There is a reason why you shiver when you are cold. Shivering produces heat. In the absence of shivering, nature has devised a solution to prevent body temperature from dropping too low. It is called brown adipose tissue (BAT), more commonly known as brown fat.  All mammals have brown fat, but it tends to be most prevalent in cases where shivering, or other forms of movement for maintaining body temperature are not possible, such as newborn infants, or hibernating animals.

Most people are familiar with white fat. The function of white fat cells is to store excess calories. Brown fat, on the other hand, does the opposite. It does not store calories, but rather converts nutrients directly into energy, generating heat in the process (thermogenesis).

The mitochondria is the organelle that produces energy within cells by oxidizing nutrients. It is the cell’s power plant. Compared to white fat cells, brown cells have a greater number of mitochondria, as well as a much higher concentration of thermogenesis-activating agents in the mitochondrial membrane, and more channels within the cell for enhanced  oxygen delivery. Brown fat is turbo-charged for turning nutrients into energy and heat.

This is of interest to extreme fitness buffs, because increased brown fat activation can help the metabolism burn more fat, and use more calories. Brown fat can be activated by the sympathetic nervous system (fight or flight response to emergencies), but frolicking with grizzly bears or lions may not be the best way to get your brown fat working. :-)

Recent studies have presented evidence of two types of brown fat cells, the classical brown fat, and a second type known as beige fat. Beige fat cells can be isolated from white fat, and while they have some characteristics of white fat, they have been shown to act like classical brown fat in thermogenic response. Studies have suggested exposure to cold, as well as certain compounds such as green tea, ginger, capsaicum, and L-arginine may help support formation of beige fat cells within white fat deposits.

Green tea, ginger and capsaicum (from cayenne pepper) are common ingredients in many fat burners, and now we may better understand some of the reasons why they are so effective at helping us to burn off those extra calories.


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Cypress AM, Chen YC, Sze C, Wang K, et al. Cold but not sympathomimetics activates human brown adipose tissue in vivo. Proc. Natl. Sci. U.S.A. 109 (25) 10001 - 10005, June 2012.

Lidell ME, Betz MJ, Leinhard OD, et al. Evidence for two types of brown adipose tissue in humans. Nature Medicine 19, 631 - 634, 2013.

Wu J, Bostrom P, Sparks LM, Ye L, et al. Beige adipocytes are a distinct type of thermogenic fat cell in mouse and human. Cell, 150 (2) 366 - 376 July 2012.