DEAR DOCTOR K: I see trans fats listed on food labels, and I've read that the FDA may ban them. Can you remind me what trans fats are, and why they're bad for me?
DEAR READER: Trans fats are a type of unsaturated fat. Once upon a time, we consumed only small amounts of naturally occurring trans fats in some meat and dairy products. But by the end of the 20th century, trans fats were everywhere. That's because chemists discovered that they could turn liquid vegetable oil into a solid or semi-solid by bubbling hydrogen gas through it (think margarine). When hydrogen is bubbled through liquid oils, they are called "partially hydrogenated" oils, or trans fats.