Trans fats are created when hydrogen is forced into liquid vegetable oil under heat and pressure, a process called partial hydrogenation. This rearranges some of the oil’s molecular bonds into a straighter shape, turning a pourable liquid into a semi-solid fat. Small amounts also form naturally in the digestive systems of cattle and sheep, and trace amounts can appear during high-heat oil refining.
What Makes a Fat “Trans”
All unsaturated fats contain at least one double bond between carbon atoms in their molecular chain. In the natural “cis” configuration, the carbon chains on either side of that bond angle in the same direction, creating a bent, kinked shape. This bend is what keeps oils like soybean or canola liquid at room temperature.
In a trans configuration, those carbon chains sit on opposite sides of the double bond, pulling the molecule into a straighter line. That straight shape lets trans fat molecules pack together more tightly, raising the fat’s melting point and making it solid or semi-solid. Structurally, trans fats behave more like saturated fat (butter, lard) even though they still technically contain double bonds.
Partial Hydrogenation: The Industrial Process
The main source of trans fats in the modern food supply has been partial hydrogenation of vegetable oils. The goal is to take a cheap liquid oil and give it the firmness and shelf stability needed for products like margarine, shortening, and packaged baked goods.
The process works by bubbling hydrogen gas through heated vegetable oil in the presence of a metal catalyst, typically nickel. The hydrogen atoms attach to some of the double bonds in the oil’s fatty acid chains, converting them to single bonds and “saturating” those positions. Because the process is deliberately stopped partway (hence “partial”), not every double bond gets saturated. The ones that survive often flip from their natural cis arrangement into the trans arrangement as a side effect of the heat and catalyst exposure. Conventional industrial conditions produce more than 10% trans fatty acids by weight in the finished product.
The result is a fat that stays solid at room temperature, resists going rancid, and holds up well in frying. For decades, food manufacturers considered it an ideal ingredient.
Natural Trans Fats in Meat and Dairy
Bacteria living in the stomachs of ruminant animals like cows, sheep, and goats carry out their own version of hydrogenation. As these microbes break down the polyunsaturated fats in grass and feed, they produce small amounts of naturally occurring trans fats, primarily trans-vaccenic acid and rumenic acid. These end up in the animal’s meat and milk.
The quantities are much smaller than what industrial hydrogenation produces. A glass of whole milk or a serving of beef contains only a fraction of a gram. Because of this natural pathway, trans fats can never be entirely removed from the human diet, even with a complete ban on industrial sources.
Trans Fats From Oil Refining and Cooking
Even without deliberate hydrogenation, high heat can flip cis bonds into trans bonds. This happens in two settings most people don’t think about.
During commercial oil refining, vegetable oils go through a deodorization step at high temperatures to remove off-flavors. Industry guidelines cap this step at 235 to 240°C to limit the formation of trans fats, and properly refined bottled oils contain no more than 2% trans fatty acids on a fat basis. That’s a small amount, but it’s not zero.
In home and restaurant cooking, the effect depends heavily on method. Research comparing different cooking techniques with corn oil found that baking, pan-frying, and deep frying did not meaningfully increase trans fat content compared to raw oil. Stir-frying at 170°C, however, roughly doubled the trans fat levels. The combination of high heat and continuous stirring accelerates lipid oxidation, which is one of the basic mechanisms that flips cis bonds into trans bonds. Repeated heating of the same oil compounds this effect over time.
How the Food Industry Replaced Hydrogenation
After regulators began restricting trans fats, food manufacturers needed alternatives that could make solid fats without partial hydrogenation. The primary replacement is a process called interesterification, which rearranges the fatty acids already present in solid and liquid fats rather than adding hydrogen.
In practice, manufacturers blend palm oil fractions (which are naturally solid) with liquid oils like rapeseed or linseed, then use either chemical or enzyme catalysts to shuffle the fatty acid positions within the fat molecules. This changes the melting point and texture without creating trans bonds. The process actually reduces saturated fat content by about 10% compared to a non-interesterified fat with the same firmness. For spreads, bakery products, and confections, interesterified blends have largely replaced the partially hydrogenated oils that dominated the market for decades.
Regulatory Status in the U.S. and Globally
In 2015, the FDA determined that partially hydrogenated oils are not generally recognized as safe. The compliance deadline for most food manufacturers to stop adding them was June 18, 2018, with extended deadlines through January 2021 for certain limited uses to allow products to clear distribution channels. The FDA also denied a petition from the Grocery Manufacturers Association requesting approval for continued limited use of partially hydrogenated oils.
Globally, nearly 60 countries now have best-practice elimination policies in effect, covering 46% of the world’s population. The WHO has validated nine countries, including Denmark, Lithuania, Poland, Saudi Arabia, Thailand, and Oman, for fully implementing these policies.
Why “0 Grams Trans Fat” Isn’t Always Zero
U.S. labeling rules allow a product to declare “0 g” of trans fat if a single serving contains less than 0.5 grams. A product with 0.4 grams per serving can legally say zero on the label. If you eat multiple servings, those fractions add up. Products that contain less than 0.5 grams of total fat per serving aren’t even required to list trans fat at all, unless they make claims about fat or cholesterol content. In that case, they can simply note “Not a significant source of trans fat” at the bottom of the nutrition label instead.
With partially hydrogenated oils now banned from the U.S. food supply, the trans fats that remain come from natural sources in meat and dairy, trace amounts formed during oil refining, and small quantities in imported products. The amounts are far lower than a decade ago, but they haven’t disappeared entirely.