Partially hydrogenated oil is a manufactured fat created by forcing hydrogen gas into liquid vegetable oil under high pressure, converting some (but not all) of its unsaturated fat into saturated fat. This process transforms the oil from a liquid into a solid or semi-solid at room temperature, but it also produces trans fats as a byproduct. Those trans fats are the reason partially hydrogenated oils became one of the most scrutinized ingredients in the modern food supply, linked to a 21% increase in coronary heart disease risk.
How Partial Hydrogenation Works
Vegetable oils like soybean, cottonseed, and canola oil are liquid at room temperature because their fat molecules contain double bonds between carbon atoms. These double bonds create kinks in the molecular chain, preventing the molecules from packing tightly together. During partial hydrogenation, the oil is heated to high temperatures and exposed to hydrogen gas in the presence of a metal catalyst, typically nickel. Hydrogen atoms attach to some of those double bonds, straightening portions of the fat molecules and making the oil firmer.
The key word is “partial.” Because the process doesn’t saturate every double bond, two side reactions happen along the way. First, some of the remaining double bonds flip from their natural curved shape (called “cis”) into a straighter configuration (called “trans”). Second, double bonds can shift position along the fatty acid chain. Both of these changes are what produce trans fatty acids. Full hydrogenation, by contrast, eliminates all double bonds and does not create trans fats, but it produces a very hard, waxy fat that isn’t useful on its own for most food applications.
Why the Food Industry Used It
Partially hydrogenated oils solved several practical problems at once. Liquid vegetable oils go rancid relatively quickly because their double bonds react with oxygen. Hydrogenation increases resistance to oxidation, extending shelf life significantly. The process also raises the oil’s melting point, turning it into a solid fat that could replace butter or lard in manufactured foods. That solid texture was essential for making margarine, shortening, and the flaky or creamy consistencies consumers expected in baked goods.
Beyond texture, these fats contributed to volume, aeration, and a smooth mouthfeel in products that needed to hold their shape at room temperature. They were also cheap and widely available, making them an economic staple of the packaged food industry for decades. A 2012 CDC analysis of U.S. packaged foods found that over half of commonly consumed food categories contained at least one product with partially hydrogenated oils. Cookies were among the worst offenders, with 35% of products containing them. Seasoned processed potatoes hit 50%. Crackers, frozen entrees, snack foods, and baked goods of all kinds relied heavily on these fats.
How Trans Fats Harm Your Body
The trans fats generated during partial hydrogenation affect your cardiovascular system through several overlapping mechanisms. In liver and fat cells, industrial trans fats activate a pathway that ramps up cholesterol production. Compared to diets rich in natural unsaturated fats, a diet high in trans fats raises LDL (“bad”) cholesterol by about 14% and lowers HDL (“good”) cholesterol by about 12%. That combination is particularly dangerous because it simultaneously increases the cholesterol that clogs arteries and reduces the cholesterol that helps clear it.
Trans fats also promote inflammation throughout the body. Studies have found that higher trans fat intake correlates with elevated blood levels of C-reactive protein, interleukin-6, and tumor necrosis factor receptors, all markers of systemic inflammation that contribute to arterial damage over time. On top of that, trans fats encourage fat storage in the liver rather than in normal fat tissue, a pattern associated with metabolic dysfunction.
The cumulative effect is striking. According to the World Health Organization, high trans fat intake increases the risk of coronary heart disease by 21%, coronary heart disease death by 28%, and death from any cause by 34%.
Labeling Loopholes to Watch For
U.S. food labeling rules allow a product to list “0 g” of trans fat if a single serving contains less than 0.5 grams. That means a food can technically contain partially hydrogenated oil and still display zero trans fat on the Nutrition Facts panel. If you eat multiple servings, or consume several of these products throughout the day, those small amounts add up. The most reliable way to check is to scan the ingredient list for the words “partially hydrogenated” followed by any type of oil.
Products with less than 0.5 grams of total fat per serving can skip the trans fat line entirely, replacing it with a note at the bottom of the nutrition table that reads “Not a significant source of trans fat.”
Where Regulations Stand Now
The FDA revoked the “Generally Recognized as Safe” status of partially hydrogenated oils in 2015, effectively banning their intentional addition to food in the United States, with compliance deadlines phased in over subsequent years. Globally, progress has accelerated. By the end of 2023, 53 countries had WHO-recommended policies in effect covering 3.7 billion people, or about 46% of the world’s population. That’s up from just 6% five years earlier. In 2023 alone, seven countries including Mexico, Egypt, Nigeria, and the Philippines enacted new policies. The WHO has called for policies covering at least 90% of the global trans fat burden by the end of 2025.
What Replaced Partially Hydrogenated Oils
The food industry has turned to several alternatives: blending different natural fats, fractionation (separating fats into harder and softer components), full hydrogenation combined with interesterification, and oils from crops bred for higher saturated fat content. No single process replicates the functionality of partially hydrogenated oils on its own, so manufacturers typically combine techniques. The most common approach involves interesterified fats, where fully hydrogenated fats are blended with liquid oils and their fatty acid chains are rearranged using enzymes or chemical catalysts.
Interesterified fats are now widespread in spreads, bakery products, and confectionery. They don’t produce trans fats, which is their primary advantage. However, their long-term health effects remain poorly understood. One study using a commonly consumed palm-based interesterified fat found that it increased blood lipid levels after meals compared to the non-interesterified version, and elevated post-meal blood lipids are themselves a cardiovascular risk factor. The replacement for trans fats may turn out to be safer, but the research confirming that simply hasn’t been done yet.