Brown Fat Burns Calories

Brown adipose tissue or brown fat helps small mammals stay warm by burning calories when activated by low temperatures. In contrast, white fat is used to store energy and shows little metabolic activity. These tissues were believed to be found primarily in babies and young children. However, scientists have found recently that adults retain significant amounts of brown fat on their bodies. Until recently, it has been nearly impossible to study brown fat in live humans because finding it in people's bodies meant taking tissue samples. Therefore, scientists mostly stuck to studying it in lab animals. Recently, it was observed that some people had deposits of tissue that looked like fat but didn't act like it; this fat-like tissue was located above the collarbones and in the upper chest and consumed lots of energy. Integrated positron-emission tomography and computed tomography (PET-CT) scans identified it to be brown fat. The same techniques can also be used to measure its metabolic activity. Among women, 7.5 percent had patches of brown fat that were more than 4 millimeters in diameter, while 3.1 percent of men had similar patches. These number varies with age and body mass index as well as whether the tests were performed during warm or cold season. Brown fats were found to be more active in leaner and younger people and when test subjects were exposed to the cold. In one test, subjects spent two hours in a room kept at 63° F to 66° F. During PET scan, they submerged one foot in ice water, alternating five minutes in the water and five minutes out. The cold conditions boosted the amount of glucose the study participants' brown fat consumed by a factor of 15. These new findings are important because drugs could be developed to “activate” brown adipose tissues and help people with obesity burn calories faster.
http://www.cnn.com/2009/HEALTH/04/10/brown.fat.obesity/index.html