May 2, 2008 -- Be careful what you eat as a kid, because those extra fries could make it harder to shed pounds years later in life.
A team of Swedish researchers has found that humans determine their total number of fat cells in childhood. New cells spring up and old ones perish, but their numbers change little after adolescence.
By measuring radiation absorbed after nuclear bomb tests in the 1950s and 60s, researchers found that our fat cells quickly regenerate.
But obese people turn over far more fat cells than others, says Kirsty Spalding, a biologist at the Karolinska Institute in Stockholm. The difference could explain why people battle to keep weight off after a diet.
"The take-home message is be careful what you feed your child," Spalding says. "Do everything you can to make sure you don't blow out your fat cell number when you are young."
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. "It's been sitting there in the fat-cell field as something that is not really known," Spalding says.
To find out, Spalding's team turned to a clever technique they had used to measure the birth of brain cells, which multiply at snail's pace in adults.
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 their food. Levels of heavy carbon plummeted when above-ground nuclear testing ceased in 1963, but the molecules put a birth date onto fat cells because of their predictable decay.
"The carbon-14 levels in the atmosphere are mirrored in our body at any given point in time," Spalding says.
Using these time stamps, her team calculated the birthdays of clumps of adipocytes taken from biopsies of 25 people: some thin, some fat. Surprisingly, her team found little difference in the turnover of fat cells between skinny and obese people.
We recycle about 10% of our fat cells each year, and every 8 years, half our adipocytes have been replaced.
But Spaulding did find that young obese people add twice as many fat cells each year as others, on average. "This could be part of the reason it's so hard to keep weight off," Spaulding says.
After plugging those numbers into a mathematical model, her 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.
Blocking the birth of new fat cells with a drug might offer a treatment for obesity, Spaulding says. Conversely, turning up the signal to grow new fat cells could help cancer patients gain weight.
Stephen O'Rahilly , an obesity expert at the University of Cambridge says the study "convincingly" proves that fat cells turnover rapidly.
However, our bodies are chock full of fat cells that stay empty until we gorge ourselves and our bodies needs a place to store the extra flab. Spalding's technique would ignore these cells because they haven't recently divided, he says.
"I think it is premature to conclude that, by the time we are adolescents the game is up," he says.
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