If the sub is willing to accept this as impetus to improve their health...and they need the direction and discipline to achieve those goals...it might work. But doing this is a personal choice. They may wish to do this as a gift to their master as well as themselves. But you cannot beat someone into doing it. Either they will enjoy the beatings and continue to fail...or they will go away. Weight and body shape are a personal choice...every time the mouth opens...every time one doesn't get up and be active...it's a choice. If both parties share the goal and the sub/slave just needs help with will power...it could work.