Obese people have trouble losing weight because their "turnover" rate of fat storage and loss is slower than for average weight people, U.S. researchers say. Researchers at the Lawrence Livermore National Laboratory, writing in the journal Nature, report the turnover rate of fat is on average 1 1/2 years for normal weight people, but for the obese the fat removal rate from fat tissue decreases and the amount of fat stored each year increases. "There is a slower output of fat in obese people in this study," national laboratory researcher Bruce Buchholz said. "The fat is on average 2 years old compared to 1 1/2 years." Fat, on average, is replaced six times during the 10-year lifespan of a fat cell in the body, regulating fat storage and movement, researchers said. "We found that a combination of high storage and low fat removal rates, as in obesity, facilitates fat accumulation within fatty tissue," Buchholz said. "This promotes the development or maintenance of excess body fat mass."