I've talked about this once in the past but I figured that another post, to serve as a reminder wouldn't hurt. Pretty regularly in forums the question of consuming something that isn't good for the body comes up, usually posing the question: How bad is it for me? There are the usual suspects: cheat meals, alcohol, Marijuana, and cigars. As usual, people are probably looking for a black and white answer to the question and the answer really is... depends.
What is sure, beyond question, is the above-mentioned items aren't good for you. The more of them that you consume, the more your athletic performance and health will be jeopardized. It's just that simple. The workout doesn't lie. If you consume stuff that is bad for you, you'll be slow and week midway through your workout in a way that you wouldn't be if you had just stayed strict and abstained.
That isn't beyond question. The issue is: how much of a difference does it make in the big picture? That's one part of the "depends" answer. If you're training for a big athletic event, then you're looking to squeeze every possible bit of physical advantage that you have out of your body then that cheat could be the small detail that screws everything up. These are times when small details make the difference.
That's not the only situation where the modest, small cheat could be disastrous either. If you're new to clean-living physical culture, then these lapses in good eating could be disastrous too. This is a new start and a time to reprogram. Habits are hard things to break and allowing a cheat meal could derail the whole process, much like a single cigarette causes relapse back into smoking. I still say habit but some call it addiction. Sometimes I think that's too strong of a word. Sometimes not. If you think that you have a food addiction, then avoid cheating like the plague.
On the other hand, if you've got the motivation, the work ethic, and a finely-tuned, in-shape body, then I know for a fact that it's entirely possible to let yourself go and have some junk food or smoke something from time to time. I know because I do it and I still stay pretty fit. I said it before and I'll say it again: Don't delude yourself though. Just because you can afford the set-back doesn't suddenly not make it a set-back! It's damage that you can afford because you spent the other 90% of your life eating right and working out that allow you to fix the damage without too much long-term consequence. Don't push things!
I'll repeat this again too: the workout ultimately tells you how detrimental it was and how often you can afford to do it. If you note that you're slower, feel sickly, can't put out the reps, need more time to rest, or can't work out as long overall then it's catching up to you and you need to dial bad stuff back. You pushed it too far and it's time to get serious again.