A new Nature news story discusses the little known fact that there are two different types of adipose (fat) tissue: white and brown. White fat tissue stores excess calories that are not used for energy as lipids, and typically accumulates around the hips and thighs of the girls, and around the belly of the guys. Simply put, it's the excess inches we try to get rid of through diet and exercise. Brown adipose tissue (BAT), on the other hand, typically accumulates around the collarbone, shoulder blade, and neck area. Originally thought to only be present in human newborns and animals, BAT is unique in that it burns excess fat calories, as opposed to storing them, to keep the body warm.
However, in recent studies published in The New England Journal of Medicine, researchers found metabolically active BAT in an unexpected place -- on human adult volunteers. The studies used Positron Emission Tomography (PET), which measures where consumed radio-labeled glucose is metabolized in the body. Subjects were scanned either at room temperature, or in a cold room (17-19 deg Celsius), while their feet were repeatedly immersed in cold water (7-9 deg Celsius). It turns out that with the cold room and ice-cold foot bath, there was a significant increase in the metabolic activity of the fat tissue around the collarbone and shoulder blades, compared to scans taken at room temperature. Cold temperatures activate the sympathetic nervous system, and epinephrine (adrenaline) is released, which causes the body to warm itself. These results show that in colder temperatures, calories may not be stored on your waist or hips, but rather, metabolized by the brown adipose tissue to keep you warm.
Despite their findings, it's not suggested you take your lunch and head for the nearest walk-in freezer. But the key finding is that BAT metabolism is triggered by adrenaline, the same hormone responsible for the "fight or flight" response. Therefore, these results open the possibility that new drugs that activate the sympathetic nervous system to release adrenaline may be a viable treatment for obesity.