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What Your BMI Doesn’t Tell You

WebMD Feature

You’ve probably heard the term BMI (body mass index). It's based on your height and weight, and is widely used to determine if you’re in a healthy weight range. But as it turns out, BMI may not be the best way to size up your shape.

Taking a Closer Look at BMI

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

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

But how useful is this number really?

"Probably for 90% or 95% of the population, BMI is just fine as a general measure of obesity," says Richard L. Atkinson, MD, a researcher and editor of the International Journal of Obesity.

But some critics take a different view. Scott Kahan, who directs the National Center for Weight and Wellness, says, "Traditionally, we define obesity by a certain cutoff on the BMI scale.” But judging whether a person is obese based only on their size is “antiquated and not terribly useful,” he says.

Kahan specializes in helping people manage excess weight that can lead to health problems, including diabetes and heart disease. At his center, measuring BMI is only a starting point. He sees people who are overweight but healthy, and their BMI doesn't truly reflect their health risks.

"They're heavy. Their BMI puts them in the obesity range. And yet, on every level that we look at, their health is actually quite good,” he says. “Their cholesterol and blood pressure are excellent. Their blood sugar is excellent. They don't seem to have any health effects associated with their excess weight."

Although BMI is useful as a quick screening tool by a doctor or nurse, Kahan says, it's not enough to look at only that number.

BMI's Drawbacks

Your BMI doesn't reveal anything about your body composition -- for example, how much muscle vs. fat you have. That's why conclusions based only on this number can be misleading, especially when it comes to the following:

  • How muscular you are: A few people have high BMIs but don't have much body fat. Their muscle tissue pushes up their weight. An example: "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.
  • Your activity level: Someone who is very inactive may have a BMI in the normal range and have lots of body fat, though they may not look out of shape. "They have very low levels of muscle and bone -- often elderly people, those in poor shape, sometimes those who are sick. Their BMI can look in the normal range, even though they have quite a lot of body fat in comparison to their lean body mass," Kahan says. "Ultimately, they have similar risks as people who carry lots of body fat and have a high BMI."
  • Your body type: Are you an apple shape or a pear shape? The location of your fat makes a difference to your health. Generally, it's the abdominal fat, or the "apple" shape, that's metabolically riskier. When fat settles around the waist instead of the hips, the chance of heart disease and type 2 diabetes goes up. Fat that accumulates on the hips and thighs, or the "pear" shape, isn't as potentially harmful.
  • Your age: The notion of an ideal BMI may shift with age. "People who are older probably should have a little more fat on them, [but] they shouldn't have a BMI of 30," Atkinson says. He points out that late in life, people who are "a little bit overweight" tend to have a better survival rate than leaner people. The reasons for that aren't totally clear, but it may have to do with having reserves to draw upon when fighting off an illness. It's hard to tell for sure, since many factors affect your health.
  • Your ethnicity: Major ethnic differences exist regarding BMI. For example, Asian-Americans tend to develop health risks, including the risk of diabetes, at significantly lower BMIs than whites. A healthy BMI for Asians ranges from 18.5 to 23.9, a full point lower than the standard range. More strikingly, Asians are considered obese at a BMI of 27 or higher compared, to the standard BMI obesity measure of 30 or higher.

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