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WEIGHT LOSS: WHY IT'S HARD

Fat cells: where the action is

They store and dispense energy, expanding and shrinking but never disappearing. Like it or not, they play a part in many bodily functions. A guide to their world.

June 02, 2008|Regina Nuzzo | Special to The Times
  • UP CLOSE: Human adipose tissue, where fat cells are interspersed with collagen fibers, blood vessels and immune system cells. Fat cells store energy as liquid.
UP CLOSE: Human adipose tissue, where fat cells are interspersed with collagen… (Visuals Unlimited/Corbis )

Far FROM being simply dumb, jumbo-size refrigerators of the cellular world, fat cells are now recognized by scientists as leading surprisingly active and influential lives.

They play a role in myriad bodily functions, research suggests, such as regulating hunger and fighting off infection. But under the wrong conditions, fat cells' natural propensities can backfire -- leading to increased risk for various modern lifestyle diseases, including heart disease, Type 2 diabetes and some cancers.

"Fat cells are surprisingly complicated," says Dr. David Heber, professor of medicine at the UCLA Geffen School of Medicine and director of the UCLA Center for Human Nutrition. "They're more than a bag of fat."

Here's a brief guide to this oft-misunderstood cell.

Name

Adipocyte, so-called for the Latin word for "greasy." This cell's main purpose in life is storing energy in the form of liquid fat. Its natural state is to assemble in giant conglomerates -- called adipose tissue -- where fat cells are interspersed with collagen fibers, blood vessels and immune system cells.

For The Record
Los Angeles Times Tuesday, June 10, 2008 Home Edition Main News Part A Page 2 National Desk 1 inches; 45 words Type of Material: Correction
Fat cells: A June 2 Health section article on fat cells said that brain cells are never replaced. Although the body doesn't create new cells in the brain's cortex and cerebellum, it does continue to create cells, or neurons, in other parts of the brain.
For The Record
Los Angeles Times Monday, June 16, 2008 Home Edition Health Part F Page 9 Features Desk 1 inches; 46 words Type of Material: Correction
Fat cells: A June 2 Health section article on fat cells erroneously reported that brain cells never get replaced. Although the body doesn't create new cells in the brain's cortex and cerebellum, it does continue to create cells, or neurons, in other parts of the brain.

Habitat

Everywhere in the body except the brain and spinal cord.

Most famously, fat cells throng in the abdomen, buttocks, breasts, thighs and upper arms. But they also enrobe internal organs, cushion the eyeballs, lubricate the lungs and serve as shock-absorbing pads on fingertips and toes. The personality of a fat cell -- whether it increases appetite, for instance, or controls blood sugar responses -- depends a great deal on where it resides. Each area of the body shapes developing fat cells in its neighborhood by exposing them to different levels of hormones, nutrients and oxygen, thus influencing how the mature fat cells will store energy and interact with other cells.

Fat cells deep inside the abdomen (called "visceral fat"), for example, particularly seem to help support the immune system -- but are also associated with an increased risk of heart disease and diabetes.

And research suggests that fat cells located just beneath the skin (called "subcutaneous fat"), especially on hips and thighs, might be especially good for a body. Harvard Medical School researchers working with mice recently found that these relocated "good" fat cells can still exert a positive influence in new environments. The results, published in May, were striking: Mice who got extra subcutaneous fat transplanted into their bellies -- harvested from the haunches of other mice -- lost weight. Their fat cells shrank and their blood sugar functioning improved -- all without the mice changing their eating and exercise habits.

Population size

Billions and billions. More fat cells exist in one adult body than there are humans currently living on Earth.

Newborns come with a starter set of about 5 billion fat cells. This population increases steadily throughout childhood and levels off shortly after high school. Actual numbers can vary widely even between two people of the same weight, although scientists aren't sure why. Skinny adults typically have about 40 billion fat cells; obese people can easily pack in twice as many.

Life span

Middle-of-the-road.

Typically living about 10 years, fat cells are not as durable as brain cells, which never get replaced, nor as ephemeral as cells lining the intestine, each of which lasts only about five days.

In fact, about 150 fat cells wink out of existence every second of the day in a typical body, says Kirsty Spalding, a cell and molecular biology professor at the Karolinska Institute in Sweden and lead author of a recent study on the turnover of fat cells. That's more than 4 billion fat cells -- about 10% of a body's total population -- lost every year.

But each vanished fat cell appears to be diligently replaced, one by one. A person's fat-cell allowance seems to be set by about age 20, and even when obese people lose a lot of weight, they don't seem to lose their total number of fat cells. The cells simply shrink in size -- waiting for more excess energy to enter the bloodstream.

Appearance

Bloated and huge -- relatively speaking.

The bulk of a fat cell is given over to storing energy in a compact, liquid, ready-to-use molecular form (a fatty substance known as triglyceride). Compared with other kinds of cells, which can comfortably house their inner cellular machinery with room to spare, fat cells are unmistakably crowded. "Fat cells look a bit like signet rings with a thin circle of working cell matter surrounding a big glob of fat," Heber says. Under normal circumstances, they're also usually bigger than most human cells -- about seven times the diameter of a skin cell, for instance. But fat cells also have an amazing ability to expand and cram in to store even more liquid energy. In some people, each one can end up swelling to a diameter of 300 microns -- almost big enough to see with the naked eye. Five of these monster cells clumped together would be bigger than the period at the end of this sentence.

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