Researchers who persuaded slender volunteers to gorge themselves on sweets to gain weight said on Monday they have overturned the common wisdom that adults cannot grow new fat cells.
As they gained weight, the volunteers added new fat cells on their thighs, while fat cells on their bellies expanded, Michael Jensen of the Mayo Clinic in Rochester, Minnesota and colleagues found.
"It sort of inverts the old dogma that we don't make new fat cells when we are adults," Jensen said in a telephone interview.
Doctors have long believed that, in adults, fat cells just get bigger and bigger when people gain weight.
Understanding why this happens on one part of the body and not another may explain why gaining weight in the lower part of the body does not appear to carry as many health risks as gaining belly fat, Jensen said.
"Those people who make new leg fat cells, it may be protecting them," he said.
Writing in the Proceedings of the National Academy of Sciences, Jensen and colleagues said they recruited 28 healthy men and women and fed them controlled meals, asking them to eat until they were more full than usual for two months.
The volunteers also agreed to have their body fat sampled.
To boost weight gain, the volunteers were encouraged to slurp milkshakes, chocolate bars and energy drinks.
In the two months the volunteers gained about 4.4 pounds of upper body fat and about 3 pounds of fat on the hips and thighs.
"We found that a gain of only 1.6 kg of lower-body fat resulted in the creation of 2.6 billion new adipocytes (fat cells) within eight weeks," Jensen's team wrote.
There were some unexpected findings.
"We thought the women would be the ones who gained on the thighs but it turned out it was pretty even on the two sides, to our surprise," Jensen said.
The volunteers shed their extra pounds easily, Jensen said.
"When you take normal, healthy weight people and you overfeed them and make them gain weight, almost as soon as they stop overeating the weight just falls off," he said.
"These were all people who never had a history of weight problems. Most of us have trouble when we gain it. We don't lose it so easily," Jensen said.
Jensen is unsure what the findings may mean for most Americans. More than two-thirds of the U.S. public are overweight or obese and the Organization for Economic Cooperation and Development says residents of most developed countries are catching up fast.
"Our next step is to try to understand better what are the genetic and environmental factors that cause the leg fat cells to be able to proliferate," Jensen said.
Scientists have learned that fat cells are not just passive storage facilities for extra calories, but produce hormones and other compounds with important biological effects. This may help explain why extra pounds raise the risk of diabetes, heart disease and some cancers.