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The results, which suggest a new way
to treat obesity, were presented at The Endocrine Society's 91st Annual
Meeting in Washington, D.C
Brown fat burns off calories and generates heat in
babies and small mammals. Most of our body fat is white fat, which also
provides insulation but stores calories. It becomes "bad" fat when you
have too much. The "good" fat—brown fat—was considered essentially
nonexistent in human adults.
"We now know that it is present and functional in
adults," said the study's lead author, Aaron Cypess, MD, PhD, MMSc, of
the Joslin Diabetes Center in Boston. "Three ounces of brown fat can
burn several hundred calories a day."
For the first time, the researchers were able to
measure patches of brown adipose tissue—brown fat—in people, thanks to a
high-tech imaging method that combines positron emission tomography and
computed tomography, called PET/CT. By evaluating biopsy tissue of what
appeared to be brown fat on PET/CT scans in some patients who had neck
surgery, the authors confirmed that they were, indeed, looking at stores
of brown fat. More than 1,970 study participants had PET/CT scans, from
mid-skull to mid-thigh.
Brown fat (when it could be detected) was located in
an area extending from the front of the neck to the chest. Of the
subjects who had detectable brown fat, about 6 percent had 3 ounces or
more of the fat.
"We believe that this percentage greatly
underestimates the number of adults in the population who have a large
amount of brown fat," said Cypess, whose results were published in the
April 9 issue of the New England Journal of Medicine, along with those
of two other independent studies of brown fat in adults.
That is because one of the other studies found that
PET/CT can detect much more brown fat if people are in a room cooled to
61°F. Likewise, Cypess and his colleagues found that people who
underwent PET/CT in the winter had more brown fat activity than those
scanned in the summer.
They also discovered that brown fat is most abundant
in young women and least frequent in older, overweight men. In fact,
women were more than twice as likely as men to have substantial amounts
of brown fat.
"One theory for this is that women may have less
muscle mass overall, so they need more brown fat to generate heat and
keep warm," Cypess said.
Brown fat provides a new focus for developing
treatments protecting against obesity and its complications, according
to Cypess. However, it may not be enough to lose weight to just have
brown fat. The researcher
said, "We may have to turn it on and make sure it burns calories in a
regulated, safe manner."
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