This is just my perspective, but p90x both the resistance training and aerobic burns plenty of calories. Also, when it comes to weight lifting I find I do better when I follow-up the workout with a few carbs–it seems to help with the recovery, protein synthesis, and glycogen replenishment. I think I have read that glycogen is needed for both aerobic as well as anaerobic type efforts and I like to give myself the chance to push my limits and do my personal best–I don’t like the idea of trying my hardest and only getting 80% output. I have a CGMS and I know what folks are saying about getting a BG rise for short duration weight lifting versus a lowering effect with less strenuous cardio–for me I keep a couple of rolls of smarties with me as I don’t get a very consistent reaction with the p90x workouts–sometimes it has no effect, other times a small rise and then other times a lower effect. The p90x workouts are quite strenous (and long) so for me I just have to test and roll with the punches. Again, this is just my two cents.
