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A lot of people do it. I'm tempted and I can understand why. You want to have the worse before photo possible so you can show a really dramatic change at the end of the challenge. And...after all, you are going to be eating correctly for the next three months, so may as well cram it all in while you can.
There are a few fallacies with this line of thinking though.
The main one is, that by gorging a couple of weeks prior to your official start date of the challenge, you will pile on a lot of fat that will just take that much longer to get off again. So you really don't get an advantage. Sure, your before photo looks like hell...but your final photo won't look that great either...or at least not near as good as what it could have been if you didn't waste the first month or two getting rid of that couple of weeks of binge eating.
A second one is habit. The body for lifestyle is really pretty simple. It's just a matter of habit. It's much better to develop those good habits before you get down and dirty with a challenge rather than trying to wing it along the way. Believe me...I speak here from experience. Things go much smoother if exercise is part of your daily routine. Eating healthy is a no-brainer if you develop a taste for good foods. Staying on track is easy if that is a lifestyle rather than a diet.
And that's really the whole point. The challenge is just something we do, to set goals and track progress. Eating well, exercising, making healthy choices...that's a lifestyle that we should be interested in keeping for life, not for 12 weeks, not for a contest, not for money, not even so we can look great naked.
It's all about taking care of yourself so you can enjoy yourself and the people around that care about you.