For many years, fat has been a dirty word in the realm of healthy eating. Fat was thought to contribute to weight gain, heart disease, and metabolic diseases such as diabetes, so low-fat diets were encouraged and fearfully adhered to. But lately, the pendulum swung in the other direction. High-fat, low-carb diets are all the rage. So what are the pros and cons of dietary fats? Are there really good fats and bad fats?