Brown fat: Brown adipose tissue, a rapid source of energy for infants in whom it forms about 5% of their body weight. It is brown because the cells in it are packed full of small cellular organs called mitochondria, which are energy factories, and it has a rich supply of blood vessels. Brown fat was believed to be virtually gone by adulthood, but studies have shown that some adults do possess brown fat. There has been widespread interest in brown fat in the diet community because some researchers believe that brown fat can be stimulated to burn or consume white fat (typical adult fat). Studies are ongoing to try to determine whether stimulation of brown fat production or brown fat activity may be valuable as a weight-loss measure.