Partially hydrogenated oils create trans fat, a form of unsaturated fat that does not naturally occur in foods. In the 1950s research suggesting that saturated fat was a major driver of heart disease gained sway over the scientific community. As this idea took hold, low-fat products began popping up everywhere. Hydrogenating vegetable oil became a cheap way to ditch the saturated fat in foods like butter. However, they added trans fat. Partially hydrogenated vegetable oils exploded on the market in the 1980s when manufacturers realized they were a cheap way to increase the shelf life of processed foods.