the guy i give a lift to training is muslim and he says it takes him around 2 weeks to get used to fasting. once he is used to it he can train for the rest of ramadan. I guess that's just him, there are 6 muslim lads at our gym, and they were all off last night so Im guessing they all decided it was for the best.
as another lad said, it will be the lack of water that would do the most damage. Have some time off, and don't put any weight on haha! My coach said the wierdest thing about it is, the muslim lads at my gym seem to make up for the lack of food during the day and eat a lot on a night, then come back a stone heavier in september and have managed to put weight on whilst fasting