I've even seen this come out of studies, that people who lost weight have lower metabolism than those of the same weight who were never overweight. I've always speculated, though its never been proven, that it all has to do w/ muscle mass/ activity level. Diet has the most drastic effect on weight and lets face it their having been so overweight would make it difficult for their to be very active before. So most of them probably lose weight w/ little to no exercise and probably never had a lot of muscle to begin with since they spent their past collecting fat instead of being active. Chances are 170:1 that they have less muscle mass/ higher bf than those who never were extremely overweight to begin w/ and probably spend their down time lounging around and that is why its been documented that they have lower metabolism. Though I've never heard of a study that put the 2 under fmri to test this I'd put my life savings on it.