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Contrary To Popular Belief, Adults Can Indeed Grow New Fat Cells

Body fat distribution is an important predictor of the metabolic consequences of obesity and scientists have long sought to understand the cellular mechanisms regulating regional fat accumulation. The contention that total number of body fat cells remains constant in adults is now being widely disputed.

Fat cells

The total number of fat cells in the body was typically thought of in scientific circles to remain the same, year after year throughout adulthood. Losing or gaining weight affects only the amount of fat stored in the cells, not the number of cells.

But what determines how many fat cells are in a person’s body? When is that number determined? Is there a way to intervene so people end up with fewer fat cells when they reach adulthood? And could obesity be treated by making fat cells die faster than they are born?

Most of these answers remain mysterious to scientists around the world. However, what is known is that in that although fat cells (adipocytes) normally function as part of the immune system and help control lipid accumulation, the extracellular environment in obesity traps a vicious cycle of inflammation, increasing risk of several major disease which is why investigations into how new fat cells grow have peaked interest.

"There is a system waiting to be discovered," said Dr. Jeffrey S. Flier, an obesity researcher and dean of Harvard Medical School.

Dr. Flier and other obesity researchers cautioned, though, that even if scientists knew how the fat cell system worked, it was not clear that it would be safe or effective to treat obesity by intervening. One of the hard lessons of the past couple of decades has been that the body has redundant controls to maintain weight.

Studies continue to provide explanations for the beneficial fat-fighting effects of natural substance like black pepper, but more research is needed to pinpoint how many substances can block the formation of new fat cells.

"I suspect that the body’s regulation of weight is so complex that if you intervene at this site, something else is going to happen to neutralize this intervention," Dr. Salans said.

Adults Can Grow New Fat Cells

Michael Jensen of the Mayo Clinic in Rochester, Minnesota and colleagues found that as people gained weight, they added new fat cells on their thighs, while fat cells on their bellies expanded.

"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.

What happens to people who are thin until adulthood and then gain a lot of weight? The situation may be different for people who got fat later. They may actually grow new fat cells -- the ones they had may have become so stuffed with fat that they could hold no more.

There was a time a few decades ago, before the current interest in how the brain regulates how much is eaten, when obesity researchers spent all their time studying and discussing fat cells. Investigators discovered that fat people had more fat cells than thin people and that fat cells shrank with weight loss and bulged with weight gain.

Dr. Jules Hirsch of Rockefeller University in New York, who did many of the initial studies with humans, said he started because he could not understand why people who lost weight regained. "They should have been cured," Dr. Hirsch said. After all, he said, if you cut out a fatty tumor, the fat does not grow back. Why was fat lost from dieting different?

The result was the fat cell hypothesis, a notion that obsessed researchers. Fat cells, the hypothesis said, are laid down early in life and after that, they can change only in size, not in number. When people lose weight and their fat cells shrink, that creates a signal to fill the cells again, making people regain. "We didn’t know a lot about obesity, so that was what we talked about," Dr. Flier said.

But the discussions stalled. It was not clear what to do about those discoveries or what they meant to efforts to help people lose weight. And no one had a method to ask whether fat cells were being created and destroyed during life. Few even thought to ask that question.


In Jensen's study, 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 2 kg (4.4 pounds) of upper body fat and about 1.5 kg (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.

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.


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