There's more to fat than meets the eye. Literally. While most of the population obsesses over that which wiggles and jiggles, research suggests it's the fat we can't see that's of greater concern. And it's not just about how much fat you have, but where you tend to store it that worries most doctors.
There are two types of fat: subcutaneous and visceral. Subcutaneous fat is located beneath the skin in places like the abdomen, thighs, hips, and buttocks. You know it, you see it, you hate it. Visceral fat, better known as belly fat, is located deep within the midsection, surrounding the liver, heart, lungs, and digestive tract. And it's invisible to the naked eye. "People are self-conscious about the fat they can see," says Heather Hausenblas, associate professor of exercise and health psychology at the University of Florida's College of Health and Human Performance, but "hidden fat, in people of any size, poses the bigger threat." Why? Visceral fat churns out inflammatory substances called cytokines that can wreak havoc on the body's organs.
Subcutaneous fat—that roll of fat you can pinch between your fingers—patiently sits beneath the outermost layer of skin, and while unsightly, it's not as dangerous as visceral fat. A 2004 study published in the New England Journal of Medicine showed that the removal of subcutaneous fat through liposuction —nearly 23 pounds of it—in obese women had no effect on their blood sugar, blood pressure, or cholesterol levels after three months. Visceral fat, on the other hand, is very active metabolically. It constantly releases substances that travel to the liver and influence the production of blood fats. "[It] supplies a feeding tube to your vital internal organs, messing up the blood that is sent to those organs," says Hausenblas. That's why the subcutaneous fat on your thighs, she explains, doesn't matter as much to your health as the visceral fat in your belly.
Visceral fat makes the body more vulnerable to type 2 diabetes, heart disease, non-alcoholic fatty liver disease, high blood pressure, Alzheimer's disease, and even certain types of cancer. A recent laboratory study, for example, suggests that visceral fat may promote the spread and growth of ovarian cancer. Says Ernst Lengyel, a professor of obstetrics and gynecology at the University of Chicago who led the research: "Cancer cells can feed from visceral fat and," he adds, "there isn't necessarily a connection to obesity because lean women also get ovarian cancer." Other cancers such as breast, gastric, and colon, research shows, may also be fueled by visceral fat.
So who accumulates visceral fat? "Everyone," says John Morton, associate professor of surgery and director of bariatric surgery at the Stanford University School of Medicine. Men have more visceral fat than women, although after menopause, women tend to gain more visceral fat than subcutaneous fat. Genetics can also play a role. "Some ethnic minorities like Hispanics and Native Americans are more prone to collecting visceral fat," says Morton. If we all have it, how do you know if you should be concerned? To find out, doctors say you need nothing more than a mirror and a tape measure.
The most precise way to gauge visceral fat is through an MRI or CT scan, but these procedures can be costly and inaccessible. MRIs and CT scans have shown that waist circumference is an indicator of abdominal fat. So simply take a good look at the shape of your body. Those with an apple shape have a large percentage of their total body fat concentrated above their waist. They're more likely to have more abdominal fat, and therefore more visceral fat, than those with a pear shape, or larger lower body, where body fat settles primarily below the waist. Also measure your waist. Studies show that women with a waist circumference of 35 inches or more and men with a measure of 40 inches or higher have dangerous levels of visceral fat.
What can we do to keep it from reaching unsafe levels? "One of the most effective treatments for targeting visceral fat is exercise, maybe more so than diet," says Kerry Stewart, a professor of medicine and director of clinical research exercise physiology at the Johns Hopkins School of Medicine. "You will see a reduction in waist size, which includes both a loss in subcutaneous and visceral fat, but the visceral fat will go down even more so than the subcutaneous fat." Cardiovascular activities like running, biking, or rowing count more towards visceral fat loss, says Stewart, since they burn more calories than other types of exercise. Still, most doctors will say, something is better than nothing.
Don't discount your diet though, which can help you lose visceral fat and body weight in general. Melina Jampolis, a physician nutrition specialist with practices in San Francisco and Los Angeles, stresses the importance of cutting back on sugar, "especially sugary beverages and refined white carbohydrates," since they're high in calories and nutritionally empty. Replace them with lean protein like skinless poultry and fish or healthy fats like olives, nuts, seeds, and avocados. Eat more fiber; this includes oats, beans, and barley. High-fiber foods can help you feel full longer so you're less likely to overeat. The Institute of Medicine's daily recommendation of fiber for adults under 50 is 38 grams for men and 25 grams for women. It's suggested that men and women 51 years and older eat 30 grams and 21 grams a day, respectively. To achieve this, increase your consumption of fruits, vegetables, and whole grains—"three servings of whole grains per day have been linked with lower levels of visceral fat," says Jampolis.
Last, but not least, consider your sleep habits. A 2010 study from the Wake Forest University School of Medicine showed that too little sleep—five hours or less—or too much sleep—eight hours or more—was related to increases in visceral fat. And while diet and stress may also have played a part, researchers say sleep itself appeared to be a risk factor.
When all is said and done, shedding fat for those who are overweight, whether it's subcutaneous or visceral, is a win-win situation. The Centers for Disease Control and Prevention defines overweight adults as having a body mass index, or BMI, between 25 and 29.9, while those who have a BMI of 30 or higher are considered obese. It should be noted that BMI correlates with body fat but does not measure it directly. To determine your BMI, simply take your weight in pounds, divide by your height in inches squared, and then multiply by 703. For example, if your weight is 200 pounds and your height is 6 feet, the calculation would be: [200 / (72)²] x 703 = 27.
Losing weight—through diet, exercise, and other behavioral changes like meditation to relieve stress—can help stave off a host of problems including heart disease, stroke, cancer, gallbladder disease, type 2 diabetes, osteoarthritis, sleep apnea, infertility, and depression. Now that's something we can all live with.
Corrected on 4/30/2012: An earlier version of this article incorrectly spelled the name of Ernst Lengyel, a professor of obestetrics and gynecology at the University of Chicago.