I can tell you my specific experiance. I have always had trouble keeping my weight off after I hit about 24 years old. Before that I could eating anything and actually was taking weight gainer product to put on muscle pounds.

Anyways, now that I'm older I have found the most success in keeping the weight off when I train using weights. It usually take a few months to kick in but it's much easier for me to lose the extra 10 pounds or so of body fat if I'm training on a reqular basis and staying away from the fatting food. On the flip side if I just try to diet without training it a much harder road.

So for my specific body type mesomorph and metabalism weight trainging acts as a natural way to lose weight.