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Americans spend millions each year searching for the right diet or exercise program - all in an effort to shed some fat. But there's one type of fat that most would probably like to hold on to: brown fat.

Instead of storing excess energy from food in lumps and bumps throughout the body - like its well-known sister, white fat - brown fat helps burn incoming calories.

Because its primary purpose is temperature regulation, brown fat cells are packed with mitochondria, the powerhouses of cells. This mitochondria-heavy design is well-suited to use high quantities of sugar, the body's fuel, and then release that energy in the form of heat.

This mechanism enables small and hibernating mammals, which can't shiver, to stay warm in cold temperatures. And it enables newborn humans, who have yet to develop layers of white fat, to stay warm after exiting the stable confines of the womb.

Until recently, only these two types of creatures were known to have brown fat.

Now researchers have found that adults don't, in fact, lose all of their brown fat to the creeping ubiquitousness of white fat; with that finding, they've scrambled to learn how the substance's fat-burning abilities could be harnessed for weight loss.

If brown fat is unleashed, it could potentially "tickle" the metabolism enough to make weight loss easier and more manageable, said Sven Enerback, a researcher at the University of Gothenburg in Sweden and one of the scientists who discovered that brown fat persists in adults. As is, once a person reduces his or her starting weight, the body begins to compensate for the loss by getting better gas mileage - that is, burning fewer calories.

Enerback calculates that inserting only 50 to 100 grams of activated brown fat into a person could significantly increase their energy metabolism and eliminate 10 pounds of white fat a year.

However, these gains in metabolic rate are possible only if the brown fat is active.

Researchers are exploring avenues through which to safely activate brown fat. Cold temperatures might play a role.

Already they've learned in experiments exposing a variety of people to cold temperatures that healthy people tend to have more active brown fat than their less healthy, older or more overweight counterparts, said Dr. Aaron Cypess of the Joslin Diabetes Center in Boston. But researchers are not sure whether the brown fat helped lead to better health or whether people in better health have more brown fat.

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