The Fat that May Keep You Thin

Take note of the brown revolution—the brown fat revolution, that is. Three separate groups of scientists on two continents have independently verified that adult humans do possess this "good," slimming form of fat, previously thought only to be present in children and rodents.

The tissue is metabolically active and the more of this fat you have, the leaner you tend to be, experts say. That's because brown fat helps regulate body temperature by generating heat.

And according to experts, insights on how to tweak brown fat's magic qualities could lead to anti-obesity treatments.

"The general public and most scientists didn't know this existed," said Aaron Cypess, MD, PhD, a research associate at Joslin Diabetes Center and instructor at Harvard Medical School in Boston. "We say it's there and could be used as a treatment for obesity and diabetes."

Dr. Cypess is lead author of one of the studies, all three of which were published in the New England Journal of Medicine.

The Discovery Of Brown Fat

While too much everyday "white" fat causes obesity, brown fat actually helps burn calories and may help us lose weight. Babies have some, but most scientists studying metabolism and obesity had abandoned the idea that adults (other than rodents) might harbor this potentially useful tissue.

That is, until imaging specialists mentioned that they had been seeing this type of tissue for years while studying cancer patients, basically considering it a nuisance factor.

"It was in a comer of the scientific literature the rest of us were not necessarily aware of," Dr. Cypess said. "Nuclear medicine experts didn't like it, because it gets in the way of seeing tumors."

So scientists interested in metabolism harnessed the latest imaging technologies to investigate this lost piece of the obesity puzzle.

Study #1

Using combined positron-emission tomographic and computed tomographic (PET-CT) scans, Dr. Cypess and his team found a wide prevalence of brown fat tissue in human adults It was twice as common in women as in men (7.5% versus 3.1%).

The fat was indicative of overall leanness, the researchers noted, since heavier people, especially older, heavier people, had less brown fat than skinnier people. People taking beta-blockers-drugs used to treat high blood pressure and other conditions-also tended to have less brown fat. Beta-blockers have been linked with weight gain.

Younger adults had more brown fat, and this type of fat was more active in colder weather, stated the authors, who were supported by funds from various sources, including drug makers Pfizer Inc. and Merck & Co.

Study #2

In a second study, Dutch researchers found that all but one of 24 young men studied had brown fat. Yet in keeping with the previous study's findings, brown fat was less active in males who were carrying extra pounds.

The tissue was also active in cold temperatures only, which makes sense, given our need to keep warm in colder climes and seasons.

Study #3

The final study, from researchers in Finland and Sweden, showed that lower temperatures activated brown fat tissue in adults. This could shed light on how humans expend energy.

Implications

"This is the first time we're finding out that there is active brown fat in adult humans, but the question is if this can be applied—if we can use medication to make this brown fat more active and help people lose weight," said Spyros Mezitis, MD, an endocrinologist with Lenox Hill Hospital in New York City.

"With obesity treatments, we now can't lose more than 10 percent," Dr. Mezitis said. This finding opens the possibility of new drugs that will treat obesity in a more effective manner, he added.

Expert Reaction

One expert believes the findings from the three studies have several drawbacks. lan Murray, PhD, assistant professor of neuroscience and experimental therapeutics at Texas A&M Health Science Center College of Medicine, noted that only a small percentage of people overall had brown fat, and very few heavy people had it.

Also, the tissue's activity seemed to be concentrated around the adrenal gland, which is known to play a role in stress and body-temperature regulation, Dr. Murray added.

"It might be that lean people have better energy regulation than fat people," he reasoned. "You can modulate this early on before people get obese...so that a larger percentage of people can develop or maintain this fat. That is another (potential] therapy, more preventive than curative."

Dr. Cypess agreed that more study is needed. "As much as scientists want to be first, we really all want to be right," he said. "With three of us showing these things, we feel that this is all probably true, and we are very optimistic now for potential treatments for obesity."