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What can—and can’t—your BMI tell you?

Michele Arthurs, M.D., shares insight into measuring your BMI, and what to keep in mind.

Photo by i yunmai

Written by Michele Arthurs, M.D.

There are few things more routine than getting weighed and measured when you visit your doctor. Part of that routine, on the doctors’ end, is using the results to calculate your body mass index, or BMI, a number that tells us whether your body weight falls within a healthy range.

Like all of the things we measure, from your pulse rate to your cholesterol levels, BMI by itself does not give us a complete picture of your health. You can have a BMI that’s somewhat higher than normal and have a healthier cardiovascular profile, for instance, than someone whose BMI falls within the normal range. Plus, BMI is not a perfect measure of how much fat your body is storing or where you are storing it.

Still, it is an important part of our toolkit. It is considered a pretty reliable gauge of total body fat, especially in our most overweight patients. And I have found that, for many of my patients, it is a good way to start a conversation about health and weight.

One of the reasons we use BMI to measure body fat is that it’s the quickest, easiest and most widely accepted method. Other methods include measuring skinfold thickness with calipers; underwater weighing; bioelectrical impedance; dual-energy x-ray absorptiometry (DXA); and isotope dilution, which measures body composition at a molecular level. But as you might imagine, these methods are all less readily available and more expensive and complicated to perform.

Measuring your BMI is so simple you can do it at home. Simply divide your weight (in kilograms) by your height (in meters) squared. Of course, most Americans think in pounds and inches, so you can also divide your weight in pounds by your height in inches squared, and then multiply the whole thing by a conversion factor of 703. If, for instance, you weigh 160 pounds and are 5-foot-8-inches, or 68 inches tall, the formula would look like this: (160 ÷ 682) x 703 = 24.3.

A BMI of 24.3 falls within the normal range (18.5 to 24.9). Anything less than 18.5 is considered underweight. A BMI of 25 and above is considered overweight. A BMI of 30 and above is considered obese. (Fortunately, there are calculators you can find online—at the Centers for Disease Control and Prevention, for instance—that can do this quickly for you.)

Yet, it is one thing to know your BMI and another to interpret it. It accounts for only one thing: your weight as a function of your height. It does not account for pregnancy, for a cyst in your belly or for an athlete whose extra weight may be composed of muscle rather than fat. It does not account for gender. Women tend to have more body fat than men with the same BMI. It does not account for age, as older people tend to have more body fat than younger people with the same BMI.

And though BMI is measured the same way in children, it is interpreted differently for young people—by comparing your child’s BMI to growth charts for children of the same age and gender.

For all of these reasons, it is important to talk to your doctor about a BMI that falls outside of the normal range, and whether it is something you want or need to address.

Some patients may feel offended to learn that their BMI puts them in the obese category, with its increased risk for health conditions that include diabetes, stroke, heart disease and certain cancers. I wish the word “obese” would not lead to an emotional response, or a sense of judgement. We have an epidemic of obesity in the United States— nearly 40% of all adults, up from 15% in 1976, are obese—and obesity is a medical diagnosis, not an epithet.

Like high blood pressure or high cholesterol, a high BMI is simply part of your risk profile and is highly modifiable. It can—and should—be treated with the same attention and respect.

For more information about BMI and links to BMI calculators for adults and children, visit the CDC website.

Michele Arthurs, M.D., is a board-certified internal medicine physician. She sees patients at the Kaiser Permanente Camp Springs Medical Center.

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