Fat that keeps you lean sounds like a contradiction in terms, but that's what brown fat does. Three new studies reveal that, contrary to long-standing assumptions, this energy-using form of fat remains functional in adults. The results bolster researchers' hopes of exploiting brown fat to control weight.
White fat, or white adipose tissue, is the jiggly stuff that stores spare energy from food. By contrast, brown adipose tissue consumes energy to generate body heat. The tissue teems with mitochondria that metabolize food, hence its color. "It's like the burner for the heater," says medical geneticist Sven Enerbäck of the University of Göteborg in Sweden, who led one of the studies. Other mammals and human infants rely on brown fat to keep warm. However, physiologists long thought that the tissue largely disappeared by the time we reach adulthood.
But now, researchers have shown that the brown fat is present and working in adults. Physiologist Wouter van Marken Lichtenbelt of the University of Maastricht in the Netherlands and colleagues dosed 24 young men with radioactive glucose, which tags metabolically active tissue, and turned down the thermostat to a chilly 16°C. Combination PET and CT scans, which spot tissues that have slurped up the radioactive glucose, revealed caches of brown fat in the neck, chest, and abdomen in all but one of the men, indicating that the fat fired up during exposure to cold. "It was exciting that we saw it in so many people in so many tissues," says van Marken Lichtenbelt. When the researchers rescanned three of the subjects at room temperature, the brown fat was no longer visible. (It hadn't disappeared; it had just stopped working so hard.)
To prospect for brown fat, endocrinologist C. Ronald Kahn of Harvard Medical School in Boston and colleagues pored over PET/CT scans of nearly 2000 patients that had been taken previously for other purposes. About 5% of the subjects sported brown fat that was apparent even though the researchers hadn't chilled the subjects. Women usually had more than men, and the amount tended to dwindle with age.
In the third study, Enerbäck and colleagues used PET/CT scans to pinpoint deposits of brown fat in cold volunteers and then confirmed the tissue's identity by looking at gene activity. Samples of tissue displayed the molecular signature of brown fat, including a key heat-producing protein that was absent from white fat. All three groups report their findings today in The New England Journal of Medicine.
Kahn's and van Marken Lichtenbelt's groups also found that heavier people tended to have less brown fat. "Not only do adult humans have brown fat, but it is metabolically active and seems to correlate with being thinner," says Kahn. Why overweight people tend to pack less brown fat remains to be explained. Thin people may be slim because they have extra brown fat to help burn calories. Or heavy people might need less to keep warm because they have more insulating white fat.
"Together, these papers will be a tipping point for how we think about brown adipose tissue," says endocrinologist Mitchell Lazar of the University of Pennsylvania. Now, he says, researchers can focus on whether we can harness brown fat--perhaps by spurring its activity with drugs--to help overweight patients burn off unsightly and unhealthy white fat.