Understanding BMI: Is it a Good Predictor of Health?

Of everything you learn about yourself at a routine doctor’s visit, it can be tough to tell which information is the most important –– and which parts should motivate changes in behavior. For example, your primary care provider may weigh you and measure your height to determine your body mass index, a data point that can determine whether you’re underweight, of normal weight, overweight, or obese.

According to One Medical’s Melanie Chang, NP, the body mass index formula was created by a statistician in the 1830s to determine the characteristics of the "average man.” The formula takes a person's weight in kilograms and divides that by the square of a person's height in meters, resulting in a number your health care provider calls your BMI.

Medical research has historically associated a higher body mass index with certain health risks, from metabolic diseases like type two diabetes to heart disease and cancer. While your primary care provider may use this information as one data point to understand your health and potential risk factors, keep in mind body mass index alone isn’t a perfectly reliable predictor of your health –– and in many cases, it can even be inaccurate.

Here’s what you need to know about body mass index –– what’s normal, where it falls short, and how it all relates to your health.

For most patients, a “normal” BMI ranges from from 18.5 to 24.9. However, that’s certainly not the standard for everyone. Babies, kids, and teens have their own formula and bell curve based on data of people their own age, and along with determining health risk factors, providers usually use it to measure growth.

BMI can be a predictor of potential health problems because it defines and measures obesity. According to the Centers for Disease Control and Prevention, obesity can increase a person’s risk of:

• High blood pressure (hypertension)
• High cholesterol
• Type two diabetes
• Heart disease
• Stroke
• Sleep apnea and other breathing problems
• Gallbladder disease
• Osteoarthritis
• Some cancers, including endometrial, breast, colon, kidney, gallbladder, and liver
• Mental illness
• Early death

While body mass index is one potentially important part of a patient’s health picture, it’s not the sole indicator –– primarily because it doesn’t distinguish between weight from fat and muscle. For example, Chang says BMI may be an inaccurate measure of health for those who have a high percentage of muscle mass (and therefore weigh more), such as athletes or bodybuilders.

BMI may also be an inaccurate tool for determining the health of older adults. Because elderly people tend to lose muscle and bone mass as they age, their BMI could be considered “normal,” but they might actually be overweight. And since the body mass index was developed to measure mostly Caucasian body types, it could fall short for other people from other ethnic backgrounds.

According to the Centers for Disease Control and Prevention, body mass index alone isn’t a perfect predictor of a person’s health or risk factors, even if a person is overweight. If a provider thinks a patient is at risk for obesity-related health problems, they will further assess factors like a patients’ diet, physical activity, and family history. Think of BMI as a starting point for understanding health, rather than the whole picture.

Instead of solely relying on body mass index measurement, Chang also suggests paying close attention to waist circumference, or the measurement slightly above your hip bones, which she says can be a more accurate predictor of metabolic disease.

“Evidence has shown abdominal obesity, or more than 35 inches for women and more than 40 inches for men, to be a greater risk factor for diabetes and heart disease than BMI, in addition to other predictive risk factors, such as high blood pressure, elevated fasting blood sugar, and low HDL cholesterol,” Chang says.

As with all data points in medicine, BMI has a number of limitations. From blood tests to body mass index, all the measurements your provider takes provide them with one, moment-in-time snapshot of how you’re doing and what you can do to get (and stay) healthy.

To ensure your long-term health, focus on evidence-based practices like routine cardio and strength exercise, a nutritious, well-rounded diet, plenty of sleep, and regular physical exams by a medical provider. And remember, health isn’t just about what you put in your body; it’s also important to take care of your emotional and mental health by seeking social support and managing your stress levels.

As you learn to take care of yourself, you may end up with a lower BMI or smaller waist circumference –– but it’s more important that you feel well and stay healthy, no matter what your measurements.

At One Medical, we know that discussions about weight and BMI are sensitive and can be triggering for many. That's why, visits do not always result in a weight check or even a discussion. Our providers will work closely with you to tailor your visit to your unique health needs and meet you where you're at. For questions about BMI and weight, reach out to your primary care provider.