Trans Fats
Industrial trans fats, made by partially hydrogenating vegetable oil, carry the clearest harm signal of any dietary fat. Countries that banned them have seen the deaths blamed on them fall steeply, but billions of people still live where they are allowed — while the small amounts that occur naturally in dairy and meat show no sign of the same harm.
Avoid industrial trans fat completely. Where it is not banned, check ingredient lists for "partially hydrogenated oil" and put the product back. The natural trans fat in milk, cheese and meat is not a reason to avoid those foods.
What they are
Trans fats are unsaturated fats with a particular shape of double bond. The industrial kind is made by partially hydrogenating vegetable oil, and appears on ingredient lists as "partially hydrogenated oil". A different group — the ruminant trans fats, chiefly vaccenic acid and conjugated linoleic acid — occurs naturally in dairy and meat. These are biologically distinct from the industrial form, and the evidence treats them separately.
The evidence of harm
Pooling 19 prospective cohort studies and about a million people, each extra 1% of daily energy from trans fat tracked with roughly 6% higher death rates, both from any cause and from cardiovascular disease.[1] This is observational, and the estimate is modest, but it is consistent with the biology: the unusual shape of the trans double bond disrupts how blood vessels work and how the body handles blood fats.
One mechanism you may read about is not established in people. In cultured human immune cells and in mice, elaidate, an industrial trans fat, shifted overall DNA methylation, the chemical tagging that helps switch genes on and off.[2] No human feeding study has tested a methylation endpoint, so this says nothing yet about trans fat and biological ageing in people.
Bans work
Between 1990 and 2021, the age-standardised death rate attributed to high trans-fat intake among adults aged 55 and over fell by about 69%, from 15.5 to 4.8 deaths per 100,000. The steepest falls came in the most developed regions, where industrial trans fat had been banned.[3]
- United States. The Food and Drug Administration's 2015 final determination on partially hydrogenated oils meant none could be added to food after June 2018, with the last extended compliance dates running to January 2021.
- European Union. Regulation 2019/649 caps trans fat, other than that naturally present in animal fat, at 2 g per 100 g of fat — not per 100 g of food — from 2021.
- The rest of the world. As of the World Health Organization's 2024 progress report, 53 countries had best-practice trans-fat policies covering 3.7 billion people, or 46% of the world's population, up from 6% coverage in 2018 and expected to save about 183,000 lives a year. More than 4 billion people remain unprotected.[4] WHO's current fact sheet attributes more than 278,000 deaths a year worldwide to industrially produced trans fat.[5]
Natural trans fats in dairy and meat
In pooled cohort studies, ruminant trans fats showed no association with coronary heart disease at the intakes people get from dairy and meat.[6] The estimates are wide, so this is no evidence of harm rather than proof of none. It is not a reason to avoid milk, yogurt, cheese or meat; whether those foods belong in your diet is a question of their other effects, covered under Dietary fats and Red and processed meat.
What to do
- Read the ingredient list where trans fat is not banned. "Partially hydrogenated oil" anywhere on it means industrial trans fat.
- Don't worry about the natural trans fat in dairy and meat at normal intakes.
- For the rest of your fat intake, what replaces saturated fat matters more than how much fat you eat; see Dietary fats.
Further reading
- Kim Y et al. Association between dietary fat intake and mortality from all-causes, cardiovascular disease, and cancer: A systematic review and meta-analysis of prospective cohort studies. Clin Nutr 2021.[7]
- Ye J et al. Impact of diets high in trans-fatty acids on cardiovascular diseases in adults aged 55 and older: insights from the Global Burden of Disease 2021 data. Front Nutr 2026.[8]
- 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 of observational studies. BMJ 2015.[9]
- Flores-Sierra J et al. The trans fatty acid elaidate affects the global DNA methylation profile of cultured cells and in vivo. Lipids Health Dis 2016.[10]