Researchers have long suspected that adults keep their fat-cell numbers in check. But no-one knew whether the cells, called adipocytes, recycle or whether they last a lifetime.

Now, a Swedish team has found that humans determine their total number of fat cells in childhood -- in fact, new cells spring up and old ones perish, but their numbers change little after adolescence, the Nature journal reported.

"The take-home message is be careful what you feed your child. Do everything you can to make sure you don't blow out your fat cell number when you are young," lead researcher Kirsty Spalding of Karolinska Institute in Stockholm said.

In their study, the researchers measured radiation absorbed after nuclear bomb tests in the 1950s and 1960s to find that the fat cells of humans quickly regenerate.

Nuclear testing during the Cold War filled the planet with radioactive -- but harmless -- heavy carbon-14 molecules, which made their way into people's bodies via food. Levels of carbon plummeted when above ground nuclear testing ceased in 1963, but the molecules put a birth date onto fat cells.

"The carbon-14 levels in the atmosphere are mirrored in our body at any given point in time," Spalding said.

Using these time stamps, the researchers calculated the birthdays of clumps of adipocytes taken from biopsies of 25 people -- some thin, some fat. Surprisingly, they found little difference in the turnover of fat cells between skinny and obese people.

After plugging those numbers into a mathematical model, the team found that obese people start building up their fat cells much faster and at a younger age -- about two years old -- than thin people.