THE MAN ON THE OPERATING TABLE is UNDER IV sedation, facedown, circles drawn around the flabbiest parts of his body. He's painted with iodine and has two small holes punched through the soft skin of his lower back. Board-certified New York City plastic surgeon David Shafer, M.D., aligns the tip of a foot-long suction wand, called a cannula, with one of the holes. He carefully places it beneath the skin. It makes a slurping sound like a straw sucking at the bottom of a milkshake. For the next hour, Dr. Shafer will be hoovering adipose fat cells from the man's back, flanks, and stomach.

This is where men tend to collect the most fat, explains Dr. Shafer—above the waist and over the ab muscles. He reaches down to grab a handful of the loose flab on the man's torso. "This is what we call subcutaneous fat," he says. It's the soft stuff you feel when you pinch your own belly. Then Dr. Shafer sweeps his hand across the patient's midsection, the soft cavity containing the intestines, kidneys, and liver. "And see how the gut swells outward? That's the visceral fat pushing from beneath the rectus muscles." That's the dangerous stuff.

When researchers in St. Louis tracked a group of liposuction patients after surgery, they found zero improvement in blood pressure, triglycerides, glucose tolerance, or HDL or LDL cholesterol profiles. The pale, gloppy fat that a patient pays to have removed isn't pretty, but its absence doesn't guarantee the health profile of a lean, fit person. That's because visceral fat, the kind that wraps around your organs and makes you unhealthy, is also the kind that liposuction can't reach. "There's no safe way to suck around the heart, kidney, and liver," says Dr. Shafer. "It's just too dangerous."

The more researchers learn about body fat, the more they've come to view it as a multi-faceted substance. In a sense, it's not unlike the fat in food. The artery-clogging trans fat in partially hydrogenated margarine isn't the same as the heart-healthy monounsaturated fat in olive oil, right? Well, neither is the fat around your quads the same as the fat around your liver. In terms of its impact on your health, the amount of fat your body carries is less important than where your body stores it.

"It's still not completely understood, but fat behaves very differently on different parts of the body," says Dr. Shafer. The better your grasp of this concept, the better you'll understand the need to target your body's most dangerous fat. (Get the latest nutrition tips to lose your gut without starving. Sign up for the .)

HIDDEN FAT FACT 1

Body fat saved your ancestors

Say you're a hungry caveman plodding along the tundra and you spot a 6-ton woolly mammoth wandering on the horizon. You want an energy source that kicks into action immediately so you can chase and kill the beast. That's very likely the reason men store more upper-body fat than women do, says Fredrik Karpe, M.D., Ph.D., a professor of metabolic medicine at the University of Oxford. Men were the hunters. "Upper-body fat, including visceral fat, is a kind of fight-and-flight depot that both stores and releases energy very easily," he says.

It does this through a process called lipolysis, which breaks clumps of fat into fatty acids that your muscles can use as energy. In visceral fat, lipolysis occurs at an unusually high rate. It's an ongoing process of deconstruction and reconstruction that keeps your bloodstream flooded with fat. This high concentration of fat compounds can bog down your liver and jack up LDL cholesterol and triglyceride levels.

The big problem today is that our bodies still hold on to visceral fat even though we're no longer starving cavemen. "Those fat depots are no longer useful," says Dr. Karpe. "It comes at a price to have such easily mobilized fat." But regular exercise can help neutralize those cardiovascular risks, says Dr. Karpe. When you put your muscles to work, they release enzymes that pluck circulating triglycerides from the blood and burn them off as fuel, which can help clear danger from your arteries.

HIDDEN FAT FACT 2

Body fat below the waist is not as dangerous

Visceral fat is a threat for another reason: It's highly susceptible to inflammation. "As the amount of stored fat increases, it triggers a cellular response designed to recruit immune cells," says Michael Schwartz, M.D., director of the diabetes and obesity center of excellence at the University of Washington. This leads to inflammation and can result in insulin resistance and a host of diseases associated with metabolic syndrome.

Fat below the waist behaves differently than visceral fat. "From an evolutionary standpoint, we believe that lower-body fat is intended as long-term storage. It's packed away, so it doesn't harm the rest of the body, and we use it as a last reserve," says Dr. Karpe. According to a 2010 review conducted by Dr. Karpe, below-the-belt fat produces fewer inflammatory compounds, which means less cardiovascular damage. This gives women a health advantage because they tend to store more fat in their lower bodies than men do. The fat women tend to carry on their hips? "That's one of the reasons we think women are more resistant to heart disease," Dr. Karpe says. For cutting-edge tips to lower your heart disease risk, follow these 5 New Tips to Help Your Heart.

HIDDEN FAT FACT 3

Body fat is far more than a calorie storage tank

Five or 10 years ago, researchers and physicians viewed fat merely as a storage system for energy—a soft balloon filled with calories. But they've since come to recognize it as an instrument that plays a critical role in your body's metabolic function. "Fat is the largest endocrine organ in the body," says David Piston, Ph.D., a professor of molecular physiology and biophysics at Vanderbilt University.

Even a 160-pound man with 13 percent body fat (that's a lean guy) has more than 20 pounds of fat. And that fat—or more specifically, the adipose cells that store fatty triglycerides and keep them out of the blood—is extremely important to his body's hormone regulation.

Consider leptin. This hormone is produced inside fat tissue, and without it you could theoretically eat until your stomach burst. Leptin regulates how responsive your body is to the "I'm full" signals coming from your stomach. The more fat cells you have, the more leptin you have circulating in your blood, so you'll feel full on less food. But while this important signal registers well in lean people, it seems to be ineffective in overweight people.

And that's just one of about 300 compounds coming from fat, says Dr. Karpe. Alas, not all of them are as benign as leptin. "When tissue is inflamed and overfilled with fat, it can pump out a lot of nasty stuff," he says. That "stuff" can hijack your appetite, reprogram your fat-storage mechanisms, contribute to conditions like arthritis, and drive your triglyceride levels to deadly heights.

The best way to cut inflammation? Yep, pack some physical activity into each day. Researchers at Appalachian State University recently determined that highly fit people who reported frequently engaging in moderate exercise such as cycling, swimming, or jogging had nearly 50 percent less C-reactive protein, a marker for inflammation, in their blood than people who were unfit and rarely exercised.



HIDDEN FAT FACT 4

Visceral fat undermines your manhood

There's a concrete connection between testosterone and visceral fat, and it works in two ways, says Farid Saad, Ph.D., head of scientific affairs for Men's Healthcare at Bayer Pharma in Berlin, Germany.

First, inadequate testosterone levels direct muscle cells to turn into (or "differentiate" into) fat, and second, visceral fat produces substances that suppress testosterone production. So as the visceral bulge grows in your belly, testosterone drops, and your body is less likely to grow muscle. If the spiral goes unchecked, you can end up overweight with no motivation to change. "Men with testosterone deficiency are also quite low on energy," says Saad. "You can tell them a thousand times to exercise, and they won't do it."

According to Saad, short-term testosterone supplements may be a viable solution. A 2012 study published in the Journal of the American Medical Association, for instance, found that men who received 20 weeks of testosterone supplementation gained fat-free, lean mass. "For men with deficiencies, 1 or 2 years of supplementation might completely break the cycle," says Saad. (Don't let this happen to you! Protect your sperm and hormones—that's What More Testosterone Can Do For You.)

HIDDEN FAT FACT 5

Skinny people are not immune

Low body fat is a pretty good indicator of health, but a dangerous clump of fat can still hide behind a flat belly. In a study published in Nature Genetics last year, researchers discovered a gene that causes those with it to carry less body fat than those without it. Surprisingly, though, people with that gene (especially men) had a higher ratio of visceral fat to subcutaneous fat. They also had higher triglycerides and lower HDL cholesterol—a risky combo that can contribute to heart disease.

What's more, it might take no more than a couple of ounces of some types of body fat to threaten your life. Researchers at the University of Cincinnati have recently begun looking into a type of fat called perivascular fat, which clumps around the arteries leading into your heart. "For so long the dogma has been that all of the disease was coming from within the artery and traveling outward," says David Manka, Ph.D., lead researcher. "What we're showing is that the fat growing around these arteries is causing the disease on the inside. This perivascular fat tends to be loaded with inflammatory cells in a way that even visceral fat isn't."

You can't tell how much perivascular fat a man has by looking at him, so it's not easy to diagnose. And even though it may seem related to overall body fat, Manka's collaborators found plenty of perivascular fat on otherwise lean organ donors when they harvested samples. His team recently received government funding for further research into this heart flab, but in the meantime, eating smart and exercising are always good ideas even if you're at your ideal weight. "Perivascular fat seems very sensitive to changes in the nutritional state," he says. "Keep an eye on your overall fitness level. That's going to have a big impact."

Clint Carter Clint Carter is a reporter and editor with a magazine journalism degree from the University of Missouri.

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