When vegetable/seed oils are turned into solid fats, like shortening or margarine, trans fats (in particular, partially hydrogenated oils) occur. They have only recently been restricted in the US, at least to some extent. They cause heart disease.
https://en.wikipedia.org/wiki/Trans_fat#Health_risks
> Partially hydrogenated vegetable oils were an increasingly significant part of the human diet for about 100 years, especially after 1950 as processed food rose in popularity.[66] The deleterious effects of trans fat consumption are scientifically accepted.
> The most important health risk identified for trans fat consumption is an elevated risk of coronary artery disease (CAD).[72] A 1994 study estimated that over 30,000 cardiac deaths per year in the U.S. are attributable to the consumption of trans fats.[30] By 2006 upper estimates of 100,000 deaths were suggested.[73] A comprehensive review of studies of trans fats published in 2006 in the New England Journal of Medicine reports a strong and reliable connection between trans fat consumption and CAD, concluding that "On a per-calorie basis, trans fats appear to increase the risk of CAD more than any other macronutrient, conferring a substantially increased risk at low levels of consumption (1 to 3% of total energy intake)".[60]