Answering a question on the boards this morning I realized that I haven't talked about the P90X diet yet. This is because I haven't addressed my own diet yet at all either. I eat okay most of the time but my diet varies drastically over the year as I train for different sports and events. The bottom line is that you should always eat for what you are doing. Since most of us don't do the same things every day it doesn't make sense to eat the same foods every day.
Periodizational training has been around for a long time and most people are sold on the fact that it's the most effective way to train. You can periodize different ways but the objective is the same: to train one energy system at a time until it plateaus and then move to the next one in a way the reduces the plateau and keeps the perfromance curve moving skyward. The schedule generaly depends upon your goals. A periodized training program can be a few weeks or an entire year. Most fall somewhere in between.
"Training" your diet works the same way. The most famous phase-oriented dieters are bodybuilders but most athletes do some form of it, either planned or serendipitious. The 90X diet is just one plan that was written with our clients and this program in mind. It's not perfect for everyone but every one can use its aspects to suit their needs. Because we don't all have the same level of fitness, goals, or agenda it's impossible to write one diet that works for every single person. With X we used the most generally effective system that had worked for the largest group of our client population. There is leaniency written in, especially in regards to how long to stay in each phase.
A periodized diet, like an training program, targets different areas in each phase. With new clients I generally follow the same path. First reduce carbs until your energy stalls out, then add carbs back in until your energy peaks, and then adjust the amounts of carbs you eat based on exercise. Within the simple plans there are many variables, from obvious ones like reducing junk foods and ensuring you are getting enough nutrients from proper foods to the more subtle, like balancing weight gain/loss with performance goals. No matter what the end goal the phases attempt to work towards that goal an efficient manner. The general path is to establish a good nutrient base, target this towards a desired weight, then get the most performance out of the body at the given weight for the desired goal.
I'll talk more specifically about the X diet, and about my own diet for my upcoming goals, as the program moves along. It will all follow the above principles.