The two fats consistently linked to health problems are trans fats and, to a lesser degree, saturated fats. Trans fats are the worst offenders, raising your harmful cholesterol while simultaneously lowering your protective cholesterol. Saturated fats raise harmful cholesterol too, though the picture is more nuanced. Understanding what each type does in your body, and where they hide in your diet, helps you make smarter choices at the grocery store and the stove.
Trans Fats: The Worst of the Worst
Trans fats are the one type of dietary fat with almost no defenders in the nutrition world. They’re created when liquid vegetable oils are processed with hydrogen to make them solid and shelf-stable, a process called partial hydrogenation. In liver cells, these industrial trans fats activate a pathway that ramps up cholesterol production in ways that natural fats, even saturated ones, do not. The result is a double hit: your LDL (“bad”) cholesterol goes up and your HDL (“good”) cholesterol goes down.
The health risks were serious enough that the FDA determined partially hydrogenated oils are no longer Generally Recognized as Safe. The agency set January 1, 2021, as the final compliance date for manufacturers to remove them from foods, and completed the last regulatory revocations in December 2023. That means partially hydrogenated oils have been stripped from the standards for products like peanut butter, canned tuna, margarine, and shortening.
Trans fats haven’t completely vanished, though. They occur naturally in small amounts in meat and dairy products, and refined vegetable oils can contain trace amounts as an unintentional byproduct of processing. In practice, you’re unlikely to encounter dangerous levels in the modern food supply, but imported or older packaged products could still contain them.
Saturated Fat: Where the Debate Gets Complicated
Saturated fat has been a dietary villain since the 1960s, and while recent headlines have questioned that reputation, the overall body of evidence still supports limiting it. Dietary patterns with lower average intakes of saturated fat are consistently associated with better cardiovascular outcomes in large observational studies. Beyond raising LDL cholesterol and increasing the concentration of harmful lipoprotein particles in your blood, higher saturated fat intake may also promote inflammation, disrupt heart rhythm, and interfere with the protective function of HDL cholesterol.
The Dietary Guidelines for Americans recommend keeping saturated fat below 10% of your total daily calories. For someone eating 2,000 calories a day, that’s about 22 grams, roughly the amount in three tablespoons of butter. For people who already have high cholesterol, some experts recommend going further, down to 5 to 6% of daily calories.
What makes the saturated fat conversation complicated is that not all sources behave identically in the body, and the food you eat instead matters enormously. Replacing saturated fat with polyunsaturated fats (found in fish, walnuts, flaxseed, and most liquid vegetable oils) is tied to a significantly lower risk of death from cardiovascular disease, cancer, neurodegenerative disease, and respiratory disease. Replacing saturated fat with refined carbohydrates like white bread and sugar, on the other hand, doesn’t appear to help. The swap matters as much as the cut.
Common Sources of Saturated Fat
Saturated fat is solid at room temperature, which is a useful mental shortcut. Butter, cheese, whole milk, fatty cuts of beef and pork, poultry skin, and cream are the biggest contributors in most Western diets.
Tropical oils deserve special attention because they’re often marketed as healthy alternatives. Coconut oil is 92% saturated fat, higher than butter or lard. Despite its popularity in wellness circles, it raises LDL cholesterol in the same way other saturated fats do. Palm oil is also high in saturated fat and appears in countless processed foods, from crackers to frozen pizza, often under names like “palm kernel oil” or simply “vegetable oil.” Checking ingredient lists is worth the few seconds it takes.
How Cooking Can Turn Good Fats Bad
Even oils that start out healthy can become problematic depending on how you use them. When any oil is heated repeatedly or pushed past its tolerance, it oxidizes, breaking down into compounds that can damage cells. Oxidative stability, meaning how well an oil resists this breakdown, matters more than smoke point alone.
Some seed oils with high smoke points actually have low oxidative stability, which means they break down internally before they visibly smoke. Oils rich in polyunsaturated fats (like soybean, corn, and sunflower oil) are more vulnerable to this kind of degradation. For high-heat cooking like frying or roasting, oils with higher stability, such as extra virgin olive oil or avocado oil, tend to hold up better. Using any oil only once for deep frying, rather than reheating it multiple times, also reduces the formation of harmful byproducts.
How to Spot Bad Fats on a Label
Trans fat now has its own line on the Nutrition Facts panel, making it relatively easy to find. The main thing to know is that “partially hydrogenated oil” in the ingredients list was the telltale sign of artificial trans fats. With the FDA’s ban fully in effect, you should rarely see this anymore in products manufactured in the United States. If you do spot it on an imported product or something that’s been sitting on a shelf for a long time, put it back.
Saturated fat is also listed on every Nutrition Facts label, measured in grams per serving. A quick rule of thumb: if a single serving delivers more than 3 or 4 grams and you eat that food regularly, it’s worth checking whether a lower-saturated-fat option exists that you’d actually enjoy. Small substitutions, like using olive oil instead of butter for sautéing, or choosing chicken thighs over a ribeye a few nights a week, add up over months and years without requiring a dramatic diet overhaul.
The Practical Takeaway
Trans fats are genuinely harmful and largely eliminated from the food supply, though not entirely gone. Saturated fat isn’t poison, but eating a lot of it, especially from processed and fried foods, raises your cardiovascular risk over time. The single most effective dietary shift you can make is replacing saturated fat with unsaturated fat rather than with sugar or refined starch. That means more fish, nuts, seeds, olive oil, and avocado in place of butter, cream, and fatty processed meats. It doesn’t mean perfection at every meal. It means tilting the balance in a better direction, consistently.