Well, it may be different everyone, but when I went on a 45 day Water Fast and lost 50 lbs, it was water, fat and maybe a tiny bit of muscle. I know this because I had a personal trainer that measured my fat (no, he didn't know I was fasting, I didn't go to the gym for that time), and it went from 28% to 17%. I really leaned out. My muscles flattened out, but when I reintroduced food they came right back.