Comment by xurukefi

It's genetics. I think it's called the set point theory. You're genetically programmed to keep a certain body fat percentage. Anything below (or above) that set point and you're body will regulate your appetite with hormones. You can manipulate this set point with things like diet or exercise, but there is only so much you can do. If you're genetically programmed to keep a body fat percentage of 25% for example you're not gonna have a lot of fun staying below 15%.