Here’s a new diet you certainly don’t want to try at home. Extreme stress—like the kind that accompanies large third-degree burns—can make the body turn ordinary fat cells into super–calorie-burning brown fat, scientists report. Their observation is the first to confirm that the human body can convert white fat into “healthy” brown fat, and it may eventually lead to drugs that can accomplish the conversion.

“The convincing thing in this study is that these people weren’t selected for any prior ability to produce these cells, and yet it seems they all showed this increase in brown fat,” says Sheila Collins, a biochemist at the Sanford Burnham Prebys Medical Discovery Institute in Orlando, Florida, who was not involved in the new work.

Most fat in mammals is a type called white adipose (fat) tissue, yet small animals and newborn humans have large amounts of brown fat, which is loaded with energy-burning mitochondria that cause it to use more calories and produce more heat than white fat. Until recently, scientists believed that in people, this brown fat disappeared after infancy. But in 2009, researchers showed that adult humans also have brown fat at the base of their necks, raising the possibility that drugs could help people produce more of it to combat obesity, diabetes, and high cholesterol.

Since then, studies in mice and isolated human cells have found that exposure to extreme cold may turn white fat cells into supercharged brown ones that can burn hundreds more calories a day than usual. But, until now, that conversion hadn’t been shown to take place inside the human body.

After noticing that victims of severe, bodywide third-degree burns require more calories than normal to maintain their weight during recovery, researchers led by geriatrician Labros Sidossis at the University of Texas Medical Branch in Galveston started studying their fat cells. They followed 48 child and adult burn patients, collecting fat samples within a few days of their arrival at the hospital and throughout the following weeks of treatment. Although the fat samples started out as normal white adipose tissue, some cells started resembling brown adipose tissue over the recovery period, the scientists report this week in Cell Metabolism. The cells showed increasing levels of energy-burning mitochondria and a protein present in brown fat called UCP1.

“We think in these patients who have lost much of their skin, which normally helps keep us warm, white fat is turning into brown fat in an effort to increase the ability of the body to produce heat,” Sidossis speculates. He and his colleagues believe that high levels of adrenaline—the stress hormone responsible for the fight-or-flight response—released after burns initially spur the conversion. Animal studies have previously shown a link between adrenaline and brown fat. “This finding provides a proof of concept that it is possible to turn white fat into brown fat in humans,” Sidossis says. “The next step is to find the mechanism.”

Although the white-fat-turned-brown-fat cells weren’t as efficient as regular brown fat, there were enough of them to burn an average of 263 extra calories each day in burn patients, without any change in exercise or diet. That amount is enough to make it relevant to disease treatment, Collins says. “263 calories a day is a candy bar,” she says. “If you do that math, over the course of a year, that adds up to significant weight loss.”

Sidossis’s team is now conducting further experiments, exploring which molecular pathways are spurring the conversion and monitoring the burn patients to find out how long the brown fat cells remain active.