Even After Weight Loss, Fat Cells Remain

Slimming down makes fat cells smaller but doesn't get rid of them.


May 5, 2008— -- There might be new vindication for those who claim the "big-boned" defense for being overweight, a new study shows.

Adults have about the same number of fat cells in their body constantly, even after losing a significant amount of weight, Swedish researchers say.

Researchers at the Karolinska Institute in Stockholm, Sweden, made their determination by studying fat samples from liposuction and abdominal reconstruction surgery in lean and obese volunteers, leading them to conclude that the number of fat cells is determined in childhood.

The researchers made their discovery by studying levels of radioactive materials locked inside of fat cells in people who had lived through the period of Cold War nuclear bomb testing from 1955 to 1963.

People whose fat cells developed before the onset of testing still had radioactive matter in the cells, showing that their fat cells were being replenished. The researchers were able to estimate that the body replaces cells at a rate of roughly 10 percent a year.

The fact that fat cells are constantly dying and being replaced could potentially offer an opportunity to develop new anti-obesity therapies, the researchers wrote in their study, which was published Sunday in the journal Nature.

Lead study investigator Kirsty Spalding said that because the study highlights the fact that fat cell population in humans is dynamic -- as in, our fat cells are consistently dying and being replaced with new fat cells -- researchers should now focus on developing weight-loss drugs that modulate the number of fat cells so that there is more cell death than cell growth.

"Potential treatments could change the balance of the birth rate and death rate of fat cells, and could compensate loss by decreasing the overall number of fat cells to help keep this weight off," Spalding explained. "Or we could develop therapies to affect this balance in the opposite direction, like to aim therapies at increasing the number of fat cell growth in cancer patients."

The study highlights some important facts about the typical life of a fat cell. Once a fat cell evolves into a mature fat cell, it cannot return to its roots, even if one loses substantial weight.

"Therefore, though we have a seemingly infinite capacity to recruit new fat cells, we cannot get rid of them once they have been recruited -- sort of George Bush's ideal army," said Michael Rosenbaum, associate professor of clinical pediatrics and medicine at Columbia University's College of Physicians and Surgeons.

Once fat cells reach a certain size -- that is, they become filled to capacity with fat content -- then new fat cells will begin to form.

"Thus, in most cases, weight gain initially reflects ... enlargement of existing fat cells followed by [an] increased growth of new fat cells," Rosenbaum said.

Obesity, the researchers say, is therefore determined by the number and size of the fat cells, which grow or shrink based on deposits from food.

So does this mean that every overweight dieter should throw out their jogging shoes and diet books? Not so, experts overwhelmingly say.

Although experts said that the more fat cells you accumulate in childhood the harder it is to remain lean and fit in adulthood, all diet experts agree that just because you may develop more fat cells in childhood does not mean that you are stuck being overweight your whole life.

"I hope people don't think that they're doomed if they were overweight in childhood," said Keith Ayoob, associate professor in the department of pediatrics at the Albert Einstein College of Medicine. "A 'typical' weight may not be appropriate for everyone. Go for a better weight and one that you can maintain. Even a 10 percent reduction [in weight] will improve your health risk."

Moreover, experts urged that those concerned about their weight should focus on obesity prevention for the future rather than a crash diet for today.

"Weight loss still makes sense, but prevention makes better sense," said David Katz, associate professor adjunct in public health practice at Yale University.

"Preventing obesity in kids is vastly better than trying to fix it later," Katz added. "We need to focus on keeping kids healthy and preventing obesity more. Once established in childhood, obesity is very hard to fix."

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