New research in the Sept. 25 online edition of the journal Nature shows that the turnover (storage and loss rate) of fat in the human body is about 1 1/2 years compared to fat cells, which turnover about every 10 years, according to Bruce Buchholz of Lawrence Livermore National Laboratory and one of the authors of the report. See the movie for a description.
And while the turnover rate of fat is on average 1 1/2 years for normal weight people, the news is worse for the obese -- the fat removal rate from fat tissue decreases and the amount of fat stored each year increases. In contrast, fat storage and removal rates balance in non-obese people for no net increase in fat.
"There is a slower output of fat in obese people in this study," Buchholz said. "The fat is on average 2 years old compared to 1 1/2 years."
The team, which included researchers from Karolinska University Hospital, University of Lyon, Uppsala University, University of Vienna, RIKEN Yokohama Institute, Technische Universit Munich, Churchill Hospital and the Karolinska Institute, applied carbon dating to fat content found within subcutaneous adipose tissue, the major fat depot for humans.
Carbon dating is typically used in archaeology and paleontology to date the age of artifacts. However, in this application, the scientists used the pulse of radiocarbon to analyze the age of fat and how fast it turns over in humans.
Radiocarbon or carbon-14 is naturally produced by cosmic ray interactions with air and is present at low levels in the atmosphere and food. Its concentration remained relatively constant during the past 4,000 years, but atmospheric testing of nuclear weapons from 1950-1963 produced a global pulse in the amount of radiocarbon in the atmosphere, Buchholz said.
Since the nuclear test ban treaty, the amount of carbon 14 in the atmosphere has decreased significantly. The carbon 14 diffuses out of the atmosphere and oxidizes to form carbon dioxide, which is taken up by plants. Since we eat plants or animals that live off of plants, the carbon 14 content in the atmosphere is directly mirrored in the human body.
Earlier research by Buchholz and colleagues at the Karolinska Institute showed that the number of fat cells in a human's body, whether lean or obese, is established during the teenage years. Changes in fat mass in adulthood can be attributed mainly to changes in fat cell volume, not an increase in the actual number of fat cells.
The new study found that fat, on average, is replaced six times during the life span of the 10-year fat cell, enabling a dynamic regulation of fat storage and movement over time.
"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."