Summary: Exercising during a fast is acceptable, so long as the activity is not too strenuous on the body and as long as the body stays hydrated and energized. Exercise lightly during a fast or cleanse with health information from a chiropractor and addictionologist in this free video on fasting.
"Hi, my name is Doctor Kim Makoi, I'm a holistic chiropractor from San Francisco, and this is how to exercise during a fast. During a fast is definitely not the time to be doing very strenuous exercising, so you're not going to be doing a lot of heavy weight lifting, strength training, that kind of thing during a fast. The most beneficial exercises during a fast are gentle energy raising types of exercises such as chi gong, tai chi, things like that. It's okay to get some gentle aerobic exercise such as biking, maybe a little bit of a light jogging but definitely make sure that when you go to exercise you bring some water with you. If it's a master cleanse, lemonade diet type of exercise make sure you bring some of the lemonade drink along with you because you may find yourself getting depleted much faster than usual and it's handy to have something immediately to drink right there and then. Definitely make sure to not push yourself if you're exercising while you're fasting. It's always better to go a little on the light side than to overdo. And that's how you exercise during a fast."
eHow Article: How to Exercise During a Fast