Does BMI Really Matter? What the Science Says

iStock/nensuria

You know your weight, your height, and your blood pressure—but you may or may not know your body mass index (BMI) off the top of your head, although medical professionals have recommended that you do. What is BMI exactly? It’s a value based on your height and weight that falls into four possible weight categories, ranging from underweight to obese. (To measure your BMI, divide your weight in pounds by your height in inches, squared, or use an online BMI calculator.) The National Health Institute considers a healthy BMI to be between 18.5 and 24.9; a number between 25 to 29.9 is considered overweight, and any number over 30 is considered obese. But how important is this number really?

UCLA researcher A. Janet Tomiyama has found that when used alone to measure health, BMI may be a somewhat arbitrary calculation. In her study comparing individuals’ BMI with other measures of health, such as blood pressure, triglycerides, cholesterol, and insulin resistance, she found that almost half of those considered overweight based on their BMI were actually metabolically healthy, as were 29 percent of those whose BMI classified them as obese. Tomiyama, an assistant professor of psychology in the UCLA College, told the UCLA Newsroom, “Many people see obesity as a death sentence, but the data show that there are tens of millions of people who are overweight and obese and are perfectly healthy.”

Indeed, doctors are continuing to explore how to measure the true impact of BMI on overall health. Most medical professionals now think it should be one of many measurements used to assess health. For example, some doctors are concerned less with your BMI number than with the distribution of the fat that is used to calculate it. Michael Roizen, MD, author of You: On a Diet, told webmd.com, “Fat around your waist is more biologically active and can do more damage to your body than the weight around your hips. The data shows that waist circumference is more reliable and more closely correlated with diseases associated with obesity.”

The bottom line is that a health measurement that once was considered to be a black-and-white indicator of health now resides in the gray area. To get a well-rounded assessment of your health, talk to your doctor about requesting lab tests such as a full metabolic panel, and consider discussing your BMI as it fits into the bigger picture of your health.