Brown Fat

Brown adipose tissue (BAT) or brown fat is one of two types of fat or adipose tissue (the other being white adipose tissue) found in mammals. It is especially abundant in newborns and in hibernating mammals. Its primary function is to generate body heat without requiring the body to shiver. In contrast to white adipocytes (fat cells), which contain a single lipid droplet, brown adipocytes contain numerous smaller droplets and a much higher number of mitochondria, which contain iron and make it brown. Brown fat also contains more capillaries than white fat, since it has a greater need for oxygen than most tissues.

Typically, mammals generate heat by shivering. Low temperatures signal muscle groups around vital organs begin to shake in small movements in an attempt to create warmth by expending energy. It produces heat because the conversion of the chemical energy (food)  into kinetic energy (motion) is not 100% efficient, causing some of the energy to show up as heat. Brown fat cells use their mitochondria to  convert chemical energy directly into heat by altering the movement of protons that pass through them. It was discovered recently that some adults retain a portion of their juvenile brown fat deposits.

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