You can fast and restrict too much and become too thin and malnourished, and ruin your organs. I was once anorexic and at 5'7 went down to 77 lbs. My skin turned yellow, and I smelt like rotting flesh, I couldn't walk. Luckily, I got the help I needed. But I would be careful that you don't become anorexic because it is real and you can be too thin.