Brown fat, white fat, too fat

August 14, 1996|By Sue Goetinck

WHEN IT COMES to body fat, brown can be beautiful.

The body stores calories as white fat. It's the stuff that broadens hips and thickens waists. Brown fat, on the other hand, burns up those calories and turns them into heat.

People, at least adults, don't have much brown fat. But scientists have learned that, under certain conditions, animals can replace white fat with brown.

Humans probably can, too. So researchers are trying to figure out the body's trick, to come up with ways to treat obesity.

"The fact that that system is there," said biologist Les Kozak, "suggests it could be a very effective mechanism for burning off calories and shifting energy balance in obese people."

Scientists have shown that in mice, for example, drug treatment can convert white fat into brown. The mice lose weight. Studies have also shown that cold temperatures can cause the conversion.

While Mr. Kozak, of the Jackson Laboratory in Bar Harbor, Maine, said he doesn't recommend turning down the thermostat to lose weight, his results and others are helping researchers piece together how brown fat fits into the body's weight-control techniques.

White fat and brown fat are actually terms for tissues that store fat, rather than for the fat itself. Both tissues consist of fat-containing cells and blood vessels.

Brown fat is brown because the fat-containing cells are packed full of energy-generating structures called mitochondria. White fat cells don't have as many so they don't look brown.

Brown fat's job is to regulate body temperature. While the mitochondria in most cells produce energy in the form of a chemical called ATP, the mitochondria in brown fat cells turn fat into heat. When the body heats up -- as it does during a fever -- brown fat's mitochondria slow down.

Babies are born with a lot of brown fat, which presumably offers protection from any cold temperatures they might encounter. But after about a year, most of the brown fat has disappeared.

A chemical signal known as norepinephrine that originates in the nervous system can cause brown fat to replace white fat, lab research on mice suggests. Pharmaceutical companies are trying to develop drugs that will produce the same effect in people.

Sue Goetinck writes for the Dallas Morning News.

Pub Date: 8/14/96

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