“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.