Brown fat - the calorie-burning, "good fat" that is plentiful in babies – has been shown to also be present in adults, and scientists are now working on harnessing its mysterious powers to help people lose weight.
Brown fat's usefulness has been emerging over the past decade. Studies have shown exactly where it's located in the body, how it is stored and what activates it to torch calories while keeping us warm.
Now, as the surprising functions of brown fat just keep piling up, researchers are investigating its possible future uses as an alternative to surgical weight loss. Quickly becoming known as the "good fat," brown fat is proving that not all fat is created equal.
Brown fat first began mystifying scientists three decades ago, when it was determined to act as a calorie-burning heat source. It looks brown because it contains huge numbers of mitochondria, which are tiny organelles inside cells that contain iron, giving the fat tissue a rusty tinge.
Mitochondria are found in smaller numbers in most body cells, and are the cells' "powerhouses" – their job is to convert the energy from sugars into a form of energy that cells can use. Therefore, tissues that are loaded with mitochondria, such as brown fat and muscle, act as calorie furnaces.
"This is a fascinating area that is opening up rapidly following the identification of functioning brown fat in humans," said Tom E. Hughes, CEO of Zafgen, Inc., a biotech firm based in Cambridge, Mass. Hughes' company is running the first clinical trial testing the effects of drugs called MetAP2 inhibitors, which putatively work by improving the way fat is processed by obese people's metabolism.
Not just baby fat
Plentiful in babies and small rodents because of their lack of an efficient shivering mechanism in their bodies, brown fat kicks into calorie-burning, heat-generating action when a person becomes cold. Scientists previously theorized that once babies' muscular and nervous systems have developed enough for them to shiver, brown fat is no longer needed and is converted into regular white fat.
But a landmark 2009 study published in the New England Journal of Medicine showed that some adults do indeed retain deposits of this baby fat – typically between their shoulder blades. In fact, the researchers also found that thinner people are more likely to have brown fat, suggesting that it may play a role in regulating body weight. Boosting growth of this fat could potentially become a new way to treat obesity, the researchers said.
Later in 2009, other scientists conducted PET-CT scans on 1,972 people and found that adults also have brown fat deposits on the side of the neck, in the nook between the collarbone and shoulder and along the spine, as well as in the upper back.
"There is a lot more interest in trying to figure out how brown fat can be induced," said that study's senior author, Ronald Kahn, head of the section on obesity and hormone action at the Joslin Diabetes Center in Boston.
Activating brown fat
But making more brown fat cells would have no effect on calorie burning or on weight loss if those cells are not activated, Hughes said. This would lead to an "all dressed up with nowhere to go" situation, with brown fat lying dormant in the body instead of scorching calories. So how can brown fat be activated?
Brown fat is normally activated by adrenaline or thyroid hormones, both of which are released when the body is exposed to cold temperatures. That's why infants have a sheet of brown fat cells covering their backs; because it is vital for their tiny bodies to stay warm, Mother Nature has rigged the brown cells to raise their body temperature as soon as they feel a chill.
Cold temperatures may have a similar effect on adults. In one experiment, researchers at the Maastricht University in the Netherlands were almost able to watch the emergence of brown fat. They took scans of 24 men, 14 of whom were overweight or obese, before and after they were asked to sit in a chilly room for two hours. Scans done prior to the experiment showed no brown fat activity, while scans taken after they were exposed to cold temperatures showed brown fat activity in all but one subject, who was obese.
Obese people have less active brown fat, according to Kahn. Because there is a difference between having less brown fat and having brown fat that is not activated, researchers have been experimenting with ways to measure, as well as activate, brown fat amounts.
Researchers are now looking for ways to increase the amount and activity of brown fat, especially in obese people. The search is on to find drugs that could activate the fat, without unsafe side effects. But not much is known about how these drugs might work, Hughes said.
"As far as we know, if one could increase the amount of brown fat and activate it, the result might be to increase energy expenditure and increase food intake," Hughes said. "This might have a net zero effect on body weight."