Can brown fat be used to ‘cure’ obesity? Or is it the new weight-loss buzzword, no less a fad than multivitamins and protein diets that end up causing more harm than good?
The answer to both questions is ‘maybe’ — based on the most current information scientists have at hand, including the results of two recent studies on brown fat.
Brown fat, the brown-colored fatty tissue found mainly in patches along the neck and between the shoulders of newborn mammals and in adults of hibernating animals, exists to warm the body.
Unlike ordinary fat cells that burn glucose to power various life processes, brown fat cells directly release as heat the energy they get, in a process known as “nonshivering thermogenesis.”
For a long time, scientists thought brown fat was found only in rodents that don’t shiver and need the heat-generating brown fat to keep warm.
A key survival mechanism for some animals living in cold environments — as well as newborn babies — brown fat allows the body to keep warm without depleting precious energy stores, which happens when muscles shiver in the cold.
Scientists thought that adult humans, who shiver and had no need for brown fat, did not have it.
But three years ago, three separate teams of researchers, working independently, found brown fat in adults. Brown fat showed up in scans of people wearing light clothes like hospital gowns in cold rooms.
Just a few ounces of brown fat in the upper back, on the side of the neck, in the dip between the collarbone and the shoulder, and along the spine showed up in scans detected brown fat absorbing glucose.
Now, a recent study from Canadian researchers shows that cold triggers one form of brown fat to burn calories like a furnace, sucking fat out of the rest of the body to fuel itself.
Another new study from Dana-Farber Cancer Institute, in Boston, Massachusetts finds that exercise can convert ordinary white fat into a second form of brown fat.
Brown fat burns calories
In the first study, researchers from Quebec, Canada’s University of Sherbrooke embarked on a study to find out if the brown fat detected in previous studies by scans in adult humans burned calories to raise body temperature — as it did in mice and hibernating animals.
Just because studies showed the brown fat in adults taking up glucose did not mean it burns calories, researchers of a previous Swedish study on brown fat note.
“We did not know what the glucose actually did: Glucose can be stored in our cells, but that does not mean that it can be combusted,” says Barbara Cannon, author of the earlier study.
Setting out to find answers to this questions, a team led by Dr. André Carpentier, an endocrinologist at Quebec, Canada’s University of Sherbrooke, recruited six healthy men ranging from 23 to 42 years old.
The men were then kept chilled by cold temperatures—but not to the point of shivering, which burns calories. Researchers then used a different kind of medical scan to show the metabolism of fat.
What Dr. Carpentier’s team found was that brown fat burns white fat to create heat. Their findings are published in late January in The Journal of Clinical Investigation.
Their study also showed that glucose is not a major source of fuel for brown fat cells — instead brown fat can burn ordinary fat and when brown fat cells run out of their own small repositories of fat, they suck fat out of the rest of the body.
Researchers also found that men who were thought to have more brown fat actually shivered less. At the same time, the more brown fat a man had, the colder he could get before he started to shiver. This two findings suggested that brown fat was kicking in to burn calories.
The men were at rest but the amount of calories they burned increased as they were exposed to cold, researchers also found. On average, Dr. Carpentier said, the brown fat burned about 250 calories over three hours. The metabolic rates of men increased by 80 percent.
Brown fat “is on fire,” write Dr. Carpentier and Jan Nedergaard, Dr. Cannon’s husband, in an accompanying editorial.
But Dr. Carpentier himself notes that larger studies that replicate and fine-tune the current findings are needed before brown fat can be used to combat obesity.
“We have proof that this tissue burns calories — yes, indeed it does,” Dr. Carpentier said. “But what happens over the long term is unknown.”
For one, brown fat may trigger a loss of calories in men who are kept in the cold, but then, they may become hungrier and eat more to make up for the calories their brown fat burns.
More, Dr. Carpentier’s study did not employ best practices in experimentation: at best, scientific studies are large studies involving a significant number of participants and a control group, and are double blinded — ensuring that both the participants and experimenters don’t know which of two similar treatments is genuine and which is a control procedure. This is a guarantee that the results of an experiment are not biased.
But the Canadian study involved only six participants, did not make use of a control group, and the authors were not blinded to the results. The study was also not designed to see the brown fat directly — only to measure its activity indirectly.
Second Study: From white to brown fat
Exercise can turn white fat into a second type of brown fat, the second study showed. That is, at least, in mice.
Years ago, scientists found this second type of brown fat in mice. Distinct from the type of brown fact being examined by Dr. Carpentier in the “chilled” men, this type did not appears in large, distinct derived from muscle-like masses but was found interspersed in the white fat.
In the second study, published last July (2011), Dr. Bruce Spiegelman, professor of cell biology and medicine at the Dana-Farber Cancer Institute, and his colleagues report that when mice exercise, their muscle cells release a newly discovered hormone that the researchers named irisin.
Irisin converts white fat cells into brown ones — and brown fat cells burn extra calories.
Researchers were then able to turn white fat into brown fat by blocking irisin — and this led to weight loss, improved blood sugar levels and insulin tolerance.
“What I would guess is that this is likely to be the explanation for some of the effects of exercise,” Dr. Spiegelman says, such as the fact that calories burned during exercise exceed the number actually used to do the work of exercising. That may be an effect of some white fat cells turning brown, he surmises.
Because humans also have irisin in their blood, and human irisin is identical to mouse irisin, Dr. Spiegelman proposes that humans, like mice, make brown fat from white fat when they exercise.
So what do these studies mean for those of us who want to lose weight?
Will drug companies be able to create a wonder weight-loss treatment that taps irisin to switch on brown fat in people—without chilling them or making them exercise for long hours?
For Dr. David Katz, founding director of Yale University’s Prevention Research Center in New Haven, Connecticut, it’s too early in the game to know.
“The ultimate question is, ‘how big a factor is this when it comes to weight?’” he asks. “As best I can tell, we can’t answer that questions yet; we’re looking at studies that are very small,” he tells ABC News.
But while the findings on brown fat aren’t conclusive, they’re intriguing — starkly different from what experts thought just a few years ago.
“Up until now, my inclination has been to say the role of brown fat (in weight loss) is very small,” Dr. Katz says. “If that role isn’t so modest, then that becomes an important topic. I don’t think we’re quite there yet, but this paper points intriguingly in that direction.”
And can we just subject ourselves to arctic temperatures if we wanted to lose weight?
“There is still a lot of research to do before this strategy can be exploited clinically and safely,” says Dr. Carpentier of the Canadian study.
Dr. Katz also warns, “The history of better weight management through pharmacology is obviously littered with unintended consequences.”
“As we explore esoteric means of weight loss, we run into one debacle after another,” he concludes.