ABOUT BROWN FAT

"Brown adipose tissue: the body's own defense in the fight against obesity, diabetes and cardiovascular disease?"

  • Brown adipose tissue (BAT; also known as brown fat) converts energy stored in fat into heat.
  • It was only recently discovered that brown adipose tissue is present and active in humans adults, and its activity is lower in obese individuals.
  • There are various methods to activate brown adipose tissue, including exposure to cold.
  • Stimulating brown adipose tissue could potentially be used to lose weight and combat related cardiometabolic disorders.

White and brown adipose tissue

W hile the main function of white adipose tissue (WAT) is to store energy in the form of lipids (known as triglycerides), brown adipose tissue (BAT) combusts these lipids into heat, a process referred to as ‘adaptive thermogenesis’. Brown fat depots are strategically localized in the scapular area near the large arteries (Figure 1), where heat production appears to be essential for the survival of small mammals in cold environments and for arousal of hibernators. In addition to its crucial role in nonshivering thermogenesis, BAT is probably also required for maintaining energy balance and is activated upon overeating, a process called ‘diet-induced thermogenesis’. However,the existence of such a process is a matter for debate.
Figure 1. Visualization of brown adipose tissue in  a cold-exposed human volunteer using 18F-FDG PET-CT scanning
Visualization of brown adipose tissue  using PET-CT scanning
Figure 2. Morphology of white adipocytes (left) and brown adipocytes (right).
Morphology of white adipocytes and brown adipocytes.

Physiology of brown adipose tissue

BAT is characterized by multilocular intracellular lipid droplets, numerous mitochondria, and regulated expression and activity of uncoupling protein1 (UCP1) (Figure 2). Clusters of UCP1-expressing adipocytes with thermogenic capacity also develop in WAT in response to various stimuli, including cold exposure and beta-adrenergic agonists. These adipocytes have been named beige, brite, or recruitable brown adipocytes and have characteristics that distinguish them from classical brown adipocytes. Controversy exists around the nature of these beige adipocytes and whether, in adult humans, BAT represents mainly brown or beige adipocytes.

Detailed description of heat production by brown adipocytes

The thermogenic activity of brown fat is primarily driven by the sympathetic nervous system, reflected by a high density of nerve endings in the tissue (Figure 3). Norepinephrine-induced adrenergic receptor signaling enhances expression of proteins involved in thermogenesis and stimulates intracellular lipolysis. Liberated fatty acids are directed to the mitochondria to be combusted, or may allosterically activate UCP1. This results in the uncoupling of complexes I–IV of the respiratory chain and ATP synthesis by proton leakage from the mitochondrial innermembrane space into the mitochondrial matrix, thereby generating heat instead of ATP. Longterm sympathetic outflow
towards the fat depots additionally leads to an increased mass of BAT depots and so-called ‘browning’ of WAT. Since activated BAT has a high capacity to take up and burn lipids and glucose, BAT is considered a promising target to combat obesity and associated diseases, including diabetes and cardiovascular disease.
Figure 3. Histological image of brown adipose tissue. The purple round structures are nuclei, white round structures indicate the spots of the intracellular lipid droplets and the brown (tyrosine hydroxylase) staining represents sympathetic nerve endings in close proximity to the brown adipocytes. 
Histological image of brown adipose tissue
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