Visceral and subcutaneous fat are like "two separate organs, each with its own function," Hamdy says. Underscoring the difference are the disappointing results of efforts to improve patients' metabolic states through liposuction. When surgeons took out large amounts of abdominal fat—in one case 20 pounds of it—the patients experienced no improvement in insulin sensitivity. The type of fat that liposuction sucks out happens to be subcutaneous and hence benign; in some cases, the liposuction actually increased the amount of visceral fat, elevating the patients' risk. "It looks as if there's a messenger," Hamdy says. "The two types of fat have a language. Each senses the other."
Unless you have a CT scan or MRI, you cannot know for sure how much visceral fat you have, because it is hidden—but you can get a pretty good idea by measuring your waist. To assess the reliability of this extremely simple diagnostic tool, a committee that included Després supervised an International Day for the Evaluation of Abdominal Obesity in 2005, during which 6,400 primary care physicians in 63 countries gathered the waist measurements and health statistics of 180,000 patients. (Each physician had been sent an instructional video on how measuring should be done: Use bony landmarks, not the navel, because the waist lies midway between the bottom of the lowest rib and the top of the hip bone.) The results, according to Després, were "overwhelmingly clear": The correlation was 97 percent. "The greater the waist circumference, the greater the prevalence of diabetes and heart disease."
Physicians generally recommend that women have waists less than 35 inches around, men less than 40 inches. With the rapid rise in obesity in the United States, the average American woman's waist grew 1.3 inches in the six years between 1994 and 2000, the average man's 1.1 inches. Sumo wrestlers are a colorful and instructive exception. Big, big eaters and artificially obese, they look like prime candidates for heart disease and diabetes and would definitely fail the belt test. Nevertheless, Hamdy reports, they are "extremely insulin sensitive and don't have hypertension." Body scans reveal that sumo wrestlers typically have little visceral fat, presumably because they exercise six to eight hours a day. Most of what hangs over their mawashis is subcutaneous fat. When they retire, however, if they keep eating, their visceral fat balloons.
So what is it about abundant fat deep inside the belly that inclines a person to diabetes, heart disease, stroke, and other ills? Why should having a beer belly be so much worse than having humongous hips, thunder thighs, a well-padded bottom, loglike arms, or an opera-singer bosom? Until recently, adipose tissue was considered passive and inert, simply a place to store energy. Anatomy books often didn't even show fat because it obscured the view of the structures underneath. That changed in 1994 with the discovery of leptin, an appetite-inhibiting hormone, and with the revelation that it is secreted by fat. The following year Philipp Scherer and others discovered adiponectin, a hormone that is protective against diabetes, also secreted by fat.
Since then investigators have found dozens of biologically active substances that are released by fat cells or by cells residing in fat. The list includes immunomodulators, coagulation factors, hormones and prohormones, inflammatory and proinflammatory markers, enzymes, and lipids. Together these substances "have a profound effect on the whole system," Scherer says. Fat is now considered to be an active, complex endocrine organ, like the pancreas.
Of the fat depots, visceral fat is the most active—"very lively," Després says—secreting and mobilizing substances in the greatest quantity. It releases a lot of fatty acids (breakdown products of fat) into the bloodstream through the portal vein and liver, a phenomenon that until recently had been thought of as the major reason why fat promotes insulin resistance. Lately, though, the focus has shifted to fat's link with inflammation.
"When people get older or more obese," Hamdy says, "some of the visceral fat cells mature and become large, lazy, and dysfunctional." As people continue to eat and the fat pads expand, some cells get too full and rupture. Immune cells called macrophages, the janitors of the body, invade the site to clean up. In the process they induce inflammation; the cells also secrete other inflammatory chemicals, like interleukin-6 and tumor necrosis factor-alpha, which are known to adhere to the endothelium of the blood vessels, an early event in atherosclerosis.
"The inflammation isn't what you'd measure in a patient with bacterial infection, which would be 1,000 times higher," Scherer says. "It's subclinical, just a bit higher than background, little needle pricks rather than a sledgehammer blow. But over many, many years of chronic exposure, it can have a negative impact on cardiovascular health and insulin sensitivity."
