The “bad” fats are trans fats and saturated fats, with trans fats being the more dangerous of the two. Both raise levels of harmful cholesterol in your blood, but trans fats go a step further by also lowering your protective cholesterol at the same time. Understanding the difference between these fats, where they hide, and what to eat instead can meaningfully lower your risk of heart disease.
Trans Fat: The Worst Type
Trans fat is widely considered the single worst type of fat you can eat. Most trans fat is artificial, created through an industrial process that pumps hydrogen into vegetable oil to make it solid at room temperature. The result, called partially hydrogenated oil, is cheap to produce and has a long shelf life, which made it a staple of the processed food industry for decades. Harvard’s School of Public Health has called it “the worst type of fat for the heart, blood vessels, and rest of the body.”
What makes trans fat uniquely harmful is its double effect on cholesterol. It raises LDL (the kind that builds up in your arteries) while simultaneously lowering HDL (the kind that helps clear cholesterol from your bloodstream). No other dietary fat does both. This combination accelerates the buildup of plaque in your arteries and significantly increases the risk of heart attack and stroke.
In the U.S., the FDA moved to phase out partially hydrogenated oils from the food supply, but trans fats haven’t disappeared entirely. They still show up in some commercially baked goods, frozen pizzas, microwave popcorn, refrigerated dough products, nondairy coffee creamers, stick margarines, and deep-fried foods. In many developing countries, trans fat intake remains high. In India, for example, a partially hydrogenated oil called vanaspati is still widely used as a low-cost cooking fat.
The Labeling Loophole to Watch For
Food labels in the U.S. can legally say “0 grams trans fat” if a single serving contains less than 0.5 grams. That sounds like a small amount, but it adds up. If you eat multiple servings, or several products throughout the day that each contain just under that threshold, you could be taking in a meaningful amount of trans fat without realizing it.
The workaround is simple: check the ingredient list. If you see the words “partially hydrogenated oil” anywhere on the label, the product contains some trans fat regardless of what the nutrition facts panel says. The closer that ingredient appears to the top of the list, the more of it the product contains.
Saturated Fat: Not as Bad, Still a Problem
Saturated fat doesn’t carry the same level of risk as trans fat, but it’s still linked to heart disease when consumed in excess. It’s found naturally in animal products like red meat, butter, cheese, whole milk, and cream, as well as in tropical oils like coconut oil and palm oil.
Saturated fat raises your LDL cholesterol through a different mechanism than trans fat. When saturated fatty acids reach the liver, they cause a redistribution of cholesterol inside liver cells that reduces the liver’s ability to pull LDL cholesterol out of the bloodstream. The result is more LDL circulating in your blood, where it can contribute to artery-clogging plaque over time.
The numbers back this up. For every 5% increase in total calories coming from saturated fat (replacing carbohydrates), overall mortality risk rises by about 8%. People who consume the highest amounts of saturated fat face an 18% greater risk of heart disease compared to those who consume the least.
Not All Saturated Fats Are Equal
Saturated fat isn’t a single substance. It’s a family of fatty acids, and they don’t all behave the same way in your body. The major ones found in food are lauric acid, myristic acid, palmitic acid, and stearic acid. In clinical trials, lauric acid has the strongest effect on raising LDL cholesterol. Palmitic acid and stearic acid are associated with the highest heart disease risk overall.
Stearic acid, found in cocoa butter and some animal fats, has a largely neutral effect on LDL levels in isolation. But when researchers looked at real-world dietary patterns rather than isolated nutrients, all major types of saturated fat were linked to increased heart disease risk. The practical takeaway: you don’t need to memorize which fatty acids are in which foods. Reducing your overall saturated fat intake is what matters.
Palmitic acid deserves a specific mention because it’s the most common saturated fat in the typical diet, found in palm oil, fatty cuts of red meat, and dairy fat. Replacing palmitic acid with plant proteins or polyunsaturated fat reduces heart disease risk by 11% to 12%.
How Much Is Too Much
U.S. dietary guidelines recommend keeping saturated fat below 10% of your total daily calories. On a 2,000-calorie diet, that’s about 22 grams, roughly the amount in three tablespoons of butter or a fast-food cheeseburger. For trans fat, the goal is as close to zero as possible.
Hitting the 10% target is harder than it sounds. Saturated fat is concentrated in many staple foods: cheese, pizza, grain-based desserts, and dishes made with butter or cream. Even people who think they eat relatively well often exceed the limit without realizing it.
What to Eat Instead
The benefit of cutting back on bad fats depends heavily on what you replace them with. Swapping saturated fat for refined carbohydrates like white bread or sugary snacks doesn’t improve your heart disease risk. What does help is replacing those calories with unsaturated fats, particularly polyunsaturated fats.
People who replace just 5% of their calories from saturated fat with linoleic acid, the main polyunsaturated fat in vegetable oils, nuts, and seeds, lower their risk of coronary heart disease events by 9% and their risk of dying from heart disease by 13%. Good sources include walnuts, sunflower seeds, flaxseeds, soybean oil, and fatty fish like salmon and sardines.
Monounsaturated fats, found in olive oil, avocados, and almonds, are also a solid replacement. The simplest strategy is to cook with olive oil or canola oil instead of butter, snack on nuts instead of cheese, and choose fish or poultry over fatty cuts of red meat a few times a week. These swaps don’t require a dramatic diet overhaul, but over time they meaningfully shift the balance of fats in your diet toward the ones that protect your heart rather than damage it.