mobster is right. 500 calories a day is a crash diet. it isn't fasting
they are NOT the same thing. when you fast it is a completely different effect on the body then eating in a deficit. you will not get anything close to the benefits on a starvation diet vs. fasting.
when you eat 500 calories your calories out also drops down quickly. so that is why people who try it spin their wheels. you may lose weight initially yes, but after a week or month it will catch up to you. plus you will be miserable with no energy and you can't think
fasting this does not happen. when you fast the body goes into flight or fight mode. your adrenaline goes up and your metabolism stays strong. your energy actually goes up and you feel great on a fast.