Trans fat is the solidified form of partially hydrogenated vegetable oil. Food manufacturers started using this synthetic substance in the 1950s because it's cheap and extends products' shelf life. But over the last few decades, numerous studies have shown that it has serious health effects, most notably raising LDL (bad) cholesterol and lowering HDL (good) cholesterol, which jacks up heart-disease risk.
Even with a ban, trans fat won't disappear completely. Small amounts occur naturally in meat, dairy products, and some vegetable oils. Those sources aren't what have the FDA worried. The biggest culprits are convenience foods like frozen pizzas, TV dinners, baked goods, potato chips, and margarine, which can contain unnaturally high amounts of trans fat and junk food. Since the FDA requires that trans fat be listed on the Nutrition Facts panel on food labels, at least you know what you're getting into. But what about when you grab a cinnamon roll at the coffee shop or order nachos at the bar? There's no way to tell how much trans fat you're ingesting.
It turns out even foods whose labels tout little or no trans fat may be fooling us. According to a recent FDA Consumer Update, companies can claim "zero trans fat" if the product has less than 0.5 grams per serving. Oftentimes, those "serving" sizes are minuscule, so you're actually getting a good amount. Also, if you see "partially hydrogenated oil" on the ingredients list, that's another red flag. As far as the FDA is concerned, hydrogenated oil basically is trans fat.
Although the ban's been set in motion, it will likely take years to phase out trans fat. So read food labels carefully to avoid it, and go easy on the bar food and pastries until restaurants can no longer cook with ingredients packed with trans fat.