May 4 (Bloomberg) -- About 10 percent of fat cells in the body die off yearly and are replaced naturally, according to Swedish scientists who say that finding ways to interrupt the renewal process may one day offer a treatment for obesity.

Researchers have long thought that the number of fat cells remains constant in adults. The newest study, published today in the journal Nature, is the first to report that some cells break down each year, and that new ones are created that may affect a person's efforts to lose weight.

The number of obese adults has skyrocketed worldwide in the past 25 years. In the U.S., more than one in three adults are obese and another third are overweight, which puts them at increased risk for heart disease and diabetes, according to the Centers for Disease Control and Prevention. Efforts to develop weight-loss treatments have been largely unsuccessful.

``The results may, at least in part, explain why it is so difficult to maintain the weight after slimming,'' said Peter Arner, senior author of the study and a professor of medicine at the Karolinska Institute in Stockholm, in a statement. ``The new fat cells generated during and after weight reduction need to fill up their lipids rapidly.''

Fat cells shrink and stretch as people eat more or less, leading to changes in girth. In the study, researchers determined the age of the cells, and confirmed their continued production, by looking for radiation in people born before and after nuclear bomb testing began during the Cold War in 1955.

Radioactive Cells

The scientist examined fat cells taken during liposuction or stomach reconstruction from 35 people, including those who were adults when the testing began. Fat cells that were removed had signs of radioactivity, ``providing a first indication that there is a continuous and substantial turnover'' in adults, the researchers said.

Using computer models, they calculated that about half of a person's fat cells are replaced every 8 years.

``The total number of fat cells in the body is stable over time, because the making of new fat cells is counterbalanced by an equally rapid breakdown of the already existing fat cells,'' Arner said.

The study also included an analysis of fat cells, called adipocyte cells, in 687 adults and compared the findings to earlier work with children and teens. The researchers showed the total number increased through age 20, and remained the same during adulthood regardless of the person's size. That means the difference in the number of fat cells in lean and overweight people is established during childhood, they concluded.

Bariatric Surgery

Major weight loss didn't change the findings, researchers said. They analyzed the number of fat cells in patients who lost an average of 18 percent of their body weight after bariatric surgery. While the amount of fat per cell and total body fat dropped precipitously after two years, the overall number of cells remained constant, the researchers said.

The findings don't rule out an increase in the number of fat cells in people with prolonged weight gain during adulthood, they cautioned.

``If you overeat, you're going to gain body fat,'' said David Herber, director of the Center for Human Nutrition and the risk factor obesity program at the University of California, Los Angeles. ``We should not be giving people a biological reason that they might regain weight, even though they are trying their best.''

A second study, in the journal Nature Genetics, found certain gene variants can account for more than 8 pounds per person. The mutations are close to a gene called MC4R that is the most common cause of inherited, severe obesity, said researchers led by Ruth Loos, from the Institute of Medical Science at Addenbrooke's Hospital in Cambridge.

To contact the reporter on this story: Michelle Fay Cortez in Minneapolis at mcortez@bloomberg.net

Last Updated: May 4, 2008 14:15 EDT