Every year, whether you are fat or thin, whether you lose weight or gain, 10 percent of your fat cells die. And every year, those cells that die are replaced with new fat cells, researchers in Sweden reported Sunday. The result is that the total number of fat cells in the body remains the same, year after year throughout adulthood. Losing or gaining weight affects only the amount of fat stored in the cells, not the number of cells. The finding was published online Sunday in the journal Nature. Obesity investigators say the study raises tantalizing questions: What determines how...