Last updated: Feb 05, 2015
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Let's face it—fat has gotten an unfortunate rap. We curse the dimpled cellulite that has settled on our thighs and survey the pudge around our belly with a quick poke and a disapproving eye. But here's the thing: Fat isn't just a place where your body dumps extra calories. It's an organ that can help—or harm—your health. (One type, brown fat, can actually turn your body into a calorie-burning machine!) "Everyone has fat—even Olympic marathon runners," says Osama Hamdy, MD, medical director of the Obesity Clinical Program at Harvard University's Joslin Diabetes Center. "Simply put, we need it to survive." The trick is understanding the difference between the kinds of fat and keeping them in balance with diet, exercise and some plain old common sense. Get ready to go deep.

Fat type No. 1: Subcutaneous fat
Where it is: Directly underneath your skin. Subcutaneous fat can be anywhere: not just in your belly and tush but your arms, legs—even your face.

What it does: In addition to storing energy and providing essential padding for your body, it has another important job: It generates the hormone adiponectin, which helps regulate insulin production. "Paradoxically, the fatter you are, the less adiponectin you produce, which means that your body has trouble regulating insulin, increasing the risk of heart disease and diabetes," Dr. Hamdy says.

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How to blast it off: Cutting calories is crucial for overall weight loss, but getting moving counts, too: Women who walked, cycled or took public transportation to work had about 1.5 percent less body fat than those who drove, according to a U.K. study published this past August. "It's proof that those little bursts of activity count when it comes to burning fat," notes Pamela Peeke, MD, author of Body for Life for Women. "Even just walking from the train station or bus to your office can burn on average an extra hundred calories."

Already active? Ramp it up. "When you take your workout up a notch, you reach VO2 max—that's the level of exertion where you have the optimal breakdown of body fat," Dr. Peeke explains. "It also fools your body into thinking that you're working out minutes after you've stopped, so you're still burning calories."

Fat type No. 2: Visceral fat
Where it is: Nestled deep within your belly, where it pads the spaces around your abdominal organs. You can't feel or grab it.

What it does: Visceral fat has been dubbed "toxic" fat, and for good reason: "It secretes inflammatory proteins called cytokines that affect insulin production and increase inflammation throughout the body, which raises the risk of developing type 2 diabetes and heart disease," Dr. Hamdy says. You can't directly measure visceral fat unless you undergo an MRI or a CT scan. The next best thing? Grab a tape measure and wind it around your waist; if your midsection is more than 35 inches, you most likely have too much visceral fat, Dr. Hamdy says. A Mayo Clinic study published last March found that Caucasian women with waist sizes above 37 inches were more likely to die from heart or respiratory disease. Another sign of trouble: Your numbers are off, meaning you've got low HDL (good) cholesterol and elevated blood glucose and triglyceride levels. "When a woman who has been lean most of her life gains 10 to 20 pounds at age 40 or so, she may not even be technically overweight, but it's usually visceral fat that's adding the extra weight," explains Caroline Cederquist, MD, a bariatric physician based in Naples, Fla., and author of The MD Factor Diet.

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How to blast it off: "To mobilize visceral fat, a balanced diet is essential," Dr. Cederquist says. "Eat lean protein throughout the day, while controlling your carb and fat intake." For keeping visceral fat off, cardio is the way to go: A 2011 Duke University study found that regular aerobic exercise—the equivalent of jogging about 12 miles a week at 80 percent max heart rate—was the best workout for losing visceral fat in particular.

Fat type No. 3: Brown fat
Where it is: Mainly around your neck, collarbone and chest. For years, researchers assumed that it was present primarily in infants, helping to keep them warm, and that it gradually disappeared during childhood. But in 2009, studies revealed that some adults still have brown cells.

What it does: This buzzed-about "good" fat becomes metabolically active when we're exposed to cold temperatures, burning up energy. "Since brown fat is used to generate heat, it burns more calories at rest," says Ruth Loos, MD, professor of preventive medicine at Mount Sinai Hospital in New York City. Fifty grams (about 4 tablespoons) of brown fat, if maximally stimulated, could torch about 300 calories a day.

How to beef it up: Since brown fat is activated by cold, prepare to shiver. According to a study in Cell Metabolism, folks who spent 10 to 15 minutes in temperatures below 60 degrees produced a hormone called irisin, which appears to make white fat cells act like brown fat; they got a similar boost from an hour of moderate exercise at warmer temps. And keep your thermostat low: An Australian study showed that men who lived in homes set to 66 degrees generated 40 percent more brown fat than when they lived in higher temps.

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The best way to measure your fat
The best way to determine whether you're carrying too much flab: Check your body fat percentage. A healthy range is between 20 and 25 percent for young women and up to 30 percent after age 50, Dr. Peeke says. Skip the skin calipers (they pinch the skin around your upper arm, thigh and stomach); results can be off by as much as 5 percent. Dr. Peeke's rec: an at-home bioelectrical impedance scale, which sends an electrical current through your body and measures how fast it returns. (Brands like Tanita can cost as low as $35.) Other methods, such as underwater weighing (you're dunked into a pool to measure your body fat) and DEXA scans (which use an X-ray to deter-mine body com-position, including body fat, lean muscle and bone density), are more accurate, but you'll have to go to a health clinic to get measured.

The skinny on cellulite
Skin dimples happen when subcutaneous fat cells collect in pockets and push against the connective tissue under your skin. Minimize their appearance by staying at a healthy weight and exercising—more muscle equals tauter skin. Caffeine-infused creams (like Clarins Body Lift Cellulite Control) can temporarily de-puff fat cells for a smoother look.

RELATED: Myths and Facts About Cellulite

The scary place fat can hide
Ectopic fat has the same metabolic properties as visceral fat, but instead of padding your abdominal organs, it settles in your heart, liver, pancreas and muscles. "Most of us have only a few pounds of ectopic fat," Dr. Hamdy says. "Even so, it's dangerous because it's inside vital organs and can increase the risk of heart disease, liver damage and type 2 diabetes." The best way to tell if you have ectopic fat is by getting an MRI or a CT scan. Keep it at bay by staying active. The more you sit, the more likely you are to have this fat around your heart, according to a University of California, San Diego study.

3 ways to burn flab faster
Resistance train: A strength workout that incorporates high intensity interval training (HIIT) will help burn calories—and fat stores—at a higher rate than straight sets, says trainer Gunnar Peterson. The drill: Do eight reps of different types of body-weight resistance activities (chin-ups, squats, burpees, etc.), alternating 20 seconds of work with 10 seconds of rest.

Rest: A Brigham Young University study found that women who slept between 8 and 8 1/2 hours a night had the lowest levels of body fat. Why? Lack of sleep can wreak havoc on hormones that control fat metabolism.

Related: 5 Fat-Burning Body Weight Exercises  

Relax: Doing yoga can reduce cortisol levels (high levels of the stress hormone are linked to belly fat). Obese postmenopausal women who practiced yoga for 16 weeks reported significant reductions in visceral fat, according to a Korean study.

The best fat-fighting foods
Dairy: People who downed more dairy while on a low-calorie diet lost 2 1/2 more pounds of fat than those who didn't, according to a review published in the International Journal of Obesity.

Protein: Research has shown that folks lose more visceral fat on a low-carb diet than on a low-fat diet that's high in carbs, Dr. Hamdy says.

Oatmeal: For every 10-gram increase in soluble fiber a day, visceral fat was reduced by 3.7 percent over five years, per a study at Wake Forest University in Winston-Salem, N.C.

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Vegetable oil: Polyunsaturated fats (including omega-6-rich oils like safflower, soybean and corn) may help prevent you from gaining visceral fat—and even help you build muscle, according to a Swedish study.

Fatty fish: Like their omega-6 polyunsaturated cousins, omega-3s (found in salmon, tuna, mackerel and sardines, as well as flaxseed and walnuts) may help whittle your middle.

Can you be heavy and healthy?
Some research says yes, but a 2013 study found that while overweight people may have a higher life expectancy than normal-weight folks, they spend those years in poorer health. "While certain types of fat—particularly subcutaneous fat—may not have the impact on your heart that visceral fat does, they can affect your quality of life," Dr. Peeke says. "If you're heavy, you're more likely to develop osteoarthritis or just have trouble moving around."

7 ways fact affects your whole body
Your head: The risk of migraines increases by almost 40 percent in younger women with general or belly obesity, according to the American Headache Society.

Your brain: Being underweight was associated with a 36 percent higher risk of developing dementia, while being obese conferred a 42 percent higher risk, per a study done at the Johns Hopkins Bloomberg School of Public Health in Baltimore.

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Your bones: A Japanese study found that postmenopausal overweight women had a higher risk of getting vertebral fractures; superskinny women were more likely to suffer fractures in the neck and long bones (like the shin, thigh or forearm).

Your breasts: After menopause, women who are overweight have a 30 to 60 percent higher breast cancer risk than those at a normal weight.

Your heart: One study found that overweight folks have a 32 percent higher risk of coronary artery disease than those at a normal weight.

Your ovaries: Six percent of infertility cases are due to being overweight and 6 percent to being underweight, says the American Society of Reproductive Medicine.

Your blood sugar: According to a Canadian study, women who had BMIs higher than 30 had a twelvefold higher risk of developing type 2 diabetes.

Your knees: A U.K. study found that overweight women had six times the risk of developing knee osteoarthritis as their lean counterparts.

How fat changes as you age
Childhood: The number of fat cells you have is set early on, so you don't actually get more of them when you gain weight. Instead, those fat cells swell as triglycerides (i.e., fatty acids) are stored in them. If enough fat cells expand in size, your body begins to, well, expand too. Boys tend to be born with more fat cells in their belly, while girls are born with more fat cells in their hips, thighs and butt, in preparation for storing fat during pregnancy.

Adolescence: Between the ages of 9 and 19, the volume of fat in girls more than doubles, due in part to a surge of the female hormone estrogen. "Your body starts producing estrogen in preparation for having a baby and nursing years down the road. That estrogen helps fuel the growth of fat cells," Dr. Peeke says.

Post-pregnancy: Yep, it's hard to shed that extra fat after having a baby. "It's reserve storage for the demands of breast-feeding," Dr. Peeke says. A throwback to caveman days, it's a safety device to ensure new mothers can feed their babies in times of famine. So even though breast-feeding burns about 500 calories a day, your body is hardwired to hold on to some fat deposits—and you may not lose those last 5 or 10 pounds until after you wean.

Menopause: Until this point, most women still store fat around their lower half, but once they go through menopause, their fat storage patterns mimic men's—which means you may start sporting what looks like a beer belly.