Anecdotal but it was simple for me. The cause was seed oils and sweeteners and they are in most processed foods. I stopped eating them and then, a few months later, I stopped gaining weight. Instead my body now seems resistant to gaining weight, even when I eat poorly for a short interval.
It is interesting because I actually eat more macros (carbs, fats, etc) now than before but don't gain weight, which indicates to me there is a qualitative component that often isn't accounted for.
It is interesting because I actually eat more macros (carbs, fats, etc) now than before but don't gain weight, which indicates to me there is a qualitative component that often isn't accounted for.