What is Trans fat?

What is Trans fat?

Trans fat is an unsaturated fatty acid that carries at least one double bond in the trans geometric configuration, meaning the hydrogen atoms sit on opposite sides of the carbon-carbon double bond. That trans arrangement makes the fat molecule straight and rigid, so it behaves more like a saturated fat: solid at room temperature, stable, and slow to spoil.

Two very different streams feed the trans fat you eat. The large one is industrial, or artificial, trans fat, produced when food manufacturers bubble hydrogen through liquid vegetable oil in a process called partial hydrogenation. This turns cheap oil into a semi-solid partially hydrogenated oil (PHO) that gives margarine its spread, keeps fried foods crisp, and lets packaged baked goods sit on a shelf for months.

The small stream is natural, or ruminant, trans fat, formed by bacteria in the stomachs of cows, sheep, and goats and found in low amounts in butter, milk, cheese, beef, and lamb. The distinction matters because the industrial version is the one health agencies target: it is the artificial, avoidable fat linked most tightly to heart disease. The World Health Organization estimates that industrially produced trans fat causes more than 278,000 deaths worldwide every year, and it has driven a global push to erase PHOs from the food supply entirely.

How it works

Trans fat damages health chiefly by shifting your blood lipids in two harmful directions at once, a pattern no other dietary fat matches. It raises low-density lipoprotein (LDL), the cholesterol carrier that deposits plaque in artery walls, while at the same time it lowers high-density lipoprotein (HDL), the carrier that clears cholesterol out. Saturated fat raises LDL too, but it tends to nudge HDL up as well; trans fat pushes both the wrong way, so gram for gram it does more arterial harm than any other fat.

Beyond cholesterol, trans fat promotes systemic inflammation, worsens the function of the endothelium lining your blood vessels, raises triglycerides and lipoprotein(a), and increases insulin resistance. Together these effects accelerate atherosclerosis, the slow narrowing and stiffening of arteries that underlies most heart attacks and ischemic strokes. The dose response is steep and there is no safe threshold: analyses find that each 2% of daily energy taken as trans fat is tied to roughly a 23% higher rate of coronary heart disease.

Because the risk is high and the fat serves no nutritional purpose, the World Health Organization treats industrial trans fat as an additive to be removed rather than a nutrient to be moderated. That is the logic behind national bans and mandatory limits: eliminating PHOs delivers a large drop in heart disease at almost no cost to the food supply.

How to apply it

• Read the ingredient list, not just the label: In the United States a product can claim 0 g trans fat if it holds under 0.5 g per serving. The word to hunt for is partially hydrogenated oil in the ingredients; if it is listed, the food contains trans fat even when the panel reads zero.
• Cut the classic PHO foods: The heaviest industrial trans fat historically sat in stick margarine and shortening, commercially fried foods like doughnuts and fries, microwave popcorn, and packaged cakes, biscuits, and pastries. Swapping these for whole or minimally processed foods removes most artificial trans fat from a diet.
• Replace, do not just remove: Trials and the WHO agree that the benefit comes from replacing trans fat with unsaturated fat, chiefly polyunsaturated oils like soybean, sunflower, and olive oil. Swapping trans fat calories for refined carbohydrate or sugar gives a much smaller heart benefit.
• Favour countries with a ban: Denmark first capped industrial trans fat in 2003; many nations followed, and the WHO best-practice model limits it to 2 g per 100 g of total fat or bans PHOs outright. In those food supplies the artificial version is largely already gone.
• Keep total intake under 1% of energy: The WHO ceiling for all trans fat, industrial plus natural, is under 1% of daily calories, about 2.2 g on a 2,000 calorie diet. Cutting industrial sources gets almost everyone under this line without eliminating dairy or meat.
• Do not fear the trace natural amount: Ruminant trans fat in dairy and meat sits at low levels and, in typical amounts, is not clearly linked to the heart harm seen with industrial trans fat. The public-health target is the artificial PHO source, not butter or beef in normal servings.

Types

Worked example

Trans fat intake adds up fast from a few processed foods. This table shows a day of older-style PHO products against the WHO ceiling of under 1% of energy, roughly 2.2 g on a 2,000 calorie diet. The point is how quickly artificial sources blow past a limit that whole foods never reach.

That single day of legacy PHO foods reaches about 7.5 g, more than three times the WHO ceiling. Replacing them with whole foods, or buying in a country that has banned PHOs, drops the same day close to zero.

Trans fat vs saturated fat

Both raise LDL, but only trans fat also lowers HDL, which is why agencies aim to remove industrial trans fat entirely while merely capping saturated fat at about 10% of calories.

By goal

• Heart health and longevity: Treat industrial trans fat as a zero-target. Scan ingredient lists for partially hydrogenated oil, replace fried and packaged PHO foods with unsaturated-fat sources like olive oil, nuts, and fish, and you remove the single most harmful fat in the diet.
• Athletes and body composition: Trans fat offers no performance or physique benefit and displaces useful calories. Spend your fat budget on unsaturated oils, oily fish for omega-3, and whole-food saturated fat instead; there is no training reason to include any artificial trans fat.
• Families and everyday eaters: You do not need to track grams. Base meals on whole and minimally processed foods, cook with liquid vegetable oils rather than shortening, and check labels on packaged baked goods. In countries with PHO bans, most of the work is already done for you.

Common misconceptions

• "A 0 g trans fat label means the food has none." In the United States a serving under 0.5 g can legally be rounded to zero. Eat several servings and the grams add up. The reliable check is the ingredient list: if it names partially hydrogenated oil, the food contains trans fat regardless of the panel.
• "All trans fats are equally dangerous." Industrial trans fat from partially hydrogenated oils drives the heart-disease risk that health agencies target. The small amount of natural ruminant trans fat in dairy and meat has not shown the same clear harm at typical intakes, so the two are not treated the same.
• "Cutting trans fat means switching to sugar or refined carbs." The benefit comes from replacing trans fat with unsaturated fat, not with refined starch or sugar. Swapping fried snacks for olive oil, nuts, and fish lowers heart risk; swapping them for sugary processed foods gives a much smaller gain.
• "Trans fat is just another name for saturated fat." They are chemically different. Trans fat is an unsaturated fat with a trans double bond, mostly man-made by hydrogenation; saturated fat has no double bonds and occurs naturally in animal and tropical fats. Only trans fat both raises LDL and lowers HDL.
• "Trans fat is a natural, traditional fat." The trans fat that matters for health is industrially produced, invented for shelf life and texture, not a traditional food. Because it is artificial and offers no nutritional value, the WHO aims to eliminate it rather than simply recommend eating less.

Related terms

Trans fat FAQ

What is trans fat in simple terms?

Trans fat is a type of fat, mostly man-made by adding hydrogen to vegetable oil, that turns liquid oil into a solid with a long shelf life. It is the most harmful fat for your heart because it raises bad LDL cholesterol and lowers good HDL cholesterol.

Why is trans fat so bad for you?

Trans fat harms the heart in a way no other fat does: it raises LDL cholesterol while also lowering protective HDL cholesterol. It fuels inflammation and artery narrowing, and each 2% of energy from it is linked to about a 23% higher risk of coronary heart disease.

What foods contain trans fat?

Industrial trans fat is found in some stick margarines, shortening, commercially fried foods like doughnuts and fries, microwave popcorn, and packaged baked goods made with partially hydrogenated oil. Small natural amounts also occur in dairy and red meat from cows, sheep, and goats.

What is the difference between trans fat and saturated fat?

Trans fat is an unsaturated fat with a trans double bond, mostly made industrially, and it both raises LDL and lowers HDL cholesterol. Saturated fat has no double bonds, occurs naturally in animal and tropical fats, and raises LDL but tends to keep HDL steady or higher.

Is trans fat banned?

Industrial trans fat is banned or tightly limited in many countries. Denmark restricted it in 2003, and in 2015 the US FDA ruled partially hydrogenated oils are no longer generally recognized as safe, removing them from the food supply. The WHO is pushing for a worldwide ban.

How much trans fat is safe per day?

There is no safe level of industrial trans fat. The World Health Organization advises keeping total trans fat, artificial plus natural, under 1% of daily calories, which is about 2.2 grams on a 2,000 calorie diet. Aim for as little industrial trans fat as possible.

Does a 0 g trans fat label mean there is none?

Not always. In the United States a food with under 0.5 grams per serving can be labelled 0 grams. If you eat several servings, the grams add up. Check the ingredient list for partially hydrogenated oil, which reveals hidden trans fat the panel hides.

Is the trans fat in butter and meat harmful?

The small amount of natural, or ruminant, trans fat in dairy and red meat has not shown the clear heart harm linked to industrial trans fat, at least at normal intakes. Health agencies target artificial partially hydrogenated oils, not butter or beef in typical servings.

What should I replace trans fat with?

Replace trans fat with unsaturated fats for the biggest heart benefit. Cook with liquid vegetable oils like olive, soybean, or sunflower oil, and eat nuts, seeds, and oily fish. Swapping trans fat for sugar or refined carbohydrate gives a much smaller improvement.

How does trans fat affect cholesterol?

Trans fat pushes both cholesterol carriers the wrong way. It raises LDL, the type that deposits plaque in arteries, and lowers HDL, the type that clears cholesterol away. This double effect is why gram for gram trans fat is the most damaging dietary fat for heart disease.

References

1. Trans fat — Fact sheet. World Health Organization, 2024
2. Mozaffarian D, Katan MB, Ascherio A, Stampfer MJ, Willett WC. Trans Fatty Acids and Cardiovascular Disease. N Engl J Med, 2006. PubMed 16611951
3. Final Determination Regarding Partially Hydrogenated Oils (Removing Trans Fat). U.S. Food and Drug Administration
4. Shining the Spotlight on Trans Fats. Harvard T.H. Chan School of Public Health, The Nutrition Source
5. Trans Fat. American Heart Association
6. REPLACE trans fat: an action package to eliminate industrially produced trans-fatty acids. World Health Organization
7. de Souza RJ, et al. Intake of saturated and trans unsaturated fatty acids and risk of all cause mortality, cardiovascular disease, and type 2 diabetes: systematic review and meta-analysis. BMJ, 2015. PubMed 26268692