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 totaled 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. |