Why are trans fats so bad for you?

Cause many illnesses: Most trans fats come from partially hydrogenated oils found in many processed & fried foods like french fries & donuts. Simply put, trans fats are highly toxic.They cause inflammation, raise cholesterol & may cause up to 100, 000 deaths in the us from heart disease each year. They also increase risk of alzheimer's disease, diabetes, obesity, depression & infertility. See my comment for some links.

Foreign Biochemistry: The issue: the carbon-carbon spatial bond pattern when double bonds, between carbons, are converted to single; lookup in google images: "cis-fat vs. Trans-fat". The vast majority of c-c bonds in fats, & other carbon backbone molecules, are single, aka saturated (no room for more hydrogen side groups). Living cells all break double bonds to cis-bonds; trans-bonds are foreign to our genes/enzymes.