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How Accurate Is Body Mass Index, or BMI?

Is BMI still the best way to measure fatness? Some experts aren't so sure.
By
WebMD Weight Loss Clinic-Feature
Reviewed by Louise Chang, MD

What's your number -- under 25 or over 35? Body mass index (BMI) may not be a term that's on everyone's lips, but it's important for your health to understand what it is and to know your number.

Essentially, BMI is a simple mathematical formula, based on height and weight, that is used to measure fatness. You should be aware of your BMI because of the health risks of being overweight (that is, having a BMI of 25 or over). According to a report in the August 2006 New England Journal of Medicine, excess body weight during midlifeis associated with an increased risk of death. 

On the other hand, being too thin and having a BMI that's below the healthy range (18.5 to 24.9) can also be a health concern.

Many health care experts think BMI is a useful tool to measure weight and health risks, but others question its accuracy. Some believe a better way might be to take out the tape measure and check your waist circumference. Or is there a place for both methods?

What Is BMI?

In June 1998, in an effort to make sure doctors, researchers, dietitians, and government agencies were all on the same page, the National Institutes of Health announced its BMI guidelines. They replaced the old life insurance tables as a method to gauge healthy weight.

To calculate your BMI, divide your weight in pounds by your height in inches squared, then multiply the results by a conversion factor of 703. For someone who is 5 feet 5 inches tall (65 inches) and weighs 150 pounds, the calculation would look like this: [150 ÷ (65)2] x 703 = 24.96. An easier way is to use WebMD's BMI calculator.

(This BMI calculator is for use in the evaluation of adults, not for children.)

According to the NIH definitions, a healthy weight is a BMI of 18.5-24.9; overweight is 25-29.9; and obese is 30 or higher. 

The Measurement of Choice

BMI is the measurement of choice for most health professionals. 

''I think BMI is a very good and easy screening tool,'' says obesity expert, Cathy Nonas, MS, RD, a spokeswoman for the American Dietetic Association. 

But while it is a simple, inexpensive method of screening for weight categories, it is not a diagnostic tool. Health professionals need to do further assessments to fully evaluate health risks. These assessments would include measurements of body fat percentage, diet history, exercise patterns, and family history.

Further, BMI does not take into account age, gender, or muscle mass. Nor does it distinguish between lean body mass and fat mass. As a result, some people, such as heavily muscled athletes, may have a high BMI even though they don't have a high percentage of body fat. In others, such as elderly people, BMI may appear normal even though muscle has been lost with aging. 

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