Study shows fat cells die and are replaced

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 many fat cells are in a person's body? When is that number determined? Is there a way to intervene so people end up with fewer fat cells when they reach adulthood? And could obesity be treated by making fat cells die faster than they are born?

"There is a system waiting to be discovered," said Dr. Jeffrey Flier, an obesity researcher and dean of Harvard Medical School.

Flier and other obesity researchers cautioned, though, that even if scientists knew how the fat-cell system worked, it was not clear that it would be safe or effective to treat obesity by intervening. One of the hard lessons of the past couple of decades has been that the body has redundant controls to maintain weight.

But the discovery is also leading to new ways to address other questions about obesity. For example, what happens to people who are thin until adulthood and then gain a lot of weight? The study focused on people who had been fat since childhood, the usual route for adult obesity. The situation may be different for people who got fat later.

Another question is whether fat cells removed with liposuction grow back.