Trans fats are the most harmful type of dietary fat, and saturated fats come in second. Both raise levels of LDL cholesterol (the kind that damages arteries), though they do so through different mechanisms and to different degrees. The good news: the worst offenders, artificial trans fats, have been largely removed from the U.S. food supply. The more nuanced picture involves saturated fats, which aren’t all equally bad, and cooking oils, which can become harmful when overheated.
Trans Fats: The Worst of the Worst
Artificial trans fats do something no other dietary fat does: they simultaneously raise your LDL (“bad”) cholesterol and lower your HDL (“good”) cholesterol. That’s a double hit. LDL particles deposit cholesterol in artery walls, while HDL particles act like cleanup crews, hauling bad cholesterol to the liver for disposal. Trans fats boost the harmful particles while disabling the protective ones.
Trans fats are especially damaging because they increase the small, dense type of LDL particle, which penetrates artery walls more easily than larger LDL particles. This accelerates the buildup of plaque that leads to heart attacks and strokes.
The FDA determined in 2015 that partially hydrogenated oils (the main source of artificial trans fats) were no longer safe for use in food, and the final compliance date for manufacturers to remove them was January 1, 2021. That regulatory action eliminated most artificial trans fats from packaged foods. However, trans fats haven’t disappeared entirely. They occur naturally in small amounts in meat and dairy products, and trace levels exist in other edible oils.
Where Trans Fats Still Hide
Despite the ban on partially hydrogenated oils, trans fats can still show up in fried, packaged, and processed foods. Categories to watch include:
- Anything fried and battered
- Shortening and stick margarine
- Commercially baked cakes, pies, cookies, and donuts
- Refrigerated dough products
- Frozen dinners and frozen pizza
- Nondairy creamers
A labeling loophole makes this trickier than it should be. Foods containing less than 0.5 grams of trans fat per serving can list “0 grams” on the nutrition label. If you eat multiple servings, or several of these products in a day, small amounts add up. Checking the ingredient list for “partially hydrogenated” anything is more reliable than trusting the number on the label.
Saturated Fat: Harmful, but Not All the Same
Saturated fat raises LDL cholesterol, and current dietary guidelines recommend keeping it below 10% of your daily calories. For someone eating 2,000 calories a day, that’s about 22 grams. The largest Cochrane review on the topic, covering more than 53,000 participants, found that reducing saturated fat intake lowered the risk of cardiovascular events by 17%. That’s a meaningful reduction, though the same review found little effect on overall mortality or heart attack deaths specifically.
What makes saturated fat complicated is that different types behave differently in your body. Palmitic acid, the saturated fat most abundant in palm oil, red meat, and dairy, raises LDL cholesterol significantly. Stearic acid, found in cocoa butter and some animal fats, does not. In a controlled trial placing older women with elevated cholesterol on different diets for five weeks, those eating stearic acid had LDL levels comparable to those eating oleic acid (the monounsaturated fat in olive oil). Both groups had significantly lower LDL than the women eating palmitic acid.
This doesn’t mean you need to memorize which saturated fatty acid is in which food. But it does explain why chocolate (high in stearic acid) doesn’t seem to raise cholesterol the way butter or cheese (high in palmitic acid) does, and why blanket statements about saturated fat can be misleading.
The Coconut Oil Question
Coconut oil has been marketed as a health food, but the clinical data tells a different story. A meta-analysis published in the American Heart Association’s journal Circulation found that coconut oil raised LDL cholesterol by about 10.5 mg/dL compared to plant oils like olive, soybean, or canola oil. It also raised HDL cholesterol by about 4 mg/dL, which is why some advocates point to coconut oil as heart-healthy. But the LDL increase outpaces the HDL benefit.
Compared to palm oil, coconut oil raised LDL by over 20 mg/dL. The one trial comparing coconut oil to butter found that coconut oil actually lowered LDL relative to butter, despite having a higher proportion of saturated fat. So coconut oil is better than butter but worse than most liquid plant oils. If you enjoy it occasionally in cooking, that’s different from using it as your primary fat.
Overheated Cooking Oils
Even healthy oils can become harmful when heated past their stability point. Polyunsaturated oils like sunflower and canola oil are particularly vulnerable because their chemical structure makes them prone to breaking down into toxic byproducts called aldehydes when exposed to prolonged high heat. These compounds are chemically reactive enough to damage DNA, and they carry mutagenic and genotoxic properties.
The amount of aldehydes produced increases with the oil’s polyunsaturated fat content and with cooking time. In lab conditions, sunflower oil heated for 90 minutes produced substantially higher levels of toxic aldehydes than oils with less polyunsaturated fat. For everyday cooking, this means a few things: don’t reuse frying oil repeatedly, don’t heat delicate oils like flaxseed or unrefined sunflower oil to high temperatures, and choose more stable options like olive oil or avocado oil for high-heat cooking.
Seed Oils and Inflammation
You may have seen claims that seed oils (soybean, corn, sunflower, canola) cause chronic inflammation because they’re high in omega-6 fatty acids. The theory is that omega-6 fats get converted into inflammatory compounds in your body. The clinical evidence doesn’t support this. A 2017 meta-analysis of randomized controlled trials found that higher dietary intake of linoleic acid (the primary omega-6 fat in seed oils) does not significantly affect blood levels of inflammatory markers. The reason is straightforward: only about 0.2% of the omega-6 you eat actually converts to the inflammatory compound in question.
That said, seed oils become problematic when they’re used to fry processed foods or when they’re heated repeatedly at high temperatures. The issue in those cases isn’t the omega-6 content itself but the oxidation byproducts described above, plus the fact that foods fried in these oils tend to be nutritionally poor overall.
What to Replace Bad Fats With
The replacement matters as much as the reduction. Swapping saturated fat for polyunsaturated fat (found in walnuts, flaxseed, fatty fish, and most liquid plant oils) reduces mortality risk by about 19%. Replacing it with monounsaturated fat (olive oil, avocados, almonds) reduces mortality by about 11%. Both types of unsaturated fat are equally effective at reducing coronary heart disease events specifically.
Replacing saturated fat with refined carbohydrates like white bread, sugary cereals, or sweetened drinks does not improve heart health and may make things worse. The type of calorie you swap in is the critical variable. A practical approach: cook with olive oil instead of butter, snack on nuts instead of cheese, and choose fish or poultry over processed red meat a few times a week. These substitutions shift your fat intake in the right direction without requiring you to count grams.