Forever Young, Fat Cells Complicate Weight Loss

May 4, 2008

By Jason Socrates Bardi
ISNS Contributor

If you are trying to lose weight, the good news is that half the fat cells in your body will be gone in less than a decade. The bad news? Your body will be replacing them with new ones even as they go.

Overturning conventional scientific wisdom on how the body stores fat, a new study published online today by the journal Nature suggests a reason why keeping weight off is so hard for many people who diet, even successfully.

Led by scientists the Karolinska Institute in Stockholm, Sweden, the study compared abdominal fat samples from 687 adults of all ages who varied in body type from lean to morbidly obese. This group also included several people who were once obese and had undergone major weight loss.

The study shows that about 10 percent of the fat cells in the body—billions of them—die every year but the total number of these cells is remains constant. People who are obese have up to twice as many fat cells as people who are lean, on average. Also, this level does not change with age or diet. Even after people have undergone stomach-reducing bariatric surgery, their bodies still produce just as many fat cells as they did before.

Research in the 1970s showed that the number of fat cells in a person's body remains constant, but scientists didn't know if or how new fat cells were produced in adults. The new study suggests that the body is constantly replacing dying fat cells, holding the overall number constant.

The situation is analogous to a tub of water. Once filled, the water level in the tub remains the same if the drain is stopped and the tap is turned off. But the level also remains constant if the stopper is removed and the tap running just enough to keep the water level steady. The new research suggests that everybody's tub stays filled because our bodies are constantly gaining and losing fat cells and that some people have a much bigger tub.

Two factors determine the amount of fat in someone's body: the number of cells and the amount of fat stored in these cells. When someone exercises more, eats less, and reduces their weight, they are not changing their number of fat cells but reducing the amount of fat in those cells.

"Your fat cell number is determined before adulthood," says Kirsty Spalding, who led the research with her colleagues Peter Arner and Jonas Frisen. Dieting and exercise don't seem to affect this number, she adds. "If you significantly drop your caloric intake," says Spalding, "the body still maintains this number of cells."

Unfortunately for dieters, the body is efficient at packing away calories in the form of fat. When you eat a meal, your gut absorbs dietary fat from the food into the bloodstream. The liver converts sugars and carbohydrates into another form of fat, and this is absorbed into the bloodstream as well. The blood transports the fat to the body's collections of fat cells, called "adipose" tissues, which store it for later use.

What allowed Spalding and her colleagues to demonstrate that the number of fat cells is constant was a new technique that looks for traces of a particular isotope of carbon. Concentrations of this isotope in Earth’s atmosphere have decreased steadily since 1963, when the United States and other countries signed a test ban treaty prohibiting atmospheric detonations of nuclear bombs.

Measuring the amount of this isotope in a cell's DNA is an accurate way of telling the age of that cell because the amount is directly proportional to how much of the isotope was present in the atmosphere when the cell was created. Atmospheric carbon enters cells because it is absorbed by plants in the form of carbon dioxide, wends its way through the food chain, is introduced into the human body, and is incorporated in the building blocks of human cells. Once in a fat cell's DNA, the carbon isotope stays locked in place there.

This dating revealed that the average fat cell in the body is relatively young—about 10 years old. The bad news for obese people who are trying to keep off the weight is that their own fat cells may thwart their weight loss success. Even after significant weight loss, they still may have twice as many fat cells as someone who has always been slender. After the weight loss, they will have smaller fat cells with less fat stored in them.

This is important for people who are trying to lose weight because the number of fat cells is not controlled by how many calories a person takes in or the amount of exercise they do. More importantly, driving down the amount of fat stored in the fat cells can throw caloric intake and exercise out of whack.

"Fat cells seem to have a happy size that they want to be," says Spalding. "As they decrease, they send out signals to the brain telling it to eat."

"The research is quite significant," says Columbia University professor Rudolf Leibel, who is co-director of the Naomi Berrie Diabetes Center in New York and an expert in this field. He was not directly involved in this study. "It shows that [fat cell-containing] adipose tissue is a regulated organ and that the number of cells is tightly controlled and established early in development."

This new research also offers hope to those struggling to keep weight off because it reveals a whole new mechanism whereby the body regulates a person's weight. Further research may reveal key biological molecules that regulate the mechanism and new targets for the design of pharmaceutical drugs that will help people take off—and finally keep off—the weight.

ISNS contributor Jason Socrates Bardi is a senior science writer with the American Institute of Physics

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