How harmful are trans fats, exactly?
Industrial trans fats are the most harmful fats in the current food supply. They demonstrably worsen your cholesterol profile and are associated with higher cardiovascular mortality, although that association is not always statistically strong at low intakes. Dairy trans fats are a different category and give no reason for the same concerns.
Industrial trans fats are artificial fats that form during the partial hydrogenation of vegetable oils. You find them in older recipes for margarine, pastries, deep-fried products and ready-made meals. Trans fats from dairy and ruminant meat are an entirely different category that behaves differently in your body. Conflating the two is incorrect.
The strongest evidence concerns cholesterol. Controlled dietary studies convincingly show that industrial trans fats simultaneously raise your bad cholesterol (LDL) and lower your good cholesterol (HDL), while also increasing your triglycerides. That combined effect is more unfavourable than that of saturated fats, and is considered causally established1,2,3.
At the population level, higher intakes of industrial trans fats are associated with roughly 34% greater risk of death from all causes and 28% greater risk of death from heart disease. Each additional percent of daily energy from trans fats, in place of healthy fats, is associated with up to a 32% higher risk of a heart attack or cardiovascular death. However, this comes from observational research. A 2025 analysis warns that the association remains weak and inconsistent at low intake levels4,5.
Beyond the cholesterol effect, there are indications that industrial trans fats also raise inflammatory markers and damage the inner lining of blood vessels. Laboratory research further shows that they direct more fat toward the liver than toward normal fat tissue, which points to possible liver damage at high intakes. Large human studies confirming this are still lacking1,2,6.
For type 2 diabetes, there are biological indications, but large analyses show no statistically significant association. A small increased risk of endometriosis has been found, but that evidence is weak. Dairy trans fats show no association with heart disease at normal consumption levels. One specific dairy trans fat was even linked to a lower risk of type 2 diabetes. 'Trans fat' is therefore not a homogeneous category4,1,6,7.
The claims are based on multiple systematic reviews and meta-analyses of cohort and intervention studies (including PMID 26268692, 19424218, 18996687, 19916363, 31782488, 40588677, 36138433). The evidence for the cholesterol effect is strong, for mortality and heart disease moderate, and for diabetes, liver damage and endometriosis limited. The 2025 analysis (PMID 40588677) urges caution about the magnitude of the effect at low intakes.