Eh. Finally a doctor in here. Sort of. In training. 

Anything you eat in excess amounts is turned to fat.  For example sugar is not fattening but it is easy to eat in excess amounts which turns into fat.  And most sugar products are already high in fat anyways.  There is also a difference between healthy fats and unhealthy fats.  Which I will also post more about later.  

This might seem off topic, but can anyone who had experience in P90X recommend a good diet for gaining muscle mass?

I eat some carbs in the morning (weatbix) and then eat protein and veg for lunch and dinner. Works great like weight just comes off you without the need for extensive exercise. Obviously your weight will flatten out eventually and you won't loose much.

P90x is not that great for gaining muscle mass.  it is more geared for losing weight, you can gain weight but there is better ways of doing it.  Anyways if you are trying to gain muscle mass, just like losing weight 90% of it is done in the kitchen.  You need to consume more calories than you burn and of course workout.  A high Protein diet is the best for this but the most important thing is to eat an excess amount of calories. The main reason for a high protein diet while gaining muscle is because protein is the building blocks for muscles.  When you work out you are tearing your muscle and protein is needed to make torn muscle fibers grow. You will gain fat while gaining muscle but that is ok.  There are two routines that you will be following.  the first would be to bulk and then lose your body fat once you are finished bulking. 

Lol the guys in here are funny.

P90x will give u some muscle but its not optimal for muscle gains.

P90x will give you more lean and toned muscle. Insanity is cardio and endurance.

YOU DON'T EVEN LIFT BRO

So do you have any suggestion what kind of nutrution plan should i follow within P90x, since i already have a low body fat?