BMI: The Complete Instructions

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November 26, 2018
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Here’s what those numbers can—and can’t—reveal about your health.


BMI: The Complete Instructions
Say you just calculated your body mass index (BMI) online and the result wasn’t exactly pretty. It may have indicated that you’re overweight, if the number landed anywhere between 25.0 and 29.9, or (gasp!) obese if it was 30 or above.

But if you eat well and exercise regularly, are you really out of shape just because an ominous online calculator says so?

What BMI Really Means

According to the National Institutes of Health, BMI is a “useful measure of overweight and obesity.” It’s calculated by dividing a person’s weight by his or her height. Here’s the formula:

BMI = (Weight in Pounds / (Height in Inches x Height in Inches)) x 703

Health experts have traditionally used the BMI scale to estimate a person’s body fat, as well as their risk of chronic diseases, such as heart disease, high blood pressure, type 2 diabetes, breathing problems, and certain cancers. The general rule of thumb is that the higher your BMI, the greater your risk of certain conditions.

But here’s the catch—and one of BMI’s big, glaring inaccuracies: It doesn’t take into consideration your unique body composition.

“Overweight does not always mean unhealthy,” says Ara Jo, Ph.D., a clinical assistant professor at the University of Florida College of Public Health and Health Professions. “This especially applies to certain people, such as serious athletes or people who serve in the armed forces, who have more muscle mass than others.”

And since your personal BMI number doesn’t account for lean body tissue, an individual with a great deal of muscle mass on a shorter frame would be considered obese, or at a higher risk of diseases, even though they are perfectly healthy, explains Tony Maloney, an ACSM-certified exercise physiologist, trainer, and fitness center manager at the National Institute for Fitness and Sport.

Take Maloney, for example: At 5-foot-7 and 180 pounds, his BMI in the low 30s. That technically makes him “obese.” However, his 10 percent body fat composition tells a different story about his health.

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According to Jo, a growing amount of research casts doubt on using the BMI calculator as a way to determine a person’s risk of chronic health conditions. One recent study from the University of Florida, in which Jo was the lead researcher, found that determining someone’s body fat percentage is a better predictor of diabetes risk than BMI.

In addition to categorizing healthy, muscular people as overweight and obese, the reverse scenario is true, and even dangerous: Someone with a “normal” BMI could actually be at a high risk of diabetes or other health conditions and not know it.

The Best Way to Measure Your Health

Jo suggests using more than one body composition assessment technique, such as a bioelectrical impedance analysis (BIA), to get a solid read on your health. BIA estimates body fat by sending low-level electrical currents through the body and measuring resistance to the electricity flow in various body tissues. (Don’t worry, the electrical current itself is imperceptible and noninvasive.)

In addition to measuring fat, you can also gauge how much muscle you have with a DEXA scan. But since it relies on X-ray technology, you probably won’t want to get it done regularly. Maloney also recommends measuring your blood pressure, resting heart rate, circumference measurements (like the size of your waist), and aerobic capacity.

But should you discount BMI entirely? Jo says no.

“BMI is still widely used in clinical settings since it’s an easy and simple method,” she says. Plus, research still shows that BMI is highly associated with risk of obesity-related chronic diseases.

If a BMI calculator classifies you as overweight or obese, consider it an indication that you might need to talk to a doctor, dietician, or fitness professional about your health, Maloney says. Although the number could very well be a fluke, your weight and body fat to muscle ratio will probably need closer analysis.

There are multiple ways to measure your health, including BMI. But many of these measurements are misunderstood, or worse, dangerous. Read about two other metrics to be wary of relying on.

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