Why Is BMI Inaccurate?

BMI, or body mass index, is a commonly used diagnostic tool to classify people as underweight, normal weight, overweight or obese based on height and weight. It is calculated as weight, in kilograms, divided by height, in meters squared. BMI provides an estimate of health risk for both adult men and women; however, there are limitations to this measure.

Significance

Your body shape and where you store fat influences your health risk. Body fat is usually stored around the hips in women and in the abdominal area in men. Fat cells around the abdomen pose a greater health risk than do fat cells stored in the thighs or hips. BMI does not consider body shape or body fat distribution, and therefore may be an inaccurate measure of health risk.

Identification

A BMI score below 18.5 indicates underweight while a score between 18.5 and 25 is ideal. A score between 25 and 30 is considered overweight while a score above 30 classifies a person as obese. These classifications are arbitrary in terms of where the line is drawn between each of the categories, and some government agencies and professional health organizations disagree with the interpretation of these results.

Body Types

BMI may overestimate body fat in muscular but lean athletes and incorrectly classify them as overweight or obese. In the March 2007 issue of "Medicine & Science in Sports & Exercise," Dr. Joshua J. Ode and his colleagues state that BMI was not an accurate measure of the amount of body fat in college students. Their results "support the need for different BMI classifications of overweight in these populations."

Time Frame

BMI may also provide an inaccurate measure of overweight or obesity for older people since BMI does not distinguish between fat or muscle. Older people generally have more body fat than younger persons with the same BMI score. As a result, your BMI score may underestimate body fat and health risk as you age.

Solutions

BMI is a good measure for assessing population health risk, but it is inaccurate for determining individual health risk. Study findings appearing in the May 2010 issue of "Obstetrics and Gynecology" suggest that racial- and ethnic-specific BMI cutoff values are needed as current cutoffs fail to identify many whites and Hispanics as overweight. Other diagnostic measures such as waist circumference and blood pressure should be used in combination with BMI to accurately determine health risk. Consult your physician for additional information on individual health risks.