"Fast," in this case, is not about speed. It's about fasting.This diet, which started in the U.K., slashes your calories so drastically 2 days a week that you're basically fasting. That's not safe for everyone, so you should check with your doctor if you're considering trying it.The Fast Diet says that you shouldn't fast if you're pregnant or underweight, or if you have a history of eating disorders or diabetes, and that you should check with your...