Trans fats are a type of unsaturated fat whose molecular shape has been altered, either through industrial processing or naturally in small amounts, making them behave more like solid saturated fats in your body. They are strongly linked to heart disease: for every 2% increase in calories from trans fats, the risk of coronary heart disease rises by 23%. Most countries have moved to ban or restrict them, and the U.S. FDA ruled in 2015 that the main source of artificial trans fats, partially hydrogenated oils, could no longer be considered safe for food.
How Trans Fats Differ From Other Fats
All fats are built from chains of carbon atoms bonded to hydrogen atoms. In unsaturated fats (like olive oil or canola oil), some of those carbon atoms share a double bond, creating a kink in the chain. This kink keeps the fat liquid at room temperature. In naturally occurring unsaturated fats, the carbon chains on either side of that double bond sit on the same side, a shape chemists call “cis.” Trans fats have the same double bond, but the carbon chains sit on opposite sides, creating a straighter molecule.
That straightened shape is the whole problem. It lets trans fat molecules pack together tightly, almost like saturated fat. This is why partially hydrogenated oils are semi-solid at room temperature and why they were so attractive to the food industry: they gave products the texture and shelf life of butter or lard but were made from cheap vegetable oils.
How They’re Made
The vast majority of trans fats in the food supply came from an industrial process called partial hydrogenation. Liquid vegetable oil is placed in a pressurized reactor (typically at 120 to 250°C and 1 to 5 bars of hydrogen pressure) with a nickel catalyst. Hydrogen gas is forced into the oil, converting some of the double bonds in the fat chains into single bonds, which makes the oil more solid. The process is called “partial” because not every double bond gets saturated. The ones that remain can flip from their natural cis shape into the trans shape as a side effect of the heat, pressure, and catalyst.
The result is a fat that’s firmer than liquid oil, resists going rancid, and can be reused for frying many times. These properties made partially hydrogenated oils a staple in commercial baking, fast food, and packaged snacks for decades.
Natural Trans Fats in Meat and Dairy
Small amounts of trans fats occur naturally in beef, lamb, and dairy products. Bacteria in the stomachs of cows and sheep convert some dietary fats into trans fats during digestion, so trace amounts end up in milk, cheese, and meat. The most well-known of these is conjugated linoleic acid.
Whether these naturally occurring trans fats are less harmful than industrial ones has been debated for years. A USDA systematic review found only limited evidence for a meaningful biological difference between the two types, and even that evidence required consumption of ruminant trans fats at seven to ten times normal dietary levels. In practice, people consume so little of the natural form that it hasn’t been a major public health concern.
Why Trans Fats Are Harmful
Trans fats hit the cardiovascular system harder than any other type of dietary fat. They raise LDL cholesterol (the type that builds up in artery walls) while simultaneously lowering HDL cholesterol (the type that helps clear it). No other common dietary fat does both. A pooled analysis published in the BMJ found that a 2% increase in energy intake from trans fats was associated with a 23% increase in the incidence of coronary heart disease.
Beyond heart disease, trans fat intake is linked to impaired insulin sensitivity, a precursor to type 2 diabetes. Research published in the Brazilian Journal of Medical and Biological Research found that trans fat intake was independently associated with higher insulin resistance, even after accounting for body weight. The researchers speculated that trans fats interfere directly with insulin signaling inside cells rather than working through inflammation, as was previously theorized.
Where Trans Fats Still Hide
Even after regulatory bans, trans fats haven’t completely disappeared. In the U.S., FDA labeling rules allow manufacturers to print “0 g” of trans fat on the nutrition label if a serving contains less than 0.5 grams. That means a product with 0.4 grams per serving is legally labeled as zero. If you eat multiple servings, or eat several “zero trans fat” products throughout the day, the amounts add up.
The most reliable way to check is to read the ingredients list, not the nutrition facts panel. Look for the words “partially hydrogenated” before any oil (soybean, cottonseed, palm). That phrase means trans fats are present regardless of what the label says. Common products that historically contained partially hydrogenated oils include stick margarine, shortening, microwave popcorn, frozen pizza dough, refrigerated biscuit and crescent roll dough, coffee creamers, and commercially fried foods.
The Regulatory Shift
On June 16, 2015, the FDA issued a final determination that partially hydrogenated oils are not “Generally Recognized as Safe” for use in food, effectively banning their intentional addition to the U.S. food supply. Manufacturers were given until 2018 to reformulate, with some extensions granted through 2020. Many companies had already started switching to alternatives like palm oil, fully hydrogenated oils blended with liquid oils, or high-oleic versions of sunflower and soybean oil.
Globally, dozens of countries have enacted their own bans or mandatory limits on industrial trans fats. Denmark was the first, in 2003, and the World Health Organization launched its REPLACE initiative in 2018 with the goal of eliminating industrial trans fats from the global food supply by 2023. Progress has been uneven. Wealthier nations have largely succeeded, while many low- and middle-income countries still lack enforceable regulations, meaning trans fats remain common in street food, bakeries, and packaged goods in parts of South Asia, Africa, and Latin America.
How to Minimize Your Intake
In countries with strong regulations, your exposure is already much lower than it was 20 years ago. The most practical steps are straightforward: check ingredient lists for “partially hydrogenated” oils, be cautious with imported packaged foods from countries without trans fat bans, and limit commercially fried foods when you don’t know what oil was used. Cooking at home with liquid oils like olive, avocado, or canola oil effectively eliminates industrial trans fats from your diet.
The small amounts of naturally occurring trans fats in meat and dairy don’t warrant avoiding those foods entirely. At typical consumption levels, they represent a fraction of the intake that has been linked to health problems in studies.