Melissa J. Luther, Contributing writer

Achieving a normal body mass index rating has become a goal for some fitness enthusiasts and weight-loss seekers.

Unfortunately, this overly simplified calculation that takes your weight in kilograms and divides it by the square of your height in meters is being misused. BMI does not take into account individual body differences and was never designed to do so.

The original calculation was developed by a Belgian mathematician, Adolphe Quetelet, who was looking for a profile of the average man. He did not use it to assess health in any way, Slate.com says.

Then, in 1972, obesity researcher Ancel Keys was looking for the best way to predict body-fat percentage in population groups. Quetelet's equation was the best predictor out of those he tried, and Keys christened the equation the "body mass index." Keys never intended the BMI to be used for individual assessment, only for evaluation of population groups.

But by 1985, the National Institutes of Health was recommending BMI as an easy way for doctors to determine which patients were at increased health risks due to obesity.

What's Wrong With BMI

BMI has several inherent difficulties as a measure of health risks. First, it does not take into account body composition. Both fat and lean muscle contribute to a person's weight, but only excess fat increases health risks. BMI often incorrectly places muscular people into the "obese" category.

Second, the distribution of fat matters more than the mere presence of fat. Fat deposits in the abdominal area are the most dangerous, but a simple measure of weight and height fails to account for that difference.

Alternatives To BMI

Alternatives to the BMI may be more accurate, especially in some population groups.

Waist circumference, for example, is an especially good measure for determining abdominal obesity, the type most associated with obesity-related diseases such as diabetes and heart disease.

Scott A. Lear of the School of Kinesiology at Simon Fraser University, in a 2005 letter to the editor of Clinical Chemistry, presented arguments in favor of waist circumference. He pointed out that not only is BMI a more complicated calculation than WC, but most patients misunderstand the concept of BMI.

Lear argued that "BMI is flawed because changes in BMI may be attributable to changes in skeletal muscle rather than body fat, whereas WC changes are attributable entirely to changes in abdominal fat." It matters because central body fat is positively associated with insulin resistance, part of diabetes.

Another option is waist-hip ratio. This measurement divides the circumference of the waist by the hips, measured at the widest part, around the buttocks.

Recent studies suggest this measure is especially useful in people over age 75.

It has long been recognized that the risks associated with a high BMI appear to decrease in this group, and a high BMI no longer correlates with increased mortality. A high WHR, however, does translate to increased mortality risk in the elderly. WHR may also be a useful predictor of cardiac status, at least in overweight patients.

In an April 2001 letter to the editor of the Western Journal of Medicine, Rudolf E. Noble cites studied conducted at the Cathedral Hill Obesity Clinic in San Francisco. The researchers looked at blood pressure, cholesterol ratios and triglyceride levels. WHR was significantly linked to all three measures, while BMI was linked only cholesterol.

"Waist-to-hip ratios foretell more regarding cardiac status in these patients than measures of height and weight," Nobel wrote.

WHR has fallen out of favor as a measure of risk, and the American Heart Association Web site states that it is less accurate than either BMI or waist circumference.

BMI is useful as a statistical measure of population groups. It does not, however, provide useful information on individuals and should not be used to assess individual risk.