[ponzao]

It isn't even just the trans fats in vegetable oils it is the amount of polyunsaturated fats (omega 3 & 6) which are present in amounts that are basically impossible to find in the wild and don't react well when heated up.

"During the last three decades, 4-hydroxy-2-nonenal (HNE), a major α,β-unsaturated aldehyde product of n-6 fatty acid oxidation, has been shown to be involved in a great number of pathologies such as metabolic diseases, neurodegenerative diseases and cancers." Source: https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3824598/

"Overall, beef fats produce less HNE and total secondary lipid oxidation products under thermal treatment, compared to pork and chicken fats as well as most vegetable oils." Source: https://conservancy.umn.edu/handle/11299/191191

Single stomach animals such as humans, pigs and chicken store whatever fat they consume somewhat in the same form whereas ruminants like cows do not, hence the fact that pork and chicken fats oxidise easier as they are nowadays fed a not very ancestral diet.

Some omega 6 amounts in oils (sourced from Fineli https://fineli.fi/fineli/en/index): - Sunflower seed oil n-6 polyunsaturated fats 62.3g - Canola oil n-6 PUFA 22.1g - Olive oil n-6 PUFA 10.4g