Adipose Tissue & Brown Fat

Adipose Tissue


There are two types of fat tissue in the body: 


  1. White fat:  This is the familiar form of fat that accumulates under the skin and around organs. Its purpose is to store fuel for future use.  White fat cells (adipocytes) have a single vacuole which simply grows larger and larger as more fat is added, and have much fewer mitochondria than brown fat cells.  2,4

  2. The liver converts excess dietary carbs, fats and proteins into triglycerides, then packages them into VLDL lipoproteins for transport in the blood.  These VLDL particles arrive in the capillaries embedded in the adipose tissue. where they are taken up by the adipocytes.  There, an enzyme called lipoprotein lipase (LPL) facilitates this uptake by hydrolyzing the individual fatty acids from the triglycerides.

  3. Visceral fat, is white fat that accumulates around the waistline and belly, and has been blamed for the obesity epidemic.  It's accumulation is believed by many to be responsible for type-2 diabetes. 13

  4. Subcutaneous fat is white fat that accumulates under the skin, such as in the hips and buttocks.  It is in the news lately because there is some evidence it protects against diabetes.  That is, it improves cell sensitivity to insulin. 13

  5. Visceral and subcutaneous have different gene expressions, even tho they are both white fat.  Mercola reports:  "when researchers added more subcutaneous fat subcutaneously, there was no major difference in health outcome. But when they placed subcutaneous fat in with visceral fat, in the abdominal cavity, all of a sudden there were surprising health benefits, such as weight loss and improved metabolic function."13


  6. Brown fat: This fat is abundant in many newborn or hibernating mammals.  It is called “brown” because it is rich with blood capillaries and mitochondria, giving it a rusty or brown color. 2 Brown fat is related to muscle tissue. Researchers at Harvart University identified “a sort of master switch that promotes the production of brown fat. THis molecular switch, known as PRDM16, regulates whether immature cells will turn into brown fat or into muscle cells.” (from Mercola and Web MD)).


Adipose tissue performs complex metabolic and endocrine functions. Among the endocrine products produced by adipose tissue are:


  1. TNF-α (tumor necrosis factor alpha):  a cytokine involved in systemic inflammation, whose primary function is to regulate the immune system; 7,8

  2. IL-6 (interleukin 6):  a pro-inflammatory cytokine that stimulates immune response to trauma such as burns; 7,9

  3. acylation-stimulating protein:  a protein that stimulates triglyceride synthesis; 7,10

  4. estrogen, especially in breast tissue; 11 and

  5. leptin:  a hormone that plays a key role in regulating energy intake and energy expenditure, including the regulation of appetite and metabolism. 7,12



What is special about Brown Fat?


Brown fat’s purpose is to burn energy and generate heat in response to cold or excess calorie intake. 5  Brown fat is activated when the body is exposed to cold temperatures, boosting the metabolic rate to warm the body. Taking a daily cold shower might help one lose weight.

Unlike white fat cells, the brown fat cells contain several smaller vacuoles to contain the fat, and have a higher number of mitochondria than white fat adipocytes. 

Mitochondria are energy factory organelles within our cells, responsible for energy production, in the form of ATP.  It is believed they evolved from a single-cell organism (prokaryotes). They contain DNA material separate from that of the cell’s nucleus, and have both an outer and inner membrane.  These membranes are responsible for the electrochemical gradient that leads to ATP production, but can also be used to generate heat by recycling the protons generated by this gradient back into the mitochondria, without producing ATP, through the action of a protein called thermogenin. 2,3

What does this all have to do with obesity and weight gain?  When brown fat is present, excess dietary fuels can be burned to produce heat, rather than stored as white fat.  White fat is what accumulates under the skin, leading to obesity.



Brown Fat and Insulin Resistance


Some animals, like mice, have brown fat throughout their lives.  Others, like sheep and humans seem to lose most of their brown fat by adulthood; however, their precursors (preadipocytes) remain in the body lodged in white fat depots.


Researchers at Joslin Diabetes Center and Children's Hospital Boston developed cell lines of precursor cells that give rise to brown fat in mice.  Then, in 2005, they studied the effect of insulin on these preadipocytes to convert them to brown, rather than white, fat cells.  This led to their discovery of a group of genes that govern the genesis of brown fat.  Through genetic manipulation, they developed preadipocytes that could not form brown fat.  This led to several discoveries, including:

  1. Insulin-resistant cells lacking in IRS-1 (Insulin Receptor Substrate-1) failed to develop into mature brown fat cells; restoring the IRS-1 mostly restored the ability to form brown fat cells. 

  2. Elevated production of a protein called necdin inhibits the ability of preadipocytes to form brown fat cells.

  3. A transcription factor called CREB is essential for reducing necdin production. 4


This research is opening the doors for development of drugs to fight obesity.  But to me, this research is a very important factor in the accumulated knowledge about insulin resistance (IR), which is behind the set of disorders collectively termed Syndrome-X or Metabolic Syndrome. Rather than fighting obesity by suppression mechanisms (drugs), wouldn’t it be better to work on the underlying problem of insulin resistance?


Here are a few articles on brown fat that I have yet to investigate:


  1. www.pubmedcentral.nih.gov/articlerender.fcgi?artid=1136913

  2. www.jci.org/cgi/content/abstract/108/8/1205

  3. www.pubmedcentral.nih.gov/articlerender.fcgi?artid=1153011


and on other fat regulators like leptin:

  1. cat.inist.fr/?aModele=afficheN&cpsidt=13585923

  2. ezinearticles.com/?Set-Your-Fat-Thermostat-at-a-Healthy-Level&id=6942





Sources:

  1. 1.How to Lower Your Fat Thermostat, by Remington, Fisher and Parent, 1983

  2. 2.en.wikipedia.org/wiki/Brown_fat

  3. 3.en.wikipedia.org/wiki/Mitochondria

  4. 4.ScienceDaily: 'Brown Fat' Cells Hold Clues For Possible Obesity ...  (www.sciencedaily.com/releases/2005/06/050608060136.htm)

  5. 5.www.futurepundit.com/archives/002053.html

  6. 6.www.sciencedaily.com/releases/1997/12/971225183203.htm

  7. 7.www.ncbi.nlm.nih.gov/entrez/query.fcgi?cmd=Retrieve&db=PubMed&list_uids=10997652&dopt=Abstract

  8. 8.en.wikipedia.org/wiki/Tumor_necrosis_factor

  9. 9.en.wikipedia.org/wiki/Interleukin_6

  10. 10.www.ncbi.nlm.nih.gov/entrez/query.fcgi?cmd=Retrieve&db=PubMed&list_uids=8763517&dopt=Abstract

  11. 11.The importance of local synthesis of estrogen within the breast.

  12. 12.en.wikipedia.org/wiki/Leptin

  13. 13.http://articles.mercola.com/sites/articles/archive/2008/05/22/good-news-about-big-bottoms.aspx?source=nl



return to Health Essays Menu

  1. Adipose Tissue

  2. What is Special About Brown Fat?

  3. Brown Fat and Insulin Resistance

by Catherine M. Haug,  February, 2007