Then don't be surprised at your next checkup if the nurse whips out a tape measure and wraps it around your waistline -- no matter how thin you might be overall -- instead of just relying on your body mass index (BMI) to determine your healthy weight.

In fact, measuring waist circumference should go hand-in-hand with stepping on a scale as part of any health assessment, according to new guidelines published Thursday by the American Heart Association in the journal Circulation.

That's because research is showing that a protruding tummy may be a sign of what is called visceral adipose tissue, or VAT -- a dangerous form of fat that wraps itself around organs deep inside your body.

"Studies that have examined the relationship between abdominal fat and cardiovascular outcomes confirm that visceral fat is a clear health hazard," said Dr. Tiffany Powell-Wiley, chief of the social determinants of obesity and cardiovascular risk laboratory at the National Institutes of Health in Bethesda, Maryland.

Belly fat is different

Unlike the fat just under your skin, called subcutaneous fat, visceral fat raises your risk for heart disease, type 2 diabetes, stroke and high cholesterol. Experts think that's because visceral fat makes more inflammatory proteins that narrow blood vessels, raise blood pressure and inflame tissues and organs.

And here's the rub: You can't assume you're safe from visceral fat if your overall weight is healthy, experts stress. That's because you can have dangerous visceral fat even if you're not considered obese by BMI standards -- and not have any visceral fat even if you are obese. You are overweight if your BMI is over 25; over 30 is considered obese.

How do you know if your stomach is protruding into dangerous territory? Do a gut check.

Find your hip bone. Then take out a tape measure and -- without sucking in your tummy, please -- wrap it around your waist at the top of your hipbone (which is typically across the belly button). Exhale normally, and measure, making sure that the tape is parallel to the floor and snug, but not tight, across the skin.

Non-pregnant women with a waist size greater than 35 inches (88 cm) and men with a waist larger than 40 inches (102 cm), are at higher risk according to the US Centers for Disease Control and Prevention.

If you're of Asian descent, the benchmark for visceral fat drops to 31.5 inches for women and 35.5 inches for men, according to the Endocrine Society.

You can also compare your waist-to-hip ratio. Start by measuring your waist as instructed above, then place the tape measure on your hips at the widest part of your buttocks (as viewed from the side in a mirror).

Then divide your waist measurement by your hip measurement: For men, a ratio of greater than 0.9 indicates an increased risk of obesity-related disease. For women, it drops a tad -- a ratio of greater than 0.8 shows increased risk.

But these are crude measures. The only sure way to know is to check your visceral fat levels on a CT scan or MRI, say experts, so be sure to discuss your concerns with your doctor.

What to do?

According to the AHA committee, the most beneficial physical activity to reduce abdominal obesity is aerobic exercise.

In fact, "reaching a target of 150 minutes a week of physical activity, particularly aerobic physical activity, may be enough to help reduce abdominal fat," said Powell-Wiley, who was the chair of the AHA guidelines writing committee.

That seems to be the case even if the exercise does not produce weight loss, she added.

"This decrease in abdominal fat without weight loss may be related to increasing fat-free mass (or muscle mass) with aerobic exercise," Powell-Wiley said.

Muscle tissue burns more calories than fat tissue.

"However, we want to emphasize that more studies are needed to determine the best diet, physical activity and other lifestyle changes needed to reduce abdominal fat enough tor reduced heart disease risk," she added.

Aerobic means "with oxygen," so aerobic exercise increases your breathing rate and promotes the circulation of oxygen through the blood. This type of exercise makes the heart more efficient and improves its ability to move oxygen-carrying blood with every beat. Speed walking, jogging, bicycling, stair climbing, cycling and swimming are all examples of aerobic exercise.

Strength training is also good at building muscle to burn more fat. But remember that spot exercises, such as sit-ups, can tighten abdominal muscles but won't target visceral fat. You have to get your whole body moving.

Of course watching your weight and eating healthy are important, too.

Eat a well-balanced diet full of fruits, vegetables and whole grains and little processed meat, saturated fats and sweets.

Sugar is an especially bad actor when it comes to belly fat, according to studies. Sugar sweetened drinks are a key player because the brain doesn't register liquid calories in quite the same manner as solid calories; studies find you end up drinking more total calories.

Instead, fill up on soluble fiber-rich foods that the body can't absorb easily, like beans, oats, oat bran, rice bran, barley, citrus fruits, apples, strawberries, peas and sweet potatoes. One study found that each 10 grams of soluble fiber eaten each day was linked to a 3.7% reduction in abdominal fat.

And of course, eating fewer processed foods and empty carbohydrates will also go a long way toward reducing weight, including that dangerous belly fat.