Trans fat is a type of unsaturated fat whose molecular shape makes it behave more like a solid, artery-clogging fat in your body. Most trans fat in the food supply was artificially created through an industrial process called partial hydrogenation, though small amounts occur naturally in meat and dairy. The World Health Organization estimates that eliminating industrially produced trans fat could save more than 500,000 lives per year worldwide.
How Trans Fat Differs From Other Fats
All fats are chains of carbon atoms with hydrogen atoms attached. In most naturally occurring unsaturated fats, the hydrogen atoms sit on the same side of the carbon chain. This “cis” arrangement creates a bend in the molecule, which is why oils like olive oil and canola oil are liquid at room temperature.
In a trans fat, the hydrogen atoms sit on opposite sides of the chain. That small difference straightens the molecule out, letting it pack tightly together the way saturated fat does. The result is a fat that’s solid or semi-solid at room temperature, with a higher melting point than regular vegetable oil. This structural quirk is what gives trans fat its usefulness in food manufacturing and its danger to human health.
How Artificial Trans Fats Are Made
Most trans fat entered the food supply through a process called partial hydrogenation. Manufacturers bubble hydrogen gas through liquid vegetable oil in the presence of a nickel catalyst. The hydrogen atoms attach to some of the double bonds in the oil’s fatty acid chains, converting them from unsaturated to more saturated. When this process is incomplete (partial rather than full hydrogenation), some of the remaining double bonds flip into the trans configuration.
Food companies adopted this process for several practical reasons. It could turn a cheap liquid oil into a solid fat with a spreadable, butter-like consistency, useful for margarine, shortening, and baked goods. Processors could fine-tune the melting point by adjusting temperature, hydrogen pressure, and how long the reaction ran. The resulting fat was also more chemically stable than the original oil, giving products a longer shelf life and making them better suited for repeated deep frying. For decades, partially hydrogenated oils were considered a low-cost, versatile ingredient available to manufacturers worldwide.
What Trans Fat Does to Your Body
Trans fat is uniquely harmful because it damages your cardiovascular system through multiple pathways at once. It raises LDL (“bad”) cholesterol while lowering HDL (“good”) cholesterol, a combination that few other dietary fats produce. It also shrinks the size of LDL particles, which makes them more likely to penetrate artery walls, and raises blood levels of a compound called Lp(a) that independently increases heart disease risk.
Beyond cholesterol numbers, trans fat promotes chronic inflammation and impairs the function of the cells lining your blood vessels. Research published in Prostaglandins, Leukotrienes and Essential Fatty Acids described one mechanism: when trans fatty acids get incorporated into cell membranes, they attract excess cholesterol into those membranes. This disrupts a key protective signaling pathway in vascular cells, accelerating the buildup of plaque in arteries. Consuming more than 1% of total daily calories from trans fat is associated with increased risk of coronary heart disease and death.
Natural Trans Fat in Meat and Dairy
Small amounts of trans fat form naturally in the digestive systems of cows, sheep, and goats. These ruminant trans fats end up in beef, lamb, butter, cheese, and milk, typically at much lower levels than what processed foods once contained. You might wonder whether these natural versions carry the same risks as the industrial kind. The evidence is limited. A USDA systematic review found no substantial biological difference between natural and industrial trans fats when ruminant trans fat was consumed at seven to ten times the normal dietary level. In practice, most people eat so little natural trans fat that it’s unlikely to pose the same threat as the industrial version, but the biology appears similar at high enough doses.
Bans and Regulations
In 2015, the FDA determined that partially hydrogenated oils are not “generally recognized as safe.” Manufacturers had until June 18, 2018, to stop adding them to foods, with a final deadline of January 1, 2021, for remaining products to clear distribution. This effectively removed the largest source of artificial trans fat from the U.S. food supply.
Globally, the WHO launched its REPLACE initiative in 2018, calling for worldwide elimination of industrially produced trans fat. Since then, population coverage of best-practice elimination policies has increased more than six-fold, and the WHO has pushed for all countries to complete the effort by 2025. Many nations now cap industrial trans fat at 2% of total fat in foods or ban partially hydrogenated oils outright.
Why Labels Can Still Say “0g Trans Fat”
Even after the ban on partially hydrogenated oils, trans fat hasn’t completely vanished. It occurs naturally in animal products, and trace amounts can form during the refining and deodorizing of regular vegetable oils. U.S. labeling rules allow manufacturers to list 0 grams of trans fat if a serving contains less than 0.5 grams. That means a product with 0.4 grams per serving will show “0g” on the nutrition facts panel.
If you eat multiple servings, those fractions add up. To spot hidden trans fat, check the ingredients list for “partially hydrogenated” anything, though this is now rare in the U.S. The American Heart Association recommends keeping trans fat below 1% of your total daily calories. On a 2,000-calorie diet, that’s less than about 2 grams per day, a threshold that’s easy to exceed if you’re eating imported products, certain fast foods, or packaged goods from countries without strong regulations.
Common Foods That Once Contained Trans Fat
- Margarine and shortening: Stick margarine was one of the most concentrated sources. Many brands have reformulated to use palm oil or fully hydrogenated oils blended with liquid oils.
- Baked goods: Cookies, crackers, pie crusts, and frozen pastries relied on partially hydrogenated oils for flaky texture and long shelf life.
- Fried foods: Restaurants favored partially hydrogenated oils for deep frying because they lasted longer without breaking down.
- Coffee creamers and microwave popcorn: These shelf-stable products often used small amounts of partially hydrogenated oils as a base.
Most of these products have been reformulated, but if you’re buying food manufactured outside the U.S. or from small producers, it’s still worth reading labels carefully.