In the study, the results of which were published online ahead of the next issue of the magazine Nature, a team of scientists from Karolinska Institutet in Stockholm show that about 10 percent of the fat cells in adult humans die off annually, only to be replaced by new cell growth.
“We've shown that fat cells in adult humans are continually replenished,” researcher Dr. Kirsty Spalding said to The Local.
Because fat cells die and are replaced at about the same rate, the end result is that the total number of fat cells in adult humans remains remarkably stable over time, regardless of how many hours people spend at the gym.
“In the people we looked at, the fat cell number didn't change much,” said Spalding.
The study also found that obese people have nearly twice the number of fat cells as people of normal weight, and that the fat cells of obese adults have an increased capacity to store fat.
What changes when someone gains or loses weight, therefore, is not the number of fat cells, but the amount of fat stored in any individual cell.
“Fat cell mass is dynamic and regulated,” said Spalding.
Researchers have long doubted that adult humans actually produce new fat cells. While the study answers the question of fat cell growth, Spalding points out that the results beg further questions about what regulates the process of fat cell replenishment.
“A next step for research is figuring out what is regulating the process and how we can modify it,” she said.
The study relied on a technique involving radioactive isotopes resulting from Cold War era nuclear tests originally used for Spalding's research on brain cells.
The nuclear blasts, which took place in the late 1950s and early 1960s resulted in higher atmospheric levels of a carbon isotope, C14. The C14 eventually made its way into humans as well, and after nuclear tests were banned, C14 levels began falling.
As a result, each cell has a C14 level corresponding to the level in the atmosphere at the time the cell is created, allowing researchers to determine cells' age.