During adulthood, our bodies tightly regulate the number of fat cells, which could explain why it seems easy to gain back lost weight.

Want to get rid of some fat cells as you age? Fat chance.

You're stuck with the number of fat cells you have acquired by about age 20, a new study finds.

Researchers have known that people gain and lose weight at least in part by changing how much fat is in their fat cells. The new finding is particularly important for obese people, who the researchers say can have twice as many fat cells as their lean counterparts.

The finding also suggests that obesity in adulthood is at least partly determined by diet and exercise in childhood.

Strange study

To determine the age of fat cells in 35 subjects, researchers focused on a marker found in fat cells — radioactive carbon from above-ground nuclear bomb tests in the 1950s and 60s. More of a naturally occurring but rare type of carbon, called carbon-14, was produced during the testing.

Bruce Buchholz, a chemist at Lawrence Livermore National Laboratory in Livermore, Calif., explained how his team used this marker to make their discovery.

Our bodies incorporate carbon-14 from the food we eat, along with the vastly more abundant types called carbon-12 and 13. Since carbon-14 from the testing is decreasing with time as it mixes with the oceans, the amount of rare carbon-14 that a cell has taken up is like a timestamp for when the cell formed, Buchholz said.

The researchers knew that cells were dying and being replaced over time, because people born before the nuclear testing had fat cells that were created after the testing. The scientists also found that about 10 percent of fat cells were replaced every year whether or not a person was obese.

Despite that replacement rate, another aspect of the study with a larger sample of people revealed that the total number of fat cells per person remained relatively constant over time. Even extreme weight-loss strategies, such as bariatric surgery, did not reduce the number of fat cells in study subjects.

Aha!

The tightly regulated number of fat cells in adulthood may explain why it seems easy to gain back lost weight, Buchholz said.

If you already have more fat cells from adolescence than other people, "it's harder to become thin," Buchholz told LiveScience.

The study raises a new mystery: Something tells the body to make a new fat cell when another dies, Buchholz said. In the future, if scientists could interfere with this turnover, they might actually reduce fat-cell number in adults, he said.

The findings, detailed in the May 4 online issue of the journal Nature, suggest that the focus for controlling obesity should be on children, said Dr. Jeffrey Gimble, who studies fat stem cells at the Pennington Biomedical Research Center in Baton Rouge and was not involved in the research. The idea is that if the number of fat cells is capped by age 20, then the smart approach is to prevent their formation in children.

Obesity prevention in the early years could have "a lifetime impact," Gimble said.