[travisglines]

Typically how these things are measured is body fat and lean mass. Lean mass is all the stuff other than fat (muscle, water, organs etc).

You're right, I did assume that the lean mass lost came out of muscle in retrospect. Given his weight loss ratios I'd guess that it has to be, but it may be all water.

One interesting point here is if he lost significant water weight then he could have actually gained muscle as he lost fat. (that one's hard to tell though)