5. You look BLEHHHH.
Dull-and-dry skin, flat-and-boring hair — you may *think*you're hiding your unhealthy habits, but when you're on a dangerous diet, your body is going to tell the world what's up. "Any time you're significantly reducing calories, you're alsoreducing the essential nutrients you need to keep your hair shiny and your skin radiant," explains Kirkpatrick. And taking vitamins down in pill form won't help, either. You'll still be lacking protein, which is pretty much the building block of your prettiest parts. (Plus so many vitamins are fat-soluble, which basically means your body can't use them if you aren't getting enough fat in your diet.)