April 20, 2009

Overlooked “Brown Fat” Tied to Obesity

Globular fat cells A scanning electron micrograph of fat tissue.David Gregory & Debbie Marshall. All rights reserved by Wellcome Images.

Scientists have thought that adults lack a type of fat called brown fat. New research shows that not only do adults have it, but it may play an important role in weight control.

There are a least 2 different types of fat. Our bodies store energy primarily as white fat. When you consume too many calories, your body turns the excess into white fat. To lose weight, you generally have to use more energy than you take in, which makes your body tap into its stores and break down the white fat for energy.

Brown fat helps to maintain body temperature by burning up chemical energy to create heat. It's found in small mammals like rodents throughout their lives. Human infants have it when they're born, but we lose brown fat as we age. By adulthood, researchers thought, our brown fat was essentially nonexistent. However, it's been difficult to actually measure brown fat.

A team of scientists at several institutions in Boston set out to measure brown fat directly. They detected uptake of a labeled sugar molecule in whole-body scans that used an imaging system called positron-emission tomography and computed tomography (PET-CT). Cells that take up the sugar are metabolically active. In fat tissue, that indicates the presence of brown fat. The team, led by Dr. C. Ronald Kahn at the Joslin Diabetes Center, was partly supported by NIH's National Institute of Diabetes and Digestive and Kidney Diseases (NIDDK).

The researchers analyzed almost 2,000 patients for the presence of regions of brown fat that were at least 4 mm in diameter. In the April 9, 2009, edition of the New England Journal of Medicine, they reported that they detected brown fat in 76 of 1,013 women (7.5%) and 30 of 959 men (3.1%). Women also had more brown fat tissue and higher levels of sugar uptake. The most common area in which brown fat was detected was a region of the neck.

Notably, the less brown fat tissue the researchers detected, the higher the body mass index (a ratio of weight to height) tended to be, especially in older people. That suggests a potential role of brown fat in adult human metabolism.

Two companion papers published in the same journal issue complemented these findings. In one, a research team in the Netherlands reported detecting brown fat tissue activity in 23 of 24 men during cold exposure, but not at room temperature. The activity was significantly lower in the overweight and obese men than in the lean subjects. These results imply a role for brown fat in keeping the body warm.

In the other study, a team from Sweden found that labeled sugar uptake increased by a factor of 15 in the cold. Suspecting the presence of brown fat, the researchers took tissue biopsies and demonstrated the presence of active brown fat.

"Taken together, these studies show that a significant percentage of adults have active brown fat, and that it likely plays a role in regulating body temperature," says NIDDK researcher Dr. Francesco S. Celi, who wrote an accompanying editorial in the journal. "Stimulating this tissue to burn more energy may be a promising novel strategy for treating or preventing obesity."

—by Harrison Wein, Ph.D.

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