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Fat Cell Numbers in Teen Years Linger for a Lifetime

Researchers have found that the number of fat cells in your body is set during adolescence and remains constant through adulthood, regardless of whether you gain or lose weight. The findings may help to explain why it can be so hard for some people to drop pounds and keep them off.

Although scientists have long suspected that the number of fat cells stays stable during adulthood—shrinking or swelling as the body's weight changes—they had no accurate and safe method for tracking the longevity and turnover of fat cells in the human body.

Scientists at the Karolinska Institute in Sweden and Dr. Bruce Buchholz at Lawrence Livermore National Laboratory (LLNL) in California developed a way to solve this problem. Their method, described in the May 4, 2008, advance online edition of Nature, is based on the unexpected observation that body cells created over the past 50 years bear a "timestamp." Cold War above-ground testing of nuclear weapons in the late 1950s and early 1960s caused atmospheric levels of carbon-14 to spike worldwide, generating telltale amounts of this radioactive isotope in the DNA of newly made cells. The researchers performed C-14 analyses of DNA from fat cells at LLNL's Center for Accelerated Mass Spectrometry, supported in part by NIH's National Center for Research Resources (NCRR).

The scientists examined fat cells removed from 35 adults during liposuction or other surgical procedures. Some of the patients were born before and others after the nuclear weapons tests. With carbon dating, the researchers could determine when the fat cells were created and calculate how long they had survived. They showed that about 10% of fat cells die and are replaced each year, regardless of an individual's age or weight. This fat cell turnover may offer the possibility of blocking fat cell regeneration as a treatment for obesity, the scientists say.

The researchers also examined fat cells in biopsies from nearly 700 adults of various ages and weights. They compared their data to that from a previous study of children and adolescents and found that fat cell numbers quickly climb through the teen years. Overweight children seem to gain more fat cells than normal-weight children. But fat cell numbers level off and stabilize in adulthood.

The scientists then looked at how fat cells change after severe weight gain or loss. Non-obese men who added significant pounds had a boost to fat cell size but no change in number. Later weight loss caused the fat cells to shrink but not vanish. Likewise, people who lost weight after stomach-stapling surgery had the same number of fat cells 2 years later. The cells, though, were smaller because they contained less fat, or lipids.

“If you are overweight and you lose weight, you still have the capacity to store lipids because you still have the same number of fat cells,” says Buchholz. “That may be why it's so hard to keep the weight off.”

—by Vicki Contie