Loses elasticity.  When stretch marks are formed, the skin is damaged and heals different. Much like a scar, those skin cells are forever changed and will never go away.  The extra/stretch type of skin cells fatties have are still cells. It can't be used up like glycogen. If you've successfully lost enough weight, it'll be like crinkled papyrus. A lot of the forner fatties "loose skin" I've seen still contain fat.  That combined with no muscle mass makes them look terrible.  Never let yourself get fat in the first place.