From what I understand of the metabolism its just the basic amount of energy needed to keep your body processes working. One of the major factors is muscle mass. More muscle mass = more energy needed. So I guess if you crash diet or something to the point where your body starts destroying your muscles for energy then you regain the weight as fat that would fit. In theory though your body would adapt to the heavier weight by bulking up those support muscles that need it so your metabolism would increase again. Unless you became more sedentary to adapt.
I personally believe that lifestyle and exercise is more important than diet for weight loss. The last time I successfully lost several stone was when i jumped up and down on my modem one day and had to spend several weeks coping with real life stuff
thankfully things haven't got desperate enough to do it again.