigerated after opening, and use them within a few months. If an oil smells like crayons, paint, or old nuts, it’s rancid.
High-PUFA Oils and Heat
Polyunsaturated fats have two or more double bonds in their molecular structure, and those bonds react with oxygen when exposed to high heat. This means oils like sunflower, soybean, corn, and generic “vegetable oil” (usually soybean) are poor choices for frying, sautéing, or any prolonged high-heat cooking. When these oils break down, they produce aldehydes and lipid peroxides, both of which are toxic.
Smoke point used to be the main way people judged whether an oil was safe for cooking, but oxidative stability is more important. An oil’s actual smoke point drops as it heats, especially in seed oils with high polyunsaturated fat content like canola and sunflower oil. So an oil might not visibly smoke but still be producing harmful compounds.
The most heat-stable oils are those highest in saturated and monounsaturated fats. Coconut oil, which is over 90% saturated fat, has maintained acceptable quality even after 8 continuous hours of deep frying at 365°F. Olive oil, despite its reputation as a low-heat oil, is actually quite stable: one study found it lasted over 24 hours in a deep fryer before excessive oxidation. Other good choices for high-heat cooking include avocado oil, ghee, lard, tallow, and palm oil.
The Omega-6 Question
This is where the conversation gets more controversial. Many common cooking oils, including soybean, corn, sunflower, and safflower oil, are very high in linoleic acid, an omega-6 fatty acid. Your body needs some linoleic acid, but the modern diet delivers far more than necessary, largely because these oils are in nearly every processed food.
Research published in the BMJ’s Open Heart journal has laid out the case that excess linoleic acid drives inflammation in blood vessel walls. The linoleic acid metabolite 9-HODE promotes inflammation and may both mark and accelerate atherosclerosis. Studies in mice engineered to maintain a 1:1 omega-6 to omega-3 ratio showed significantly reduced arterial plaque compared to mice with the lopsided ratios typical of Western diets.
A two-year randomized trial found that people consuming a margarine high in linoleic acid (58% LA) saw their C-reactive protein, a key inflammation marker, rise steadily over two years. Those given a margarine enriched with omega-3 alpha-linolenic acid instead saw their inflammation drop, with a net difference of about 0.5 mg/L. One trial found that excess linoleic acid was more inflammatory to blood vessel linings than excess saturated fat.
This doesn’t mean you need to eliminate all omega-6 fats. It means that if your diet is heavy in processed foods, fast food, and restaurant meals, you’re likely consuming far more linoleic acid than your body can handle without consequences. Shifting toward olive oil, avocado oil, or even butter as your primary cooking fats is a practical way to reduce that load.
Tropical Oils: A Mixed Picture
Palm oil, palm kernel oil, and coconut oil are high in saturated fat, which raises LDL cholesterol. But they’re not all equivalent. Palm oil is about 50% saturated fat, while palm kernel oil and coconut oil are over 85% saturated. Harvard nutrition experts consider palm oil clearly better than trans-fat-laden shortenings and likely a better choice than butter, though they still recommend naturally liquid oils like olive oil as a first choice.
Coconut oil has gained a health-food reputation, but its saturated fat content is among the highest of any cooking fat. It’s excellent for heat stability, but regular heavy use will raise your LDL cholesterol. Using it occasionally for specific dishes is different from making it your everyday oil.
Practical Guidelines for Choosing Oils
The simplest framework: match your oil to how you’re using it.
- For high-heat cooking and frying: Choose oils high in saturated or monounsaturated fats. Olive oil, avocado oil, coconut oil, ghee, and animal fats are the most oxidatively stable options.
- For salad dressings and cold use: This is where high-PUFA oils like walnut, flaxseed, or sesame oil can work well, since there’s no heat to trigger oxidation. Store them in the fridge.
- For everyday cooking: Extra virgin olive oil is hard to beat. It’s high in monounsaturated fat, resistant to oxidation, and well-supported by long-term health data.
Avoid any product listing “partially hydrogenated” oil in its ingredients, regardless of what the nutrition label says about trans fat. Don’t reuse frying oil more than once or twice, and discard it if it darkens or smells off. Keep all oils sealed, away from light and heat, and pay attention to expiration dates, especially for polyunsaturated oils and fish oil supplements that are prone to going rancid before you finish them.