LONDON (Reuters) - Scientists have found out how some fat cells are turned into calorie-burning brown fat known as brown adipose tissue rather than into the white fat associated with obesity.

The discovery may help researchers develop ways to fight the obesity epidemic that is sucking up health budgets and resources in rich nations and quickly spreading to the developing world.

Stephan Herzig of the German Cancer Research Center in Heidelberg, who led the study, said scientists could now try using stem cells to generate brown fat cells in a lab dish to then implant them into the body and help speed up calorie burn.

“Now that we know some of the signals that are required to generate brown cells, we have the tools to put everything together and try it out,” he said in a telephone interview.

Stem cells are the driver cells from which all other cells develop.

Rates of obesity have risen dramatically in recent decades in affluent nations, and more Western-style diets and less exercise mean that corpulence is taking hold in developing populations too.

Already, two-thirds of U.S. adults and nearly one in three children are overweight or obese -- a condition that increases risks of diabetes, heart disease and other chronic illnesses.

FOX-2 ENZYME IS THE TRIGGER

Experimenting on mice in the lab, Herzig and a team of scientists in Germany and Switzerland found that an enzyme called COX-2 triggers development of fat cells to become brown fat, instead of white fat.

White adipose tissue hoards fat by using our bodies -- particularly our bellies and thighs -- as a large storage unit, while brown adipose tissue is a sparse form of fat that helps keep newborns warm and helps adults burn calories.

Once activated by cold temperatures, brown fat burns calories faster than regular fat.

The researchers, whose study was published in the journal Science on Thursday, also found that mice who were genetically engineered to produce high levels of COX-2 burned energy faster and were protected from obesity.

Scientists estimate that as little as 50 grams of brown adipose tissue in a normal adult human would be enough to increase energy consumption by 20 percent.

“That’s not a lot of brown fat, but a big increase in energy burn,” said Herzig. “And this type of transplantation idea has been tried out with other types of animals and other types of cells, so in principle it works.”

COX-2 is critical for the production of prostaglandins -- hormone-like substances that play a role in a wide range of functions essential to a healthy body, including regulation of the immune system.

Painkilling drugs called COX inhibitors like aspirin and ibuprofen block COX enzymes. But Herzig said this doesn’t mean that taking these drugs makes you fat, since the COX-2 function needed for brown fat to burn calories operates differently.