The number of fat cells in a person's body is determined during childhood and stays constant throughout life, with about 10 percent of fat cells dying and being replaced annually, according to study published in Nature yesterday (May 4).

Understanding the hitherto poorly characterized dynamics of fat cell production and turnover may help researchers target key processes in obesity and related diseases, such as diabetes.

"We are generating quite a few fat cells," said Kirsty Spalding, a biologist at Sweden's Karolinska Institute and first author on the study, "but it seems to be really tightly regulated."

Spalding said that both the expansion of the fat cell population and the arrival at what will be the final number of fat cells, or adipocytes, in the adult body occur at an earlier age in obese people. Fatter people experience a period of rapid adipoctye production around age two and reach their adult number of fat cells when they are about 16.5 years old, she said. Lean people, however, recruit fat cells most rapidly at about age six, with their fat cell population reaching its adult size at about 18.5 years old. "The expansion is definitely going on at an earlier age in obese children and at an increased rate," Spalding said.

The team of mostly Swedish researchers employed several methods to characterize adipoctye dynamics in the human body. To study how fat cell numbers differ in heftier or lighter people, Spalding and her team examined fat biopsies from about 680 lean and obese Swedish people. They found obese people can have as much as twice the number of adipocytes as do lean people.

The researchers also followed 20 gastric bypass patients who lost weight after their operations. Over the course of two years, their fat cells shrunk in size, but the total number stayed constant.

To characterize the turnover of fat cells in adults, Spalding used a ¹⁴C dating method she developed. The researchers measured the radioactive signature in the DNA of fat cells, and modeled adipocyte death and replacement by comparing fat samples from 35 adult liposuction or reconstructive surgery patients to existing ¹⁴C data gathered from children.

Spalding said that knowing about the turnover of fat cells could help develop future obesity treatments. She cautioned, however, that dramatic weight reduction will not result solely from some "magic pill," and will likely require reduced calorie intake and exercise. She next plans to elucidate fat cell population dynamics in people who are lean as children and becomes obese adults; a scenario not explored in her study. "I would imagine that they're going to have an increase in [adipoctye] number, but having said that, it seems to be extremely tightly regulated," she said. "It's really an open question."