Trans fatty acids (trans fats) are a third form of fatty acids. While trans fats do occur in tiny amounts in some foods (particularly foods from animals), almost all the trans fats now in our diets come from an industrial process that partially hydrogenates (adds hydrogen to) unsaturated fatty acids. Trans fats, then, are a form of processed vegetable oils. This means that in our diets, trans fats are almost exclusively found in the processed foods we eat.
The advantage of trans fats to the food processing industry is that partial hydrogenation solidifies and stabilizes vegetable oils, which otherwise tend to turn rancid relatively quickly. Because they exist in solid form instead of liquid form, trans fats can be used as substitutes for saturated fats in food products that are meant to have a long shelf life.
Trans fats were invented in the 1890s and began entering the food supply in the 1910s. However, the use of trans fats in food processing really took off in the 1970s and 1980s, when it was learned that saturated fats can be bad for your health.
Because trans fats were derived from the more healthy vegetable oils, it was assumed that they, too, would be healthy food products.