I'm holding off on anything like that, Shiroi--simply put, my father was a twig same as me until his mid-twenties, at which point his metabolism slowed and he started being able to put on weight. I have a load of biological and physiological stuff inherited from him, and it's likely this may be one of them too.
He didn't have the kinds of issues I'm having or to the extent I'm having them (to the best of my knowledge, anyway), which is a bit worrisome, so it's not as clear a case as just simply waiting for it to slow down.
I'm more or less bearing with it for another couple of years and seeing if my body starts taking the same general route as his did, or if my metabolism just slows down at its own pace, before going in to the physician's.