This post has been edited by a moderator as discussion of extended fasting is not allowed at this forum.
Review our policy and our reasons for it here.
I started this after fixing a back pain problem that kept me from running and caused about 30 lbs of weight gain. The weight gain was less from not running than it was from the stress I alleved with food instead of running. BTW, if your lower back cramps up while running, check your flexibility on both sides of your body and stretch only the least flexible side before you run.
I think the low energy and headaches many have reported are due to dehydration, it's a problem I've encountered in the field before, and it seems more likely to happen in mild weather than on hot days. I've experienced only positive side effects so far. I run or lift hard on fasting days. It kills time and my appetite and makes me crave water.