the most obvious is fatigue.. if you're obsessively excercising i'd say you should eat more than someone who doesn't excercise. unless of course, you're trying to lose weight.
a lot of running isn't good for your joints. Just think of all that impact on your joints Walking fast will save them. Plus, all that time in the gym leaves less time to read, hang out with friends and being social, do homework (if you're in school). Try sports with other people as exercise or go out dancing too.
ADRIA LYNN