It's the "keeps you moving" part that encourages a lean body.
That's not, however, to discredit your individual metabolism and it's role in your healthy lifestyle. Your metabolism is quite important and you should, in all honesty, keep an eye on it. Loosely translated: That just means you have to eat right and move more. Because if you don't encourage your body with healthy fuel, and if you don't use that fuel, well...everything backs up and your metabolism says "fine, then I don't really need to work hard."
Behold the following infographic, which dives into a few silent killers of your metabolism.