Saturated fat and trans fat are two types of dietary fat that raise your risk of heart disease by increasing harmful cholesterol levels in your blood. They differ in their chemical structure, where they come from, and how much damage they do, but both are worth limiting. The World Health Organization recommends no more than 10% of your daily calories come from saturated fat and no more than 1% from trans fat.
How Saturated Fat Works
At the molecular level, saturated fat is the simplest type of fat. Its carbon chain holds as many hydrogen atoms as physically possible, with only single bonds between each carbon. That full saturation makes the molecules pack together tightly, which is why saturated fats are solid at room temperature. Think of the white fat on a steak, a stick of butter, or a jar of coconut oil sitting on the counter.
The major dietary sources are animal products: butter, cheese, whole milk, ice cream, red meat, and processed meat like sausage and bacon. Lard, ghee, and tallow are concentrated forms. On the plant side, tropical oils are the main contributors, particularly coconut oil, palm oil, and cocoa butter. Baked goods and fried foods often contain saturated fat from shortening or butter.
How Trans Fat Works
Trans fat is structurally different. It’s technically an unsaturated fat, meaning it has at least one double bond in its carbon chain. But the hydrogen atoms around that double bond sit on opposite sides of the molecule (the “trans” configuration) rather than on the same side (the “cis” configuration found in natural vegetable oils). That opposite-side arrangement straightens out the molecule, making it behave more like a saturated fat: solid or semi-solid at room temperature, resistant to spoiling, and shelf-stable for months.
Most trans fat in the food supply was created through an industrial process called partial hydrogenation. Manufacturers pumped hydrogen gas into liquid vegetable oils under high pressure, which converted some of the natural double bonds into single bonds and flipped others from cis to trans. The result was a cheap, stable fat ideal for frying, margarine, packaged snacks, and baked goods. Small amounts of trans fat also occur naturally in meat and dairy from cows, sheep, and goats, produced by bacteria in the animals’ digestive systems.
Why Both Raise Heart Disease Risk
Saturated fat increases your total cholesterol, LDL (“bad”) cholesterol, HDL (“good”) cholesterol, and non-HDL cholesterol. The LDL increase is the primary concern. Research published in Arteriosclerosis, Thrombosis, and Vascular Biology found that overeating saturated fat changes the composition of LDL particles themselves, enriching them with certain lipids that make the particles more prone to clumping together. Clumped LDL particles are more likely to lodge in artery walls and contribute to plaque buildup.
Trans fat does something worse. It raises LDL cholesterol like saturated fat does, but it simultaneously lowers HDL cholesterol, the type that helps clear excess cholesterol from your bloodstream. That double effect on your cholesterol ratio makes trans fat significantly more harmful per gram than saturated fat. Harvard researchers found that replacing just 5% of daily calories from saturated fat sources like red meat and butter with polyunsaturated fat from vegetable oils, nuts, and seeds lowered coronary heart disease events by 9% and deaths from heart disease by 13%.
Trans Fat and Inflammation
Beyond cholesterol, trans fat triggers inflammatory responses throughout the body. A large study of women found that higher trans fat intake was associated with elevated levels of tumor necrosis factor receptors, markers that reflect chronic, low-grade inflammation. In women with higher body mass, trans fat intake was also linked to increased levels of two additional inflammatory markers, interleukin-6 and C-reactive protein. Chronic inflammation is a driving force behind atherosclerosis, the progressive narrowing and hardening of arteries that leads to heart attacks and strokes.
Where Things Stand With Regulations
In 2015, the FDA determined that partially hydrogenated oils, the primary source of artificial trans fat, were no longer “generally recognized as safe.” Manufacturers had until June 2018 to stop adding them to foods, with a final extension through January 2021 for certain products to clear the supply chain. This effectively removed the vast majority of artificial trans fat from the U.S. food supply.
That doesn’t mean trans fat has disappeared entirely. Small amounts still exist naturally in animal products, and trace amounts can form during high-heat processing of oils. Food labels in the U.S. can list “0 grams trans fat” if a serving contains less than 0.5 grams, so checking ingredient lists for “partially hydrogenated” oil remains a useful habit, though you’ll encounter it far less often than you would have a decade ago.
Practical Ways to Reduce Both
Saturated fat is harder to avoid completely because it’s present in so many common foods. The goal isn’t elimination but moderation. On a 2,000-calorie diet, the 10% guideline translates to roughly 22 grams of saturated fat per day. For context, a tablespoon of butter has about 7 grams, and a cup of whole milk has about 5 grams.
The most effective swap is replacing saturated fat sources with foods rich in polyunsaturated and monounsaturated fats. That means choosing olive oil or canola oil over butter for cooking, eating nuts and seeds as snacks, and favoring fish over red meat a few times per week. Importantly, what you replace saturated fat with matters. Swapping it for refined carbohydrates like white bread and sugary foods doesn’t improve heart disease risk. The Harvard research found that replacing saturated fat calories with the polyunsaturated fat linoleic acid, found in vegetable oils, nuts, and seeds, produced the clearest reductions in heart disease risk. Replacing those same calories with carbohydrates yielded similar benefits only when the carbohydrates came from whole grains and other complex sources.
For trans fat, the regulatory changes have done most of the heavy lifting. Your main exposure now comes from naturally occurring trans fat in meat and dairy, which exists in small enough quantities that the 1% guideline is easy to stay under without major dietary changes.