Once dismissed as a feature only babies possess, brown adipose tissue (BAT), is widely recognized as the body’s metabolism-boosting, heat-generating fat. This Time Machine episode takes a look back at early research and new discoveries about this metabolism-boosting fat.

According to a 2007 paper published in the American Journal of Physiology – Endocrinology and Metabolism, it was the ability for BAT to take up glucose that enabled its discovery in PET scans, which relied on a low dose of radioactive-glucose to capture images of organs. These scans revealed that adult humans carry small but powerful BAT depots tucked in surprising spots: in the neck, near the collarbone, alongside the aorta, above the kidneys, and around the spine as seen the images of a female patient above.

Like other mammals, human BAT is activated by the sympathetic nervous system in cold environments. Activation of BAT turns on a process called non-shivering thermogenesis in which metabolism generates heat instead of creating it from shivering. This is because BAT contains a lot of mitochondria that are normally responsible for producing energy. But when this energy-making process is disrupted in BAT, the mitochondria switch to making heat instead, which helps animals tolerate cold conditions.

The ability for BAT to soak up glucose was a welcome sign for physiologists and physicians as it meant that BAT is metabolically active and may be involved in blood sugar regulation. Increased attention on just how BAT works revealed that BAT is indeed more than a heater and can be activated by more than just cold stimuli. The BAT that surrounds blood vessels, for example, can help regulate blood pressure. Because BAT can release a variety of signaling molecules, it is now also thought to be a type of endocrine organ (Chang et al., 2017; Gavalda-Navarro et al., 2022). In fact, the molecules released from BAT can target the liver, heart, and skeletal muscles to maintain a healthy metabolism (Gavalda-Navarro et al., 2022).

Interestingly, hibernating animals such as the thirteen-lined ground squirrel (above) build up white adipose tissue as well as BAT before hibernation. Although they rely on white adipose tissue as a source of energy during hibernation, their BAT starts burning stored lipids for fuel which generates heat and causes the animals to arouse periodically. Since hibernating animals cannot shiver, BAT becomes an important tool for the animals to generate body heat. Such interbout arousal events interrupt cycles of really low metabolism, known as torpor, every few days to weeks in hibernating animals (Ballinger and Andrews, 2018).

Sources:

J Nedergaard, T Bengtsson, B Cannon. Unexpected evidence for active brown adipose tissue in adult humans. Am J Physiol Endocrinol Metab. 293: E444-E452, 2007. Doi: 10.1152/ajpendo.00691.2006

Ballinger MA, Andrews MT. Nature’s fat-burning machine: brown adipose tissue in a hibernating mammal. J Exp Biol. 221(Suppl 1): jeb162586, 2018. doi: 10.1242/jeb.162586

Chang L, Garcia-Barrio, Chen YE. Brown adipose tissue, not just a heater. Arterioscler Thromb Vasc Biol. 37(3): 389-391, 2017. doi: 10.1161/ATVBAHA.116.308909

Gavalda-Navarro A, Villarroya J, Cereijo R, Giralt M, Villarroya F. The endocrine role of brown adipose tissue: An update on actors and actions. Red Endocr Metab Disord. 23(1): 31-41, 2022. doi: 10.1007/s11154-021-09640-6

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Categories: Climate Change, Environment, Extreme Animals, Nature's Solutions, Time Machine

Tags: American Journal of Physiology, American Physiological Society, brown adipose tissue, health, hibernation, metabolism, torpor