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Why Your BMI Doesn't Tell the Whole Story

How well do BMI, waist size, and other measurements gauge obesity?
By
WebMD Feature
Reviewed by Louise Chang, MD

You may have heard of BMI -- body mass index. It's based on your height and weight, and it's widely used to say who's obese, who's overweight (but not obese), who's at a normal weight, and who's underweight.

But BMI has some drawbacks. It may not be the best way to size up your shape. And it says nothing at all about your fitness level.

So how useful is BMI really? And what else might you use?

What is BMI?

BMI dates from 1832 when Belgian statistician Adolphe Quetelet invented the Quetelet Index, now called BMI. Little did he know that it would become a modern standard for measuring overweight and obesity.

BMI, calculated from a person's height and weight, breaks down into four categories:

  • Underweight: BMI below 18.5
  • Normal: BMI ranging between18.5 and 24.9
  • Overweight: BMI ranging between 25 and 29.9
  • Obese: BMI of 30 or higher

"Probably for 90% or 95% of the population, BMI is just fine as a general measure of obesity," says Richard L. Atkinson, MD, director of the Obetech Obesity Research Center in Richmond ,Va. 

But some critics take a different view. Scott Kahan, MD, MPH, director of the National Center for Weight and Wellness in Washington, D.C., and a faculty member of the George Washington University School of Medicine, believes that BMI has merit, particularly for scientific research.

"When you take a big population -- thousands of people, tens of thousands of people -- as part of a research study, it's extraordinarily difficult to use anything more advanced than BMI," he says. "BMI is cheap, it's quick, and on the whole, on the average, it gives a reasonable measure that can be useful in those situations."

But for you, or any other individual? BMI might not be the best.

Kahan specializes in helping patients manage excessive weight that can lead to health problems, including diabetes and heart disease. He notes that although BMI is useful as a quick screening tool by a doctor or nurse, it's not enough just to look at a number. "Traditionally, we define obesity by a certain cut-off on the BMI scale," he says. That amounts, he says, to judging whether a person is obese based only on that person's size, something Kahan calls "antiquated and not terribly useful."

BMI's Drawbacks

Your BMI is a number. It doesn't reveal anything about your body composition -- for example, how much muscle versus fat you have. That's why conclusions based only on BMI can be misleading, especially for the following groups:

Muscular people: Some people have high BMIs but don't have much body fat. Their muscle tissue pushes up their weight. People in the military who exercise daily might fall into this category, Atkinson says. Other examples: "A football player or a body builder who is very muscular. Their BMI shows up pretty high and yet their body fat is actually pretty low," Kahan says.

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