BMI and Your Health: How Are They Connected?

Among all the numbers your doctor takes at your appointment — like blood pressure, cholesterol, or body temperature — your body mass index (BMI) is likely the one to stick in your memory. Maybe your doctor told you that yours means you fall into the overweight or obese category, for example, and they recommended bringing your number down. Or maybe they told you that your BMI is in the normal range, which suggests that you’re at a healthy weight, or too low, meaning you’re considered underweight.

BMI can be a sensitive topic — and maybe you don’t care to pay it any mind. Some medical professionals support that choice. “Unfortunately, there is no perfect tool [to measure one’s health],” says Anisha Abraham, MD, interim chief of the division of adolescent and young adult medicine at Children’s National Hospital in Silver Spring, Maryland. Instead, there are several measurement tools, such as blood pressure, BMI, and blood sugar, that tell us different things but are pieces of a larger puzzle, she says.

Body fat percentage is another one. Frank Contacessa, MD, an internist with Northwell Health Physician Partners in Armonk, New York, says that it’s a better factor to consider in a holistic health assessment, but it’s more difficult to measure than BMI.

All the same, you may be wondering what your BMI suggests about your present and future health — and how much stock should you put your number?

Here’s an overview of how and why BMI is used, and what yours may mean.

BMI is a common measure when considering a person’s future health issues because it’s quick, easy to calculate, and free, says Linda Anegawa, MD, a Honolulu-based board-certified physician in internal medicine and obesity medicine.

To measure health risks across a population, BMI can be advantageous, especially because it’s more widely used than other weight measurement approaches, says William Yancy, MD, a professor of medicine at the Duke University School of Medicine in Durham, North Carolina.

To calculate your BMI, take your weight in kilograms and divide it by your height in meters squared. Alternatively, take your height in inches squared, divide that by your weight in pounds, and multiply by 703.

You can avoid doing manual math by plugging your stats into an online calculator, like the one from the Centers for Disease Control and Prevention (CDC). To see how your weight stacks up against your height, you can use the body mass index table from the National Heart, Lung, and Blood Institute (PDF).

Here are the ranges and how the CDC describes each:

• BMI below 18.5 is underweight (too low).
• BMI 18.5 to 24.9 is normal weight (just right).
• BMI 25 to 29.9 is overweight (high).
• BMI 30 and higher indicates obesity (very high).

There are two sides to this argument. On the one hand, many public health experts say that knowing your BMI is information in your hands, and if you can adjust it if your BMI is considered too high or low, you may reduce your medical costs and your possible risk for future health problems.

On the other hand, some healthcare professionals choose not to focus on BMI. “I do not focus on a patient’s BMI when I counsel them on being healthy,” Dr. Contacessa says. “There is no single indicator of total health. You have to look at the totality of a person, including a family history, lifestyle review, and physical exam.”

The flaws of using BMI as a measure of health are well documented.

“If BMI is the only thing your doctor uses, you should also use other measurements,” says Yancy, who advocates for knowing your waist circumference. “That’s actually a better predictor of your health, as carrying fat around your midsection is riskier than other parts of your body,” referring to visceral fat.

The more fat you have around your middle, the more your health may take a hit, regardless of BMI, echoes Patrick M. O’Neil, PhD, the director of the weight management center and a professor in the department of psychiatry and behavioral sciences at the Medical University of South Carolina in Charleston.

These details about what makes up your body weight and where on the body it’s distributed matter and are needed to determine potential health risks. “One could have a higher BMI and yet be free of health conditions,” Anegawa says. “On the other hand, some individuals with lower BMIs have very significant weight-related diseases, such as prediabetes and type 2 diabetes, hypertension, or abnormal cholesterol.”

Put differently, when it comes to BMI and your actual risk for health issues, it’s complicated.

Something to keep in mind is that a focus on BMI in the doctor’s office or in your daily life may increase weight stigma and harm your mental health. “Overfocusing on BMI can affect self-esteem and emotional wellness,” Dr. Abraham says. “For example, I have had patients who developed body image and eating disorders as a result of being told that they had a high BMI and needed to change their weight.”

Here's a closer look at what a high BMI may be associated with:

Being underweight may leave you with lower energy reserves in the event that you do become ill. “People who are acutely ill have better survival or outcomes if they had a bit of fat storage to begin with. Those who are overweight seem to do best in these situations,” says Yancy.

Just because you’re considered overweight or obese according to your BMI doesn’t necessarily mean you’re unhealthy — your healthy eating and exercising habits mean more, despite what the scale (or BMI calculator) says.

Rather than shoot for a specific BMI, what may be most important is maintaining — or starting — healthy habits.