Published online 5 May 2008 | Nature | doi:10.1038/news.2008.800

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Even serious weight loss doesn't reduce your overall number of fat-holding cells.

Fat cell numbers set in childhood may make weight harder to shift as an adult. Punchstock

The number of fat cells in your body remains constant throughout your adult life, a new study has found. The discovery suggests that the process of weight gain may be fundamentally different in adults and in children.

Adults who gain or lose weight may do so through changes in the size of the fat cells, also called adipocytes, that constitute fatty deposits in the body. Children, on the other hand, may put on extra fat by increasing the overall number of these cells in the body.

This may mean that people who got fat during childhood may find it more difficult to shift the weight later in life, compared to those who piled on the pounds as adults, suggests Kirsty Spalding of the Karolinska Institute in Stockholm, Sweden, who led the new research.

Although the number of fat cells remains constant in adulthood, Spalding and her team found that it is not the same cells persisting for ever. There is a dynamic process of cell death and replenishment.

Fat by numbers

Spalding and her team took biopsies of belly fat from 687 people, both lean and obese, and recorded the number and size of fat cells, as well as the subjects' age, sex and body mass index. Combined with previous similar data from children, they showed that the average number of fat cells rises until the age of about 20, and then remains relatively constant, and is closely linked with body mass index.

The researchers also measured 20 people who were obese and had 'stomach stapling' surgery to reduce food intake. When Spalding and her team measured these volunteers again two years after the procedure, they found no reduction in fat-cell number: the subjects still had over 80 billion individual fat cells in their bodies, Spalding and her colleagues calculate. This despite losing an average of 18% of their body weight. It was the volume of each individual fat cell that was reduced, rather than the number, they report in Nature1.

Nevertheless, fat cells are constantly dying and being replaced, even in adults, Spalding and her team found. They determined this by studying fat extracted during liposuction procedures from 35 people who had lived through the period of Cold War atomic bomb testing, from 1955–63, when the atmosphere was briefly more radioactive than normal. Food grown and eaten during this period had elevated levels of an isotope called carbon-14.

Fewer fat cells showed a heavy dose of carbon-14 than might be expected if these cells were never replenished, the team reports. This shows that the cells have been subject to turnover in the intervening decades.

Cutting down

If cell biologists can work out exactly how this cell replenishment is regulated, it might be possible to design drugs to interfere with this process — potentially helping people to keep weight off once they have lost it.

Spalding says that such a treatment would best be given only after patients have undergone serious weight-loss therapies such as gastric surgery. "You need to be really cautious about applying this," she warns.

"It would be very dangerous to give people these drugs while they're still obese," Spalding adds. Cutting the number of fat cells while people still have a high fat volume would place extra strain on the fat cells that are left over, leading to metabolic complications such as diabetes, she explains.

"I don't think it's going to be as simple as 'take a pill, lose weight, problem solved'," Spalding adds.

Perhaps most important, Spalding says, is the confirmation that fat cells can proliferate in childhood, although not in adulthood. The factors behind this are likely to be both genetic and dietary, she says.

So although obesity tends to run in families, avoiding getting fat at a young age will help to establish a healthy number of fat cells for life. "The best take-home message is for people with kids to ensure they have a healthy lifestyle," Spalding says.