History of Trans Fats:

Why do restaurants use trans-fat oils? Animal fats were commonly used in restaurants through the early 1980s, until the public became worried about the link between animal fat and high cholesterol. By the 1990s, most restaurants had replaced animal fats with partially hydrogenated oils, the source of synthetic trans fats, which at the time weren't widely considered harmful. Source: Wall Street Journal, December 9, 2006.