I have been looking into this lately. As I understand, it used to be thought that the body had a fixed number of fat cells, however in the case of severe obesity, when all of these fat cells have expanded to their maximum extent, the body can create further fat cells. This can actually create an excess of oestrogen, in men anyway.
When dieting or fasting, it appears that the fat cells lose their fat content but do not die so easily. However, from the bits I have read so far, it IS possible to kill these cells off, and I suspect this will be by strong exercise when thin and fit.
Whatever the real truth is, I am going to lose lots and lots of weight by fasting, then eat a raw food diet and do lots of vigorous exercise. Unless of course my body tells me otherwise (I listen to it a lot since I have started my first fast!)