Fat people have less than thin people. Older people have less than younger people. Men have less than younger women.
It is brown fat, actually brown in color, and its great appeal is that it burns calories like a furnace. A new study finds that one form of it, which is turned on when people get cold, sucks fat out of the rest of the body to fuel itself. Another new study finds that a second form of brown fat can be created from ordinary white fat by exercise.
Of course, researchers say, they are not blind to the implications of their work. If they could turn on brown fat in people without putting them in cold rooms or making them exercise night and day, they might have a terrific weight loss treatment. And companies are getting to work.
But Dr. André Carpentier, an endocrinologist at the University of Sherbrooke in Quebec and lead author of one of the new papers, notes that much work lies ahead. It is entirely possible, for example, that people would be hungrier and eat more to make up for the calories their brown fat burns.
“We have proof that this tissue burns calories — yes, indeed it does,” Dr. Carpentier said. “But what happens over the long term is unknown.”
Until about three years ago, researchers thought brown fat was something found in rodents, which cannot shiver and use heat-generating brown fat as an alternate way to keep warm. Human infants also have it, for the same reason. But researchers expected that adults, who shiver, had no need for it and did not have it.
Then three groups, independently, reported that they had found brown fat in adults. They could see it in scans when subjects were kept in cold rooms, wearing light clothes like hospital gowns. The scans detected the fat by showing that it absorbed glucose.
There was not much brown fat, just a few ounces in the upper back, on the side of the neck, in the dip between the collarbone and the shoulder, and along the spine. Although mice and human babies have a lot more, and in different places, it seemed to be the same thing. So, generalizing from what they knew about mice, many researchers assumed the fat was burning calories.
But, notes Barbara Cannon, a researcher at Stockholm University, just because the brown fat in adults takes up glucose does not necessarily mean it burns calories.
“We did not know what the glucose actually did,” she said. “Glucose can be stored in our cells, but that does not mean that it can be combusted.”
A new paper in The Journal of Clinical Investigation by Dr. Carpentier and his colleagues answers that question and more. By doing a different type of scan, which shows the metabolism of fat, the group reports that brown fat can burn ordinary fat and that glucose is not a major source of fuel for these cells. When the cells run out of their own small repositories of fat, they suck fat out of the rest of the body.
In the study, the subjects — all men — were kept chilled, but not to the point of shivering, which itself burns calories. Their metabolic rates increased by 80 percent, all from the actions of a few ounces of cells. The brown fat also kept its subjects warm. The more brown fat a man had, the colder he could get before he started to shiver.
Brown fat, Dr. Carpentier and Jan Nedergaard, Dr. Cannon’s husband, wrote in an accompanying editorial, “is on fire.”
On average, Dr. Carpentier said, the brown fat burned about 250 calories over three hours.
But there is another type of brown fat. It has been harder to study because it often is interspersed in the white fat and does not occur in large masses. Investigators discovered it in mice years ago. Now, in a recent article, Bruce Spiegelman, professor of cell biology and medicine at the Dana-Farber Cancer Institute, and his colleagues report that, in mice at least, exercise can make it appear, by turning ordinary white fat brown.
When mice exercise, their muscle cells release a newly discovered hormone that the researchers named irisin. Irisin, in turn, converts white fat cells into brown ones. Those brown fat cells burn extra calories.
Dr. Spiegelman said the brown fat he studies is different from the type that appears in large, distinct masses in rodents, the type Dr. Carpentier was examining in his subjects. That brown fat is derived from musclelike cells and not from white fat.
Dr. Spiegelman suspects that humans, like mice, make brown fat from white fat when they exercise, because humans also have irisin in their blood. And human irisin is identical to mouse irisin.
“What I would guess is that this is likely to be the explanation for some of the effects of exercise,” Dr. Spiegelman says. The 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.
Many questions remain. The only brown fat that can be easily seen in people is the muscle-derived fat that shows up in scans. And that brown fat, notes Dr. C. Ronald Kahn, chief academic officer at the Joslin Diabetes Center in Boston, is visible in people only when it is turned on by making them cold.
Almost everyone of normal weight or below shows this brown fat if they are chilled, although individuals vary greatly in how much they have. But this brown fat almost never shows up in obese people. Is that one reason they are obese, or is their extra body fat keeping them so warm that there is no reason to turn on their brown fat?
There is also an intriguing relationship between the brown fat that emerges under the skin and the density of bone. Dr. Clifford Rosen, a professor of medicine at Tufts University School of Medicine in Boston, is studying mice that cannot make brown fat and was astonished by the state of their bones.
“The animals have the worst bone density we have ever seen,” Dr. Rosen said. “I see osteoporotic bones all the time,” he added, “but, oh my God, these are the extreme.”
And while exercise may induce brown fat in humans, it remains to be seen how important a source of calorie burning it is, researchers say.
As for deliberately making yourself cold if you want to lose weight, Dr. Carpentier said, “there is still a lot of research to do before this strategy can be exploited clinically and safely.”