Oprah has not been practicing what she's been preaching.

Since 2006, Winfrey, 54, a proponent of healthy eating and regular exercise, has gained 40 pounds. She now weighs 200 pounds and says she's too fat.

In fact, she's obese.

That's because she's got a body mass index (BMI) of 31.8. People with a BMI of 25 or greater are considered overweight, and those with a BMI of 30 or greater, obese.

But a scale won't tell her that. A decade ago, if you wanted to start losing weight or firm up, your first step was to weigh yourself. Today, you've got a few more choices.

Want to know how much body fat you have? Step into a Bod Pod, a small egg-shaped chamber that measures your body volume via displaced air. Curious about the muscle mass of your right leg? Hop on Tanita Corporation of America's new BC-558 Ironman Body Composition Monitor, which measures segmental body fat percentages and muscle mass.

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But with all of these numbers to calculate and interpret, experts say there's also a lot of confusion about which ones are worth paying attention to in the long haul.

"There are so many ways to measure your health or body fat now," says Amy Jamieson-Petonic, spokeswoman for the American Dietetic Association and employee wellness manager for the Cleveland Clinic. "It can be hard to make sense of it if you don't have a really good background.

Measuring Up

Almost 50 years ago, the insurance industry came up with the concept of ideal body weight, a bench mark for finding out how your weight measured up. Tables for men and women highlighted heights and weights with the lowest mortality rates. But they didn't initially accurately take into account a person's frame size or the fact that smokers might weigh less but have a host of other health risks, says Dr Donald Hensrud, medical editor in chief of Mayo Clinic Healthy Weight for Everybody.

Body mass index (BMI), a term you're probably familiar with, has become increasingly popular as a body measurement in the US since 1998. That's when the National Institutes of Health and the National Heart, Lung and Blood Institute released a report classifying weights via BMI: your weight in pounds multiplied by 703 and divided by your height in inches squared.

People with a BMI 25 or greater are considered overweight, and those with a BMI 30 or greater, obese.

Because of its formula, BMI is considered a better measure than ideal body weight. It also more closely correlates to percent body fat and risk of health problems, like diabetes, for large populations. But it has problems, such as a failure to make a distinction between fat and muscle.

A 6-foot-tall football player who weighs 250 pounds and has 8% body fat would have a BMI of 33.9, for instance. He'd technically be considered obese.

Jim Pivarnik, professor of kinesiology and epidemiology for Michigan State University, researched the issue with colleagues from Saginaw Valley State University for March's Medicine and Science in Sports and Medicine, a journal of the American College of Sports Medicine. He found that BMI frequently misclassified athletes, particularly those into resistance exercises, as well as college students.

"You've got to use it cautiously," Pivarnik says of BMI. "It's not a terrible measure, but it's just one measure."

Chewing the Fat

If you're looking for more specific information about your body, experts suggest determining your percent body fat. There are multiple methods, which range in cost from free to upward of $1,000, depending on which gym or clinic you visit.

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The cheapest technique is the skin-fold measurement, which involves using a small gadget, often available free from health clubs or for $10 on the Internet, to pinch the skin where the body stores fat, such as the abdomen. Results, however, are only approximate. Other options include high-tech body composition monitors; a dual energy X-ray absorptiometry test (DEXA) or bone scan; a bioelectrical impedance test, which uses an electrical current to take measurements; a Bod Pod, a chamber that uses an air displacement technique; and hydrostatic weighing--basically an underwater scale.

If that sounds too complicated, you can always try breaking out the measuring tape. Beyond how much fat you have, its location also is important, Hensrud says. Women with a waist over 35 inches and men whose waists measure over 40 inches may face a higher risk of disease than people with smaller measurements. Part of the problem is visceral fat, which builds up around the organs in the belly and has been linked with risk factors for heart disease.

Weighing In

Once you come up with telling numbers, Hensrud says it's essential to set realistic, not lofty, goals for what you'd like them to become in the future. Focusing on how you're going to get in shape, by walking more or upping your intake of fruits and vegetables, is what's going to help you succeed.

If you are focused on the numbers, just remember that calculations such as BMI won't change as rapidly as that old-fashioned measurement, your weight.

And while recent research from the University of Minnesota has showed that stepping on the scale every day, rather than on a more occasional basis, can better help people lose or maintain their weight, most experts say consistency is the real key. Always weighing yourself at the same time of day, such as in the morning after you've voided, will eliminate some of the fluctuations in your fluid levels that will affect your results, says Jamieson-Petonic.

The rest is up to you. "There's nothing magic about calories in and calories out," Hensrud says. "You've got to make lifestyle changes, and if they're sustainable, the numbers will change."

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