Is Body Mass Index a Bad Measure?

Q. I saw the “Obesity in America” report on MSN that tracked how overweight the population has become using the body mass index (or BMI). But I think it's misleading to use this as a measure because it is unreliable: My BMI suggests that I am overweight, yet my body fat percentage is in the 8 percent range. Does that make sense?

A. You fall into a subset of people for whom the BMI is not a reliable indicator of fatness.

One of the toughest challenges in scientific studies is being able to measure what is being studied accurately. For example, if a researcher wants to assess a person’s risk of heart disease, there are many potential indicators he could look at—blood pressure, cholesterol, fitness level and, to get more technical, c-reactive protein or homocysteine levels in the blood. However, each of these measures is only associated with a greater risk of disease, they are not proof it will develop. In addition, sometimes the technology to conduct more accurate testing does exists, but is not practical to use in a study.

When it comes to fatness, the most common way to measure that is to step on the scale. Yet looking only at body weight can be misleading. Someone who is 6 feet tall and 165 pounds is not overweight, or more accurately, over-fat, while someone who is 5 feet and 165 pounds is.

The body mass index factors in both height and weight and has been found to be highly correlated to obesity. You can figure out your BMI through an online calculator or by doing the math yourself: Take your weight in pounds and divide it by your height in inches squared. Then multiply your answer by 703.

Obesity experts classify a person who has a BMI of less than 18.5 as underweight; normal is 18.5-24.9; overweight is 25-29.9; obese is over 30; and extremely obese is over 40. These ranges were established based on the health risks associated with different BMIs. People who are underweight have higher rates of disease and death than people with normal BMI readings. People with BMIs of 25 and above are at greater risk of developing heart disease, diabetes, high blood pressure, gallstones and osteoarthritis. People with levels over 30 are at increased risk for these conditions as well as some cancers and death, especially from heart disease.

There are definite limitations with the BMI measurement. As many fit individuals like you can attest, they may have a higher-than-expected BMI because they have a big body frame or lots of muscle mass. These people may weigh heavy but still be lean and fit. For that reason, people should take their waist circumference into account as well. High levels of fat in the abdominal region are associated with increased risks of diabetes, high blood pressure and heart disease; specifically, in men with a waist size greater than 40 inches and women with a waist size of greater than 35..

The reason that BMI is used is that it’s a convenient tool. In large epidemiological studies, such as those that may have 50,000 subjects, it’s impractical to conduct body-fat tests on each and every person. (One of the most accurate tests for body fat involves weighing someone in an underwater tank—known as hydrostatic weighing.) The equipment, staffing or funding to carry out such a massive undertaking is often not available. But asking person’s height and weight and/or measuring it is simple enough to be done with large groups of subjects. And if the study is large enough, using statistics, researchers can take into account that a subset of this group may not actually be overweight when they analyze the data. (With any study, you want to carefully look at how it was designed and conducted before accepting the conclusions. If you can get a hold of the original published study, the “Discussion” section outlines problems or limitations that the researchers themselves acknowledge. You can also read reactions of reputable scientists and other researchers to a study in other studies that may cite it. Or you can learn more about similar studies by reading more related research. You'll find original published research here.)

Although BMI may have limitations, generally, it is considered a reliable way to track the rise in obesity in the U.S. population. But if you are lean and fit, don’t worry about your BMI. The BMI is meant to express increased risks in disease and death. As a regular exerciser, you are reducing your risks of many diseases and early death.

Correction, July 28, 2006:  This article originally stated the calculation for BMI as weight in pounds multiplied by 703 then divided by height in inches. The correct formula is weight (in pounds) divided by height (in inches) squared, then multiplied by 703.This error has been corrected.

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Martica is a Manhattan-based exercise physiologist and nutritionist and an award-winning fitness instructor. She has written for a variety of publications including Self , Health , Prevention , The New York Times and others. Martica is the author of seven books, including her latest, Cross-Training for Dummies . (Read her full bio.)