Rodents, unable to shiver, use calorie-burning brown fat to stay warm. So do human infants. So do human adults, it turns out; scientists just didn’t know how to look for it:
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.
A second study, led by Wouter D. van Marken Lichtenbelt of Maastricht University in the Netherlands, involved 24 healthy young men. Ten were lean, the rest overweight or obese.
The scans showed no brown fat when the men had been in a room that was a comfortable temperature. But after they were in a chilly room for two hours, scans showed brown fat in all but one, an obese man.
A third study, led by Dr. Sven Enerbäck of the University of Goteborg in Sweden, involved five healthy adults. Each had two scans — one after being in a room at a comfortable temperature, the other after being in a chilly room for two hours. The investigators saw brown fat in their chilled subjects. Three participants allowed the researchers to remove some white fat and some brown fat to demonstrate that what looked like brown fat in the scans really was that elusive substance.
The studies, investigators say, should stimulate research on safe ways to activate brown fat. It is known to be activated not only by cold but also by catecholamines, hormones that are part of the fight or flight response. That is why beta blockers, which block catecholamines, can suppress brown fat activation.
Epinephrine, or adrenaline, and ephedra, an herbal supplement containing epinephrine, can stimulate brown fat, said Dr. Rudolph Leibel, co-director of the Naomi Berrie Diabetes Center at the Columbia University Medical Center. But the drugs have too many side effects to be used for weight loss, he said, adding that while caffeine can bolster ephedra’s effects, it is easy to eat your way out of a brown fat effect.