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.
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.
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.
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.
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.
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.
Yet there's only so much one fat cell can do to supply a body's energy needs. Each fat cell harbors a typical capacity of only about 500 micro-calories, which means even a lean body still needs about 4 million fat cells to store the equivalent of a Tic Tac.
Storing and releasing fat droplets, over and over again.
Fat cells stay busy by taking in free-floating fatty acids from the bloodstream -- remnants of fat from the previous meal -- and building them into a tidy fat globule for storage. (If a meal consists of more carbohydrates than a body can immediately burn off for energy, the liver converts the leftovers to little fat packages, which the fat cells then grab and happily add to their collection.)
That fat globule is a dense bundle of energy, packing in more than twice the calories of carbohydrates or protein (mostly because fat is greasy, with no bulky water molecules to weigh things down). This makes fat storage a great energy solution for organisms that need to protect against starvation and still be lightweight enough to get around on two feet.
When the body needs extra energy -- during a heavy bout of exercise, for example -- fat cells will break down the fat globule back into small, free-floating fatty acid molecules and release them into the bloodstream. In times of famine, they will empty their cupboards, quickly at first and then in a more miserly fashion.
Although exact numbers vary, fat cells in a typical well-fed body can supply a person with enough energy to live for a couple of months without any food at all.
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When fat cells talk, the body listens
Scientists previously thought that fat cells were relatively passive and inert. Now they have evidence that fat cells are metabolically active, continuously communicating with the brain and other organs through at least 25 hormones and other signaling chemicals.
For example, fat cells seem to release hormones that inform the brain how much energy is left and when to stop (or start) eating, guide muscles in deciding when to burn fat and tell the liver when to replenish its fat stores.
All this cross talk can be a mixed blessing in the body, however. A healthy population of fat cells, for example, helps the immune system fight off infection by releasing chemicals that cause mild inflammation. But an overactive group of fat cells might keep the inflammation permanently in the "on" position, eventually leading to heart disease.
Sometimes the whole communication system takes on a life of its own. Researchers at the University of Western Ontario working with mice found that fat cells in the belly are particularly adept at producing a hormone that not only stimulates appetite but also spurs nearby immature cells to become full-fledged fat cells.
The results, published in April, suggest a vicious cycle: Fat cells in the belly cause a person to be hungry and eat more, thus producing more fat cells -- leading to even more hunger and even more fat cells.
-- Regina Nuzzo