1. Losing weight is not the same as burning fat cells. You can shrink those cells, burn muscle or other tissues. I've hard its darn near impossible to actually burn cells as opposed to shinking them, which was the point of the original post.
2. As for this whole air "pressure" thing, Ryaln is right, in that it is the atmosphere on top getting pulled to the surface by gravity which creates the "pressure" at a lower altitude and a corresponding incrase in the density of the air. I guess, if the atmosphere is really shaped differenlty than the surface of the earth, that 20,000 ft. could feel differently at different places on the earth's surface. So much for that altimeter.