July 11, 2012 -- Looking to slim down? Then beige is your color, at least as far as fat is concerned.
Scientists at Dana-Farber Cancer Institute have isolated a new type of energy-burning cell known as "beige fat," which they say may have therapeutic potential for aiding weight loss and treating obesity in adults.
According to a new report published in the journal Cell, beige fat is scattered in pea-size deposits beneath the skin near the collarbone and along the spine. But rather than storing excess calories in the form of jiggly thighs and a jelly belly as blubbery-and-prolific white fat does, this type of fat is a calorie burner.
Sweating and Freezing
"During exercise, muscles release the hormone irisin, which then converts ordinary white fat cells into beige ones – and those beige cells burn up extra calories," explains Bruce Spiegelman, the senior author of the paper.
It's long been known that the calories burned during exercise exceed the number used during the actual activity. Beige fat could be responsible for torching these extra calories. However, because the muscles also release irisin when the body is cold, Spiegelman speculates that the beige fat mechanism might have evolved as a response to shivering, which, like exercise, is a neuromuscular activity.
Spiegelman doesn't necessarily believe the conversion of cells to beige is permanent. "It's an adaptive process," he says. "They probably increase or decrease depending on physiological conditions such as age, sex and obesity."
This could be why more brown fat and perhaps more beige fat is present in people who are fit and physically active versus those who are slothful couch potatoes. An attractive hypothesis to be sure, but Spiegelman cautions there's not yet enough evidence to prove it.
Beige Versus Brown
Beige fat appears to be genetically distinct from "brown fat" another type of fat found in small amounts in the necks and collarbones of adults and in larger amounts in rodents and human infants. It's distinct from the white fat that plagues anyone who struggles to lose weight. Both brown and beige fat have an abundance of mitochondria, the tiny power plants of the cell that convert food into energy and generate heat. Both types contain iron, which gives them their distinctive brown and beige hues.
But the two fats differ in a number of ways. One key difference is that brown fat cells express high levels of UCP1, a protein required by mitochondria that burns calories and generates heat, whereas beige cells normally express low levels of it. Beige cells can, however, turn on high levels of UCP1 in response to cold or the release of irisin, enabling beige fat to burn calories nearly as effectively as brown fat. Also, brown fat cells appear to arise from stem cells precursors that also produce muscle cells, while beige fat forms within deposits of white fat cells from beige cell precursors.
The discovery of irisin, and its ability to transform white fat to brown fat was originally announced in another paper by Spiegelman that his team published earlier this year. This latest Cell report confirms that irisin specifically stimulates white fat to produce beige fat.
Dr. Dazid Katz, director of the Yale University Prevention Research Center, said the existence of a middle ground between white and brown fat makes sense.
We already knew that exercise could induce a transformation of white fat to brown," he said. "This simply characterizes the intermediate state. The functional significance of this 'beige fat' appears to be what we already knew: exercise, and cold, can raise energy expenditure in part through the activation of brown fat."
The hope, of course, is that the beige fat cells might one day lead to new treatments for obesity and diabetes. Dana-Farber has licensed both discoveries to Ember Therapeutics, a biotech company founded by Spiegelman. Keith Ayoob, associate clinical professor of pediatrics at the Albert Einstein College of Medicine, said that this research will be crucial in determining whether anything useful can come from this discovery.
"We need to know more about how much is there, why some people may have a greater or lesser proportion of it, and to see if it can be of use in helping people lose weight," Ayoob said. "There may be a way -- probably via a drug -- to stimulate either the production of beige or brown fat over white fat, or to increase fat breakdown.
Would such a drug be a good idea? Katz said he remains skeptical, saying that he believes the finding means "that the quest for a magical means of tickling both brown and beige fat into burning more calories ... without the ... inconvenience of actually exercising or eating better will continue in earnest.
"To date, our efforts to tickle the metabolic engine in this selective manner have reaped a whirlwind of unintended consequences, and frankly, I see that peril around this corner as well."
As for using cold and shivering as an alternative to exercise – some diet gurus are already recommending this – Spiegelman says forget about it.
"Anyone can write a book to say it works, but this demands serious research and clinical trials," he said. "We've never done the experiments before, and it would be a pretty uncomfortable therapy, so realistically I don't think it's a good road to travel down."