P90X is a pretty extreme program. You will notice that the fat shredder phase of the diet is low on carbs / fruit but has more then what you would have on a strict keto (30gram) type diet.
Awesome success here, started @ 204 (5'10") into the 170s now, with minimal lean mass loss. This is with minimal cardio and maintenance lifts and small carb up (one controlled meal) every few weeks....