Industrial trans fats worsen the cholesterol profile (strong evidence) and increase the risk of heart disease and total mortality (moderate evidence). Trans fats from dairy and ruminant meat do not appear to be harmful at normal consumption levels.
Industrial trans fats are artificial fats that form during the partial hydrogenation of vegetable oils. They are found in (older formulations of) margarine, pastries, deep-fried products and ready-made meals. They are distinct from trans fats that occur naturally in dairy and the meat of ruminants, because those two groups behave very differently in the body.
The clearest and strongest evidence concerns the cholesterol profile. Controlled dietary studies convincingly show that industrial trans fats simultaneously raise bad cholesterol (LDL) and lower good cholesterol (HDL), while triglycerides also rise. This combined effect on the so-called total/HDL ratio is more unfavourable than that of saturated fats and has been judged to be causal1,2,3.
At the population level, higher levels of industrial trans fat intake are associated with approximately 34% greater risk of death from all causes (RR 1.34) and 28% greater risk of death from heart disease (RR 1.28). For industrial trans fats specifically, the latter figure is 18% (RR 1.18). Each additional percentage of daily energy from trans fats replacing healthy fats is linked to up to a 32% higher risk of a heart attack or death from heart disease. The certainty of evidence for these associations is rated as 'moderate', partly because they come from observational studies. A recent 2025 analysis4 awards the IHD evidence only two out of five stars, indicating that the association remains weak and inconsistent at low intake levels5,1,4.
Beyond the direct effects on cholesterol, industrial trans fats also promote inflammation in the body: markers such as TNF-alpha, interleukin-6 and CRP are elevated in studies. At the same time, they damage the inner lining of blood vessels (endothelial dysfunction) and activate a mechanism in liver cells that ramps up cholesterol production. Laboratory research also shows that industrial trans fats direct more fat to the liver than to normal adipose tissue, suggesting possible liver damage at high intake levels, although solid evidence from large human studies is still lacking1,2,6.
For type 2 diabetes and insulin resistance, there are biological indications, but in large meta-analyses the association is not statistically significant (RR 1.10, 95% CI 0.95-1.27). A small increased risk of endometriosis has been found (RR 1.12), but the studies are highly heterogeneous and this association is weak. For dairy trans fats the story is very different: at normal consumption levels no association with heart disease has been found (RR 0.93), and one specific dairy trans fat (trans-palmitoleic acid) was even associated with less type 2 diabetes. The evidence for this is limited, but it underlines that 'trans fat' is not a homogeneous category5,1,6,7.
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 strength of evidence for the cholesterol effect is strong; for mortality and heart disease it is moderate; for diabetes, liver effects and endometriosis it is limited. The 2025 analysis (PMID 40588677) urges caution regarding the size of the effect at low intake levels.