The role of fat in energy metabolism and its related disorders is
reviewed in non-technical terms.

New York Times
  August 7, 2007

  Basics

           Its Poor Reputation Aside, Our Fat Is Doing Us a Favor
                                     
  By NATALIE ANGIER

  In this country, the most popular cosmetic surgery procedure is
  liposuction: doctors vacuum out something like two million pounds of
  fat from the thighs, bellies, buttocks, jowls and man-breasts of
  325,000 people a year. What happens to all that extracted adipose
  tissue? Its bagged and disposed of as medical waste; or maybe, given
  the recent news about socially contagious fat, its sent by FedEx to
  the patients old college chums. But one thing the fat surely is not,
  and that is given due thanks for serving as scapegoat, and for a job
  well done.

  We are now in what feels like the 347th year of the fastidiously
  vilified obesity epidemic. Health officials repeatedly warn that
  everywhere in the world people are gaining too much weight and
putting
  themselves at risk of diabetes, high blood pressure, heart disease
and
  other obesity-linked illnesses, not to mention taking up more than
  their fair share of molded plastic subway seat.

  Its easy to fear and despise our body fat and to see it as an
  unnatural, inert, pointless counterpoint to all things phat and
  fabulous. Yet fat tissue is not the problem here, and to castigate
fat
  for getting too big and to blame it for high blood pressure or a
  wheezing heart is like a heavy drinker blaming the liver for turning
  cirrhotic. Just as the lushs liver was merely doing its hepatic best
  to detoxify the large quantities of liquor in which it was doused,
and
  just as the alcoholic would have been far worse off had the liver not
  been playing Hepa-filter in the first place, so our fat tissue, by
  efficiently absorbing the excess packets of energy we put in our
  mouths, has our best interests at heart.

  Obesity is not due to any defect in adipose tissue per se; its an
  issue of energy balance, said Bruce M. Spiegelman of the Dana-Farber
  Cancer Institute in Boston. If you are consuming too many calories
  relative to what you burn off, the body must cope with that energy
  surplus, he said, and adipose tissue is the proper place for it.

  If you had no fat cells, no adipose tissue, youd still be out of
  energy balance, and youd put the excess energy somewhere else, he
  said, at which point really bad things can happen. Consider the
  lipodystrophy diseases, rare metabolic disorders in which the body
  lacks fat tissue and instead dumps its energy overruns in that
  jack-of-all-organs, the liver, causing extreme liver swelling, liver
  failure and sometimes liver-bearer death.

  Some adipose tissue is a good thing, said Barbara Kahn, chief of the
  endocrinology, diabetes and metabolism division at Beth Israel
  Deaconess Medical Center, at Harvard.

  Indeed, evolutionary biologists have proposed that our relative
  plumpness compared with our closest nonhuman kin, the chimpanzee, may
  help explain our relative braininess. Even a lean male athlete with a
  body fat content of 8 percent to 10 percent of total body mass (half
  the fat found on the average nonobese, non-Olympic American man) is
  still a few percentage points more marbled than a wild male
  chimpanzee, and scientists have suggested that our distinctive
adipose
  stores help ensure that our big brains will be fed even when our
  cupboards go bare.

  Scientists who study fat emphasize that its bland and amorphous
  appearance notwithstanding, our adipose depots represent highly
  specialized organs, as finely honed to the task of energy storage as
  muscle is built for flexing. Our body fat is made of some 40 billion
  fat cells, or adipocytes, and their supportive matrix, with most of
  the bulk stashed under the skin but also threaded viscerally, around
  and between other organs. Each fat cell is essentially a bouncing
  balloon filled with those greasy lipids we call triglycerides, three
  fatty acid chains of mostly carbons and hydrogens arrayed in
  high-energy configurations that explain why, gram for gram, dietary
  fat has more than twice the calories of meat or starch; and every
  fatty acid trio is tacked to a sugar-sweet glycerol frame.

  In most body cells, the watery cytoplasm where the labor of proteins
  takes place accounts for maybe 70 percent of the cells volume, with
  another 10 percent given over to the nucleus, seat of the cells DNA.
  In a fat cell, by contrast, lipids are king, queen and bishop, and
the
  checkerboard, too. They fill more than 95 percent of the adipocyte
  volume, crowding the cytoplasm with its proteins and the nucleus with
  its genes up against the cell wall in what Dr. Kahn calls a crescent
  moon space.

  Yet for all its lipid density, the average fat cell is ever primed to
  hoard more, to take in more fatty acids and sugars from the blood and
  stitch them into triglyceride stores, and to swell to several times
  its cellular waistline of yore. Most weight that we gain and lose in
  life is the result of our existing fat cells growing and shrinking,
  absorbing and releasing energy-rich lipids as needed, depending on
our
  diet and exercise regimens of the moment. But when exposed to chronic
  caloric overload, fat cells will initiate cell division to augment
the
  supply; and because fat cells, like muscle cells, rarely turn over
and
  die, those new lipidinous recruits will be your helpmeets for life.

  Fat is no rutabaga. It is dynamic and mercantile, exchanging chemical
  signals with the brain, bones, gonads and immune system, and with
  every energy manager on the bodys long alimentary train.

  We used to think of an adipose cell as an inert storage depot, Dr.
  Kahn said. Now we appreciate that it is an endocrine organ, in other
  words, an organ that like the thyroid or pancreas, secretes hormones
  to shape the behavior of other tissues far and wide. Squashed to the
  side a fat cells cytoplasm may be, but it nevertheless spins out a
  steady supply of at least 20 different hormones. Key among them is
  leptin, an essential player in reproduction. Scientists suspect that
a
  girl enters puberty when her fat stores become sufficiently dense to
  begin releasing leptin, which signals the brain to set the pulsing
  axis of gonadal hormones in motion.

  Fat also seems to know when it is getting out of hand, and it may
  resist new personal growth. Dr. Spiegelman and others have shown that
  with the onset of obesity defined as 25 or more pounds above ones
  ideal weight the fat tissue starts releasing potent inflammatory
  hormones. That response is complex and harmful in the long run. But
in
  the short term, said Dr. Spiegelman, inflammation clearly has an
  anti-obesity effect, and it may be the bodys attempt to restrain
  further accumulation of adipose tissue. The fat sizes up the risks
and
  benefits, and it takes its fat chance.