Experts used to believe that the sheer mass of carrying around extra weight is what caused much of the damage from being overweight. Now scientists have learned that there is some mighty complex biochemistry going on in fat cells. All fat-storage cells churn out a stew of hormones and other chemical messengers that fine-tune the body's energy balance. Roughly 25 different signaling compounds--with names like resistin and adiponectin--are now known to be made by fat cells and many of these are highly destructive to optimal health.
Fat tissue is now recognized to be the body's biggest endocrine organ, and its sheer volume is impressive even in normal-size people. A trim woman is typically 30 percent fat, a man 15 percent. That is enough fuel to keep someone alive without eating for three months. One of the clearest hazards of overfilled fat cells is their influence on the body's production and use of insulin, the hormone that instructs the muscle to burn energy and the fat cells to store it. Oversize fat cells blunt the insulin message, in part by leaking fat into the bloodstream. So the liver must compensate by making more insulin and other proteins.
Yahoo! News May 11, 2004