There is a theoretical maximum amount of fat loss your body can endure, before it starts losing muscle. That amount is 31 calories/day/lb of fat (Note that the author revised this number from 31 to 22 cal an email exchange with Redditor, Chr0me). Be aware, though, that this data comes from the Minnesota Starvation Experiment, and uses subjects under severe dietary restriction that did not strength train. So the information should be taken with a grain of salt. See Lyle McDonald’s take on this study for more.