In the quest to fight obesity, scientists are looking at an intriguing question: Is it possible for adults to lose weight by having more baby fat?
　　Babies have lots of brown adipose tissue, or brown fat, so called because of its color. It is critical to the body's heat production. Unlike white fat cells, which store energy from the food we eat, brown fat consumes calories to generate heat. Revving up this process, research has shown, may help us grow leaner by burning more of the white fat.
　　Until recently, experts believed that only babies and children had brown fat, to help keep them warm before their young bodies develop techniques like shivering to help them cope with cold temperatures. A discovery last year that adults still have at least some brown fat has spawned hope among scientists and drug developers that the calorie-burning tissue may provide one solution to curbing obesity.
　　Researchers at Harvard Medical School have identified a protein in the body that appears to spur production of brown fat, including by converting some white fat cells into brown ones, and are now working to develop a drug that would encourage that process. They expect their work could lead to a new approach to treating obesity within a few years.
　　Other researchers are seeking ways to prompt the brown fat we already have to become more active, thereby prompting our bodies to generate more heat and consume more calories. One technique being investigated: exposing people to colder temperatures, which appears to trigger brown fat to turn up the body's heat.
　　Brown fat is able to burn calories because of the cells' abundance of mitochondria, the engines of the cellular world. White fat cells, by contrast, are like fat-filled balloons that store energy and help insulate the body.
　　Brown fat is thought to have evolved to protect babies and children from hypothermia. Babies and smaller organisms are at greater risk because they tend to cool more easily than adults. And, in prehistoric times, once people became adults, the more important issue wasn't keeping warm, it was to make sure they had enough energy stored to survive times without food. But today, with food so plentiful, white fat cells' energy-storage ability has led to obesity as a significant public health issue.
　　Wouter van Marken Lichtenbelt, a professor of human biology at the Maastricht University Medical Centre in the Netherlands, estimates that boosting the body's heat-generating ability, a process known as thermogenesis, could help people burn as much as 5% to 10% additional calories. 'That can be a lot,' he says. However, whether we lose weight or not depends on whether we overcompensate by eating more food. 'We shouldn't eat too much,' he says.
　　Researchers at various institutions are seeking ways to increase the body's heat-generating ability by taking advantage of the fact that brown fat is activated by the cold. Studies with obese mice show that putting them in the cold for a few days activates their heat-production system significantly. Within a week, an obese mouse could burn off half of its white fat, even while eating 1.5 times as much as it normally does, according to Leslie Kozak, a professor of molecular genetics at Pennington Biomedical Research Center in Baton Rouge, La. 'This is a very powerful system,' Dr. Kozak says.
　　While there are data showing that people who live in colder climates tend to be leaner than people from warmer places, losing weight isn't as easy as taking a vacation in Alaska, for example. For one thing, people often compensate in colder environments by eating more; a similar phenomenon occurs when people begin to exercise more and their metabolism speeds up. It's also unclear how long people need to be exposed to cold in order to boost their heat generation.
　　Dr. Kozak has been investigating whether activation of brown fat can be increased through training people to better tolerate the cold. The idea here is that if people get used to cold, it's because their internal furnace is working better, he says.
　　Dr. Kozak says it's unclear how much cooler an environment needs to be to increase brown fat's activity level. He says he would like to test whether a school classroom, for instance, set at 65 degrees, a few degrees below normal, could increase heat generation in the students' bodies.