Brown fat is not the same as white fat. In fact, it is more closely related to skeletal muscle than it is to white fat. Brown fat is thermogenic, meaning that it burns energy to create heat, whereas white fat just stores excess energy, particularly around the thighs and abdomen. Scientists have been interested in the ability of brown fat to reverse some of the metabolic changes which precede diabetes for years. In 2009, they engineered mouse and human cells to produce brown fat cells which they then transplanted into mice. Those brown fat cells then burned energy in the host mice, protecting them against obesity.
Brown fat is mostly found in newborn mammals, who are unable to shiver and animals who hibernate. It mainly occurs on the back, neck and shoulders as well as around the heart and protects the mammal from hypothermia or becoming too cold, by burning sugar and creating heat, a process known as thermogenesis.
Brown fat is able to burn so much energy, because it has a very high number of mitochondria, little organelles within the cell which act as the powerhouses for the cell. Incidentally, this is where brown fat cells get their name from. The mitochondria contain a high proportion of iron, which makes the cell look brown. They also contain more droplets of fat than white cells, which only contain one droplet each. Brown fat has more capillaries too, as it has a higher requirement for oxygen to fuel the energy burning. White fat is associated with increased body mass, brown fat is associated with a lower body mass index (BMI).