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 jam-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, who 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 thought to even 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 launched a scramble to discover 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 can be achieved only if the brown fat is active. "When heat is not needed, the burner is off -- so we need to find good ways to activate it safely," he said.
The role of cold
Researchers are exploring various avenues through which to do this. It's possible cold temperatures may 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. Further investigation will need to determine whether brown fat actually plays a protective role against obesity.
"First, we need to know the significance of brown fat function in human adults -- we can look at how to increase brown fat -- but first we need to understand it more," said Wouter van Marken Lichtenbelt, a professor of physiology at Maastricht University in the Netherlands and a co-discoverer of brown fat in adults.
Recently, a team of researchers led by Dr. Bruce Spiegelman, a professor of cell biology at the Dana-Farber Cancer Institute in Boston, was able to increase brown fat stores in mice by turning immature muscle cells into brown fat cells, then transplanting these cells into adult mice.
Their findings were published July 29 in the journal Nature.
The brown fat made by the research team was able to actively burn off incoming caloric energy, Spiegelman said. Though the study did not demonstrate whether the animals lost weight, he said, the researchers' next step will be to implant the engineered brown fat into obese mice to observe whether or not they get thinner.
Animal models in previous studies have already demonstrated how the elimination of brown fat in rodents leads to obesity, Spiegelman said. So brown fat may have the potential to help humans in the same way, he added.
"These findings open up another avenue for treatment," Enerback said. "It could be the first step toward a cell-based therapy for obesity."
This may even lead to drug treatments that would make it possible to activate brown fat and allow it to burn up incoming calories before white fat acts to store it in the body.
Not only are such therapies a long way off, however -- brown fat was discovered in human adults just a few months ago -- such interventions would likely be effective only if coupled with lifestyle changes such as exercise and diet, Enerback added.
Though the discovery of brown fat in humans has shed light on its potential as a weight-loss strategy, many questions remain unanswered. To name a few: Can brown fat be activated in humans? Can more be implanted without causing serious side effects? Who naturally has more brown fat and why?
With all these missing pieces, researchers predict a wait of about five to 10 years before potential brown fat interventions -- such as a brown fat pill or brown fat implants -- can be tested on humans. But already the substance has changed how people should think about fat.