Jan Nedergaard of the University of Stockholm did the opposite of Dr. Kozak. He and Barbara Cannon, also at the University of Stockholm, studied mice that were genetically engineered so their brown fat could not burn calories. The animals became fat.

“Until very recently, we would have said that it is doubtful that differences in brown fat really could contribute to obesity,” Dr. Nedergaard said. Now, he said he had changed his mind, at least for mice.

The key to finding brown fat in humans was PET-CT scans. The PET scans pinpoint areas where cells are actively burning glucose and the CT scans identify it as fat. Because brown fat rapidly burns glucose to produce heat, it lights up in PET scans. In two of the three studies, investigators also studied samples of brown fat that were removed from a few subjects, confirming that the cells had a protein, UCP-1, that is unique to brown fat.

Brown fat in adult humans was in an unexpected place. Infants have it mostly as a sheet of cells covering their backs. Rodents have it mostly between their shoulder blades, just down from the neck. But in adult humans, it showed up in the upper back, on the side of the neck, in the dip between the collarbone and shoulder, and along the spine.

That may be one reason it was missed for so long, Dr. Kahn said.

“There was an interest in looking at humans 20 or 25 years ago with different scanning techniques, but people were always looking between the shoulder blades,” he said. And since there is so little brown fat — just a few grams of tissue — it can be hard to find, Dr. Kahn added.

His study, one of the three published Thursday, involved 1,972 people who had had PET-CT scans for a variety of reasons. The scans showed brown fat in 7.5 percent of the women and 3 percent of the men — an underestimate, Dr. Kahn says, because the people had not activated brown fat by getting cold.

Dr. Kahn and his colleagues also examined surgical samples taken from the necks of two patients. They concluded that what looked like brown fat in their scans was indeed brown fat.