Sentry said:
Right, I don't mean I want to get really big or anything, but I do want more muscle. I just look too skinny at the moment, and want some more leanness.
How should someone like me (a bit skinny) eat during the workout? I'm almost afraid of getting a fat stomach along with muscle gain. :s
Calculate your daily caloric need with P90X and eat that. You can go straight to the second phase of the diet. Get your daily need of protein (around 1 gram of protein per pound), and then just do whatever the hell you want with the rest, really, as long as it's not white bread and sugar. You don't have to deprive yourself of carbs, like people who want to lose fat will have to, which is great, because you'll be able to push yourself harder during the workout, and get better strength gains. If you want to start by maintaining your fat percentage, eat your daily caloric need. Simple as that.
You really, really,
really won't gain a fat stomach by eating healthy and working out like crazy, which P90X requires you to do. Eat a lot. It's hard to gain fat and muscle at the same time, lest you eat 4000 calories a day. It's nothing to worry about. Even if you do think "hmm, I could drop some body fat" after the first month, you can correct it then.
You won't get huge over night, not with muscle and not with fat. It's not like you're stuck with your diet forever once you start P90X.