Borrowed from bodybuilder.com:
The Short Answer:
It's absolutely OK to have a cheat meal! If the rest of your diet plan is tight, there's nothing wrong with cutting loose once or twice a week. In fact, the shift in calories may help you avoid plateaus. Also, it keeps you sane. After all, life is too short not to eat the occasional doughnut.
The Long Answer:
Now that you've stopped twerking for joy around the room and returned to your screen, I'll explain why the occasional cheat meal is OK. As I said, they are a good way to ward off potential plateaus. They also break up the stress commonly associated with diet and keep you on track.
Cheat meals fight plateaus.
While we live in a modern world, our bodies still operate under some very primitive rules. When you eat at a calorie deficit while working out regularly, your body doesn't get the memo that you could stop at any time. Instead, it assumes you're trudging across a desolate African plain with nary a Souplantation in sight, desperately tracking a wildebeest in hopes of feeding your starving family. (Some might argue that Plyo X offers a similar sensation.)
To deal with this stress, your body will sometimes slow down its metabolism and hold on to emergency fuel stores (body fat) to survive. We call this "starvation mode" and because you're not burning fat, it creates a weight loss plateau. While the obvious solution is to increase your calories in general, cheat meals are also effective because they give your body a little "feast" break in the middle of the "famine" to convince it to keep burning those love handles away.
In fact, we often suggest people zigzag their calories from a large deficit to a slight surplus over the course of a week to break this type of plateau. A cheat meal or two, provided the rest of your eating is extremely clean, can create such a zigzag.
Cheat meals strengthen willpower.
In their excellent book Willpower: Rediscovering the Greatest Human Strength, Dr. Roy Baumeister and science writer John Tierney discuss the concept of "decision fatigue," the idea that willpower, just like our muscles, can give out after a while. "When asked whether making decisions would deplete their willpower and make them vulnerable to temptation, most people say no. They don't realize that decision fatigue helps explain why ordinary, sensible people get angry at their colleagues and family, splurge on clothes, buy junk food at the supermarket, and can't resist the car dealer's offer to rustproof their new sedan."
Think of cheat meals the same way you think of recovery days. They give your willpower a chance to rest and restore, reducing the chance that you'll fall off the wagon in a bigger way.
Cheat meals relieve stress.
Survival mode suggests cheat meals relieve physical stress. Decision fatigue suggests they relieve mental stress. There's also a third type of stress they relieve: emotional stress. Turning around your diet can be really hard. Sometimes, eating clean means you're walking away from foods you've looked to your whole life for comfort, nourishment, celebration, and security. The fact that they are absolute cr** and they were slowly killing you is beside the point. They still have an enormous amount of emotional resonance. As long as you don't think it'll cause a relapse, there's no reason to completely walk away from cake on your birthday or a hot dog at the baseball game. Whenever I'm visiting my parents in Atlanta, you better believe I help myself to my mom's epic banana cream pie. In fact, only having it on special occasions makes it all the more delicious.