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Home » How to lose weight safely » Feel a chill? Brown fat's slimming you down

Fat people have less than thin people. Older people have less
than younger people. Men have less than younger women.

It is brown fat, actually brown in color, and its great appeal
is that it burns calories like a furnace. A new study finds
that one form of it, which is turned on when people get cold,
sucks fat out of the rest of the body to fuel itself. Another
new study finds that a second form of brown fat can be created
from ordinary white fat by exercise.

Of course, researchers say, they are not blind to the
implications of their work. If they could turn on brown fat in
people without putting them in cold rooms or making them
exercise night and day, they might have a terrific weight loss
treatment. And companies are getting to work.

But Dr. Andre Carpentier, an endocrinologist at the University
of Sherbrooke in Quebec and lead author of one of the new
papers, notes that much work lies ahead. It is entirely
possible, for example, that people would be hungrier and eat
more to make up for the calories their brown fat burns.

"We have proof that this tissue burns calories - yes, indeed it
does," Carpentier said. "But what happens over the long term is
unknown."

Until about three years ago, researchers thought brown fat was
something found in rodents, which cannot shiver and use
heat-generating brown fat as an alternative way to keep warm.
Human infants also have it, for the same reason. But
researchers expected that adults, who shiver, had no need for
it

and did not have it.

Then three groups, independently, reported that they had found
brown fat in adults. They could see it in scans when subjects
were kept in cold rooms, wearing light clothes like hospital
gowns. The scans detected the fat by showing that it absorbed
glucose.

There was not much brown fat, just a few ounces in the upper
back, on the side of the neck, in the dip between the
collarbone and the shoulder, and along the spine. Although mice
and human babies have a lot more, and in different places, it
seemed to be the same thing. So, generalizing from what they
knew about mice, many researchers assumed the fat was burning
calories.

But, notes Barbara Cannon, a researcher at Stockholm
University, just because the brown fat in adults takes up
glucose does not necessarily mean it burns calories.

"We did not know what the glucose actually did," she said.
"Glucose can be stored in our cells, but that does not mean
that it can be combusted."

A new paper in The Journal of Clinical Investigation by
Carpentier and his colleagues answers that question and more.
By doing a different type of scan, which shows the metabolism
of fat itself, the group reports that brown fat can burn
ordinary fat and that glucose is not a major source of fuel for
these cells. In the study, the subjects - all men - were kept
chilled, but not to the point of shivering, which itself burns
calories. Their metabolic rates increased by 80 percent, all
from the actions of a few ounces of cells. The brown fat also
kept its subjects warm. The more brown fat a man had, the
colder he could get before he started to shiver.

On average, Carpentier said, the brown fat burned about 250
calories over three hours.

But there is another type of brown fat. It has been harder to
study because it often is interspersed in the white fat and
does not occur in large masses. Investigators discovered it in
mice years ago. Now, in a recent article, Bruce Spiegelman,
professor of cell biology and medicine at the Dana-Farber
Cancer Institute, and his colleagues report that, in mice at
least, exercise can make it appear, by turning ordinary white
fat brown.

When mice exercise, their muscle cells release a newly
discovered hormone that the researchers named irisin. Irisin,
in turn, converts white fat cells into brown ones. Those brown
fat cells burn extra calories.

"What I would guess is that this is likely to be the
explanation for some of the effects of exercise," Spiegelman
says. The calories burned during exercise exceed the number
actually used to do the work of exercising. That may be an
effect of some white fat cells turning brown.

Almost everyone of normal weight or below shows this brown fat
if they are chilled, although individuals vary greatly in how
much they have. But this brown fat almost never shows up in
obese people. Is that one reason they are obese, or is their
extra body fat keeping them so warm that there is no reason to
turn on their brown fat?

As for deliberately making yourself cold if you want to lose
weight, Carpentier said, "there is still a lot of research to
do before this strategy can be exploited clinically and
safely."

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Feel a chill? Brown fat's slimming you down

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