Cool temperatures may activate "good" fat in the body that burns calories and ultimately leads to weight loss, according to a new study published in the journal PNAS USA Early Edition.

The "good fat" is called brown adipose tissue, or brown fat, and is naturally found in humans. It takes calories from normal white fat, which stores energy, and burns it. Past trials have shown that brown fat can be stimulated in mice, showing promising signs for future weight-loss therapies, but scientists have not shown how to successfully stimulate it in humans.

Researchers from the Joslin Diabetes Center in Boston wanted to compare the effects of ephedrine, a stimulant that is used in weight-loss drugs, versus the cool tempearatures, which scientists predicted would activate the brown fat.

The team tested 10 study participants in three different ways. They were given injections of ephedrine, or saline solution as a control, or made to wear a "cooling vest," which had 57-degree water pumping through it. After each phase, brown fat activity was measured using a PET/CT scan.

Researchers found that ephedrine did not stimulate brown fat activity. But after study participants wore cooling vests for two hours, brown fat showed significant stimulation on the scans.

Both ephedrine and the cooling vests stimulated the body's sympathetic nervous system, which controls our fight-or-flight response. Symptoms of this response include an increase in blood pressure and heart rate and the slowing of digestion. More symptoms of the response showed after the ephedrine shot than while participants wore the vest.

"We've found a way to stimulate brown fat in a way that is effective and apparently safe by using this mild cold activation and compared it to an over-the-counter weight loss medication that does not have the same effect on brown fat," said Dr. Aaron Cypess, lead author of the study and a staff physician at the Joslin Diabetes Center.

The ephedrine's inability to trigger brown fat activity may be the reason so many diet drugs do not show long-term beneficial results.

"The problem with prescribing ephedrine is that people need to stay healthy their whole lives," said Cypess. "Are we really going to be prescribing a sympathetic activating drug for someone's entire life?"

There need to be safer, long-term solutions, he said. "You've got an organ in your body that is designed to burn off calories (brown fat). There is more of a reason to focus on how to activate this."

Cypess said he is not encouraging people to stand in a refrigerator any time soon. Much more research needs to be done to figure out what is the best way to stimulate brown fat through cooling temperatures.

Cypess said the findings could making significant contributions to weight loss in the future - important since 70 percent of Americans are considered overweight or obese.

"I bet I could get more people to wear a vest for an hour and a half than exercise every day for the same amount of time," he said.

But Dr. David Katz, director of the Yale Prevention Center, said neither ephedrine nor any type of cooling vest should be the go-to intervention for a weight loss regimen.

"One is unsafe, the other is uncomfortable," said Katz. "If people are willing to put with discomfort, how about they try eating better and being more active?"