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Shifting the weight.
The body continuously makes new fat cells (shown) each year to replace those that die off.

Credit: Michael Jensen/Mayo Clinic

And the Fat Goes On

By Steve Mitchell
ScienceNOW Daily News
5 May 2008

Researchers have assumed that a human can't make new fat cells, because the total amount of the cells doesn't seem to change after reaching adulthood. The trouble with testing this assumption is that standard techniques for tracking cells in animals are either toxic to people or can't be applied to fat cells. To get around this problem, a team that included Kirsty Spalding, a molecular biologist at the Karolinska Institute in Stockholm, Sweden, used data from aboveground nuclear tests conducted from 1955 to 1963. The tests spewed vast amounts of the radioactive molecule, carbon-14, into the atmosphere, which in turn was incorporated into the DNA of the fat cells of humans. Carbon-14 concentrations in the fat cells correspond to levels present in the atmosphere when the cells were created, giving the researchers a way to date when the cell was born.

In 10 lean or obese adults born before the nuclear bomb testing began, the carbon-14 concentrations in fat cells matched levels present in the atmosphere during the tests, indicating the fat cells were generated later in life. Conversely, fat cells from 25 lean or obese people born after the testing was halted had carbon-14 concentrations lower than the levels present during the bomb testing years. The carbon-14 amounts were closer to current atmospheric levels of the radioactive compound, indicating that the fat cells were generated later in life and confirming that the cells are continuously generated during our adult years, the researchers report this week in Nature. Using the carbon-14 data, the age of the study participants and their total number of fat cells, the researchers calculated that people generate about 10% of their fat cells each year to replace an equivalent amount that die off. The total stays the same, giving the illusion that fat cells aren't dying.

The researchers also monitored fat cell numbers in 20 people who underwent gastric bypass operations. Weight loss after the surgery was due to shrinkage of their fat cells, but the total number of the cells stayed constant. This indicates that even when people eat less, their bodies continue to churn out new fat cells to ensure the levels stay the same.

How the body regulates the turnover of fat cells remains unknown. But Spalding says the fact that the body keeps up the fat cell production even during weight loss could be one reason why people find it hard to keep off any weight they lose.

Understanding how the body regulates fat cell numbers could provide a better understanding of the factors that lead to obesity, says Joel Hirschhorn, a pediatric endocrinologist at Children's Hospital in Boston, Massachusetts. Michael Jensen, an endocrinologist at the Mayo Clinic in Rochester, Minnesota, says that that although our total number of fat cells may not drop below a certain level, that doesn't mean that we can't add more. Previous research has suggested that under situations where people are consuming a high number of calories, their fat cells may increase in both size and number, he says.

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