What can brown [fat] do for you?

brown fat

Brown fat (pictured on the right) is able to freely burn off excess fat. A team at Harvard found one way to stimulate its production is through exercise.

It seems every month a radical new weight loss method is discovered. So talk of brown adipose tissue (BAT), or simply brown fat, revolutionizing the weight loss industry has to be met with a degree of skepticism. However, brown fat could be important to fighting the obesity epidemic. Brown fat is a specialized type of adipose tissue that catabolizes fat to produce heat. Most adipose tissue store fat to be released when sugar levels are too low. When the liver's glycogen stores become depleted, the body switches to utilizing its fat reserves. The adipocytes that take up, store and release fatty acids are what comprise white fat.    

Unlike white fat, brown fat takes up fatty acids and turns that fat into heat.  Thus, brown fat literally "burns calories". It accomplishes this by uncoupling the dephosphorylation of ATP to ADP. ATP produced by oxidative phosphorylation generates heat rather than powering cellular processes such as muscle fiber contraction. The brown color of brown fat stems from large amounts of mitochondria flecked throughout the adipocyte. Brown fat is interspersed with an abundance of blood vessels that dissipate heat to the rest of the body.  Brown fat has been known for many years. It exists in hibernating animals and mammalian infants as a thermoregulator. However, only in the last several years has it been known to exist in adult humans (Human Brown Adipose Tissue, 2010.  Sven Enerbäck.). The figure below shows where in the human adult and infant the brown fat is found.

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The predominating location of brown adipose tissue (BAT)  are, based on results from postmortem and PET/CT studies: (A) thyroid/tracheal, (B), mediastinal, (C) paracervi- cal/supraclavicular, (D) parathoracical, (E) supra- and perirenal. In the infant, also a kite-shaped thin layer of BAT has been described (in light brown). 

Brown fat has been shown to decrease weight in diet-induced obesity mice. The study put brown fat into obese mice and found that weight gain was severely minimized. Not only was weight lost, but insulin insensitivity was markedly reduced. Brown fat may be able to treat diabetes and obesity (Brown adipose tissue regulates glucose homeostasis and insulin sensitivity, 2013.  Kristin I. Stanford, et al.). The question then is how can we get more brown fat? Although the research goes back and forth, the latest research suggests that white adipose tissue (undesired fat) and brown adipose tissue are closely related in development.  In fact, scientists have been able to "brown" white fat both in vitro and in vivo with a hormone called irisin. As a hint to its importance, irisin is highly conserved: the sequence is 100% identical in mice and humans (by comparison, insulin is 85% identical in mice and humans).

Last year a team at Harvard found that exercise increased production of irisin in mice through the upregulation of the PGC1-alpha (A PGC1-alpha-dependent myokine that drives brown-fat-like development of white fat and thermogenesis, 2012. Pontus Bostrom, et al.). Humans also increase irisin production during exercise. Through the biochemical pathway is not completely understood, a three fold increase in irisin has notable effects on browning of adipose tissue.  Irisin may have undiscovered benefits to other tissues. It seems paradoxical that exercise, which expends energy itself, would generate brown fat. The authors speculate that muscle contraction to bring on shivering may be the evolutionary link between brown fat and exercise.

In summary, brown fat decreases obesity and increases glucose tolerance.  Whether doctors are able to harness brown fat to clinically treat obesity or diabetes remains to be seen. Next time you work out, take satisfaction in knowing that exercise continues to burn calories even after you finish.

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