There aren't too many things I disagree with in the P90X nutrition guide (it lays out a very sensible eating plan in my opinion) but one item that has caused some consternation and may have caused more than a few people to give up on P90X (or at least the nutrition portion of the program) is the advice given on page 7:
"...Try to finish your last snack approximately three hours before you hit the sack. This will help keep undigested carbs from being stored as fat. If you do need to break this rule, a small protein shake would be the snack of choice before bed, as it can help your recovery during sleep."
It is pretty rare to find agreement among all the experts in the areas of nutrition and exercise, so not surprisingly there are a number of dissenters who think the above information is based more on conjecture and myth than on science.
For example, Mark Sisson (who developed the P90X supplements) has this to say about eating before bedtime:
The Myth: Eating after 8 p.m. prompts weight gain.
The Truth: No need to feel bad about those late night snacks! In reality, your body has no idea what the clock reads, so you’re ability to metabolize food will be no different whether you’re sitting down for dinner at 6 p.m. or 10 p.m. What is more important when dining late at night, however, is to eat healthy foods - including lean meats, healthy fats, and plenty of veggies - both to ease digestion and best fuel your body for restorative sleep!
And Doctor Dr. Rob Shmerling says this:
Perhaps you've heard this advice before: Don't eat before going to bed. People most often give this advice to people trying to lose weight. The logic behind this advice sounds reasonable: If you eat and then go to sleep, your body will convert the food you ate into fat rather than using it right away as fuel. Ultimately, you'll gain weight.
But is this true? I have been unable to find any study that specifically asked and answered this question: When total calories are kept constant, does eating at night (whether just before bed or in the middle of the night) lead to weight gain?
In fact, I could find no compelling evidence that eating late at night or just before bed matters one way or the other. It is likely that total intake over a 24-hour period balanced against calories burned through one's daily activities matters much more than what time a snack or meal is consumed.
I too can't find any scientific research that supports the "no eating before bed" rule.
On the other hand in many countries, like Spain, people eat dinner very late in the evening, often after 10 pm. Yet Spain has an obesity rate that is nearly 1/3 the amount of America's.
What is important is that, as Mark mentions, you make healthy choices, but that rule applies no matter what time of the day you eat. If you are trying to lose weight and following P90X you still have to work within the calorie and portion limits specified for your phase and level in the nutrition guide. But there is no reason to go to bed hungry because you fear that whatever you eat after a certain point in the evening will turn into fat overnight!