A layer of special heat-producing fat cells found mainly around the shoulder blades and kidneys. It is more abundant in infants than adults. Brown fat cells have a high density of mitochondria. The mitochondria contain large amounts of pigmented cytochromes enabling the mitochondria to generate large amounts of heat which is transported quickly by the good blood supply. The metabolic activity of brown fat is stimulated by ingestion of food and by noradrenaline. Brown fat makes up less than 1% of adult body weight and is generally regarded as unimportant in adults. However, it has been suggested that some forms of obesity may be linked with a lack of brown fat cells.