Brown Adipose Tissue
Brown adipose tissue is so-named because it appears brown or red and it has
this distinctive color because brown adipocytes (brown fat cells) are not big
clear balloons of fat like white adipocytes, but instead are a mixture of fat and
lots of cellular "organelles"(little organs) called mitochondria that are in turn full
of reddish iron. Furthermore, unlike white fat, brown fat is richly interwoven with
blood vessels and nerves. All these differences reflect the vastly different role
played by brown fat in the body. A microscopic photograph of brown fat is
shown below and looks nothing like the large clear "bubbles" of white fat cells
show here.
Brown Fat Burns Fats For Heat

Unlike white adipose tissue which stores fats for the body to use as energy
during starvation, brown fat stores fats in order to "burn" them up and
produce heat when we are exposed to low temperatures. This is especially
significant in newborns where up to four percent of an infant's body weight
is brown fat (whereas adult humans only have several grams of the tissue).
Because it produces heat directly from within its constituent cells, brown
fat can actually feel hot and this heat can be imaged as a thermal
"hot-spot" using an infra-red camera. Heat produced by brown fat allows
babies to warm themselves without shivering.

Brown Fat and Body Weight:

Recent studies have shown in inverse correlation between the amount of
brown fat in a person's body and that person's weight; the more brown fat,
the lighter a person is, the less brown fat, the heavier. This suggests that
brown fat may be one of the important factors controlling body weight.

Brown Fat in Human Adults

Most adult brown fat is located around the shoulders and neck as shown in
the photograph below. When totalled up, most adults have no more than
one to three ounces of brown fat.
Uncoupling Protein in Brown Fat:

Brown fat has the ability to create astonishing amounts of heat very
quickly and it does this through a protein called UCP-1 (uncoupling
protein). This protein works in much the same way as a very old and
long-banned weight loss drug called "dinitrophenol", by destroying the
"proton gradient" (the power supply) of the mitochondrion. Normally the
mitochondrion uses this gradient to make high energy molecules called ATP
that can fuel cellular metabolism, but with UCP-1, they simply convert all
the energy in fat into heat. It's a bit like the difference between using five
gallons of gasoline to drive your car 150 miles on the one hand or simply
lighting the gas on fire on the other hand. One way produces useful work
(and a small amount of heat) and the other way produces no work and lots
of heat.
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