How much are you training?
IF is primarily used by athletes for losing weight while preserving as much muscle as possible. They eat in a 3-6 hour window after an intense workout typically consisting of an hour of weight lifting, and then 2+ hours of sparring and grappling.
The whole point is to get to eat as much as you can to give your muscles adequet nutrition after ripping them up, then not eating the rest of the time to get your body to use itself for energy. Gotta be fairly high protein, and combined with serious training.