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Brown adipose tissue (BAT) or brown fat is one of the two types of adipose tissue (the other being white adipose tissue) that is present in many newborn or hibernating mammals. Its primary purpose is to generate body heat. In contrast to white adipocytes (fat cells), which contain a single, large fat vacuole, brown adipocytes contain several smaller vacuoles and a much higher number of mitochondria. Brown fat also contains more capillaries since it has a greater need for oxygen than most tissues.

The mitochondria in a eukaryotic cell utilize fuels to produce energy (in the form of ATP). This process involves storing energy as a proton gradient, also known as the proton motive force (PMF), across the mitochondrial inner membrane. This energy is used to synthesise ATP when the protons flow across the membrane (down their concentration gradient) through the ATP synthase enzyme; this is known as chemiosmosis.

In endothermic animals, body heat is maintained by signaling the mitochondria to allow protons to run back along the gradient without producing ATP. This can occur since an alternative return route for the protons exists through an uncoupling protein in the inner membrane. This protein, known as uncoupling protein 1 (thermogenin), facilitates the return of the protons after they have been actively pumped out of the mitochondria by the electron transport chain. This alternative route for protons uncouples oxidative phosphorylation and the energy in the PMF is released as heat.

To some degree, all cells of endotherms give off heat, especially when body temperature is below a regulatory threshold. However, brown adipose tissue is highly specialized for this non-shivering thermogenesis. Firstly, each cell has a higher number of mitochondria compared to more typical cells. Secondly, these mitochondria have a higher than normal concentration of thermogenin in the inner membrane.

In neonates (new born babies), brown fat, which then makes up about 5% of the body mass and is located on the back, along the upper half of the spine and towards the shoulders, is of great importance to avoid lethal cold (hypothermia is a major death risk for premature neonates). Numerous factors make infants more susceptible to cold than adults:

• The higher ratio of body surface (proportional to heat loss) to body volume (proportional to heat production)
• The higher proportional surface area of the head
• The low amount of musculature and the inability or reluctance to shiver
• A lack of thermal insulation, e.g. subcutaneous fat and fine body hair (especially in prematurely born children)
• The inability to move away from cold areas, air currents or heat-draining materials
• The inability to use additional ways of keeping warm (e.g. turning up a heater, drying their skin, changing clothes or performing physical exercise)
• The nervous system is not fully developed and does not respond quickly and/or properly to cold (e.g. by contracting blood vessels in the skin)

However, the burning of the brown fat will, in fact, provide a baby with an alternative form of heat induction. This form is an adequate substitute to any and all other unconscious means of an adult.

When growing up, most of the mitochondria (which are responsible for the brown color) in brown adipose tissue disappear, and the tissue becomes similar in function and appearance to white fat - as a mere fat deposit. However, recent studies using PET scanning of adult humans have shown that it is still present in many adults in the upper chest and neck. The remaining deposits become more visible (increasing tracer uptake) with cold exposure, and less visbile if an adrenergic beta blocker is given before the scan.

• Histology at BU 04901lob - "Connective Tissue: multilocular (brown) adipocytes"