Decoding the Surprisingly Active Life Of Fat Cells

"There's a growing realization about the convergence of the medical problems associated with obesity and chronic inflammation, which wasn't appreciated for a long time but is coming to the fore scientifically," Lazar said. "Where does the inflammation come from? Surprise: It's the fat cells themselves."

But not from the fat cells alone. Scientists recently discovered that fat tissue is comprised of far more than just fat cells -- it is a complex amalgamation that includes key immune system cells called macrophages. Macrophages and fat cells produce powerful substances called tumor necrosis factor-alpha and interleukin-6, which help regulate the immune system.

Fat probably evolved a close connection to immune function because the body needs energy when it is fending off threats, scientists say.

"It's like if you are sending troops into battle," Hotamisligil said. "You have to send not just rifles but bullets."

But a surplus of fat cells and macrophages probably triggers unnecessary inflammation, which most likely explains at least part of why obesity increases the risk for so many diseases, including cancer, heart disease and diabetes.

"As fat mass increases, this is associated with a systematic stress response and inflammatory response, and that exhibits itself in a variety of diseases," Hotamisligil said.

Fat cells also send out signals that cause blood vessels to constrict, raising blood pressure, and make blood clots form, which may explain how obesity increases the risk for heart attack and stroke. At the same time, fat cells emit signals that promote blood vessel and cell growth, which could help explain why obesity increases the risk of cancer.

The more scientists learn about fat, the more intimidating it becomes. Because fat is so vital to survival, nature has created a complex system of overlapping feedback loops that make it very difficult to override the body's imperative to store energy.

"What we're trying to do is do like the physicists do: Build a theory of everything about fat tissue," Leibel said. "We'd really like to understand not only what the signals are but how they are integrated. There's so much that we simply do not yet understand about this."