Here's the thing I don't like about fasting - how are you supposed to work out while you're fasting? I can't imagine going to the gym eating 500 calories a day without passing out.
My body has adapted! Initially I couldn't lift as much nor run quite as quickly while working out on empty, but the fat melted off nicely at about a pound a week for the first 6 months. My weight then stabilized for about 3 months.
In the last few months I have been trying a very slow bulk while still keeping to the schedule. Trying to eat above maintenance level and add muscle. It seems to be working I have put back on about 7 pounds and I estimate that about 4 of those are muscle. And, my personal bests in weight lifting have just recently been matched.
I have to say that overall I am very pleased with the results of this style of intermittent fasting and the ability to keep working out.
Having said all that, I am thinking that this year I would like to experiment with some longer fasts of 24 to 36 hours. I anticipate that I will need to significantly reduce the intensity of my workout during that kind of fast.
My calorie intake on exercise days on IF is about 2800kcal (as a 6'1 male with a current weight of 108kg/238lbs and a substantially above average muscle volume), which usually meant a lunch consisting of 1800+ calories of protein heavy food.
I'd usually exercise fasted (I do power lifting) apart from a low quantity of bcaa's, and have my first meal at noon.
But in fact, you adjust to going to the gym on low calorie diets too. Intensity may suffer, but it's certainly possible.
When I was getting ready for the prom I was working out, swimming and fasting for two weeks (only protein 200-400 calories). It was uncomfortable but totally manageable.
Warriors have waged whole wars depraved of adequate nutrition.
I workout around 6-8 am and eat at 1pm and I actually find I have more energy fasted than if i go in the afternoon.