How healthy is everyday eating?How unhealthy is alternate day fasting? 24 hours.
Fasting can be good for you, and I'd suggest that everyone physically able should fast for at least a portion of the week. Those not able include some with medical conditions, the young, the pregnant, the nursing, etc. But for everyone else, spending some time in a fasted state instead of a fed state could be very beneficial for a number of health markers. But is it "healthy?" A question that broad is inevitable met with "it depends."
Here's a ye olde tyme post I made on intermittent fasting:
http://www.neogaf.com/forum/showpost.php?p=37060108&postcount=17385
It doesn't cover all methods and reasons for fasting, but you'll find a vast treasure trove of knowledge before you consider any move towards it. Speaking of, ask yourself why you want to try ADF in the first place. I've fasted on average for sixteen hours a day for the past year and a half, maybe even two years. It wasn't always easy, though now I can perform the occasional 18-23 hour fast without issue. Ask yourself if you'll even be able to fast for 24 hours every other day (I've never tried this personally, it doesn't sound appealing). I perform most of my cardio fasted. You can gain weight, lose fat, body recomp or anything else with fasts. It's a tool, not a magic bullet.