HMS Team Discovers Healthy Fat in Adults

Researchers at Harvard Medical School have discovered that adult humans do in fact possess a type of beneficial fat—previously thought to only be present in babies, young children, and other small mammals—that burns calories instead of storing them.

The discovery was made simultaneously by three independent research teams in Boston, the Netherlands, Finland, and Sweden. Brown fat and its potentially crucial role in warding off obesity has since been the subject of three articles in the latest issue of the New England Journal of Medicine.

Unlike its counterpart white fat, a mere two ounces of brown fat tissue, if fully activated, could burn an estimated of 20 percent of daily calories, according to C. Ronald Kahn, one of the lead investigators and a professor at HMS.

“It doesn’t take very much for brown fat to burn a significant number of calories,” said Kahn, who is also head of obesity and hormone action research at Harvard-affiliated Joslin Diabetes Center.

Previously, other researchers had unsuccessfully looked for brown fat between the shoulder blades, where it is located in mice and in human babies, leading to the conclusion that brown fat disappeared as people age. It was widely believed to only remain in “extraordinary circumstances,” such as living in extreme Northern climates or having a tumor in one’s adrenal glands, according to Kahn.

“We thought, maybe the reason we’re not finding any brown fat is that we were not looking in the right place,” he said.

Analyzing a combination of over 3000 PET and CT scans, the team found trace amounts of brown fat nestled under the muscles in the front of the neck.

“This was actually a bothersome thing to the radiologists,” said Allison B. Goldfine, an associate professor at HMS and one of the paper’s authors. “With the new technology that radiologists were using to find tumors, they were picking up something else with similar high metabolic activity—the brown fat.”

Because radiologists would not have biopsied the region and endocrinologists were not aware of its existence, this “momentous” discovery was made possible only when the different disciplines merged, said Goldfine, referring to the team of endocrinologists, radiologists, endocrinologists, physiologists, geneticists, and others who worked on this study for over a year.

The three independent studies showed that the amount of brown fat fluctuates with the season, temperature, age, and weight. It is also lower in people with high blood sugar levels. The Harvard team also found that women generally had more brown fat.

Establishing the presence of brown fat in adults opens up the possibilities for future investigations as to how its usefulness might be harnessed. According to Kahn, he and his team will look into creating a technique to scan for brown fat that is more practical for clinicians.

“The bottom line is, if there are ways to stimulate the activity or the amount of brown fat in humans...it’d make a big difference in the prevention and reduction of obesity,” said Kahn.

—Staff writer Helen X. Yang can be reached at hxyang@fas.harvard.edu.