Dear Doctors: I’ve heard there’s a test that’s more accurate than BMI. Do you know what that is? My daughter is 17 years old, plays tennis, lifts weights and has a sporty build. She’s not overweight, but her BMI of 26 says she is. It upsets her, and it really doesn’t make sense.
Dear Reader: The BMI is a measure known as the body mass index. It is a ratio of a person’s height and weight and is used to estimate body fat percentage. This, in turn, is used to indicate if someone is underweight, overweight or obese.
While fine in theory, in the real world, the BMI falls short. That’s not too surprising, considering the method dates back to 1832, when a Belgian mathematician set out to create a metric to define a “normal man.” Over the decades, the original calculations underwent subsequent tweaks, each time using data based solely on the male body. Those adjustments gradually led to the modern-day BMI and its role as a factor in assessing overall health, well-being and disease risk.
Using only height and weight to estimate body fat percentage leaves out important variables. These include muscle mass, bone density, frame and overall body composition. As has happened in your daughter’s case, someone who is athletic can register higher on the BMI scale. That’s because muscle tissue is denser than fat and weighs more by volume. That’s not a distinction BMI calculations can make. Also excluded are factors such as age, sex and ethnicity, each of which play a role in body fat percentage.
Another important variable regarding body fat is its location. Fat that sits deep in the body, surrounding the organs contained within the abdomen, is known as visceral fat. It is metabolically active in a way that contributes to chronic and systemic inflammation. Visceral fat, which results in a rounded belly and a large waistline, is associated with metabolic disease, insulin resistance, diabetes risk, cardiovascular complications and an increased risk of certain cancers.
The many health risks associated with abdominal fat, as well as the significance of body fat distribution, have led to the update to the BMI that you are asking about. Known as the body roundness index, or BRI, it shifts the focus to visceral fat. Instead of a ratio of someone’s height and weight, the BRI calculation uses their height and their waist circumference. The resulting measurement, which ranges from 1 to 15, reflects how round someone is around their middle.
A recent study analyzed two decades of health data gathered from 33,000 adults. It found the BRI to be more accurate than BMI at assessing whether someone is overweight, obese or underweight. Each of these is associated with an increased risk of disease and death. When BRI scores reached 7 and higher, the risk of developing cardiovascular disease, cancer and metabolic disease increased. People with low scores -- 3.4 and below -- were also at higher risk for illness and death.
Although an improvement over the BMI, the body roundness index is also just an estimate. It should be used alongside blood pressure, blood sugar and blood lipid levels, important metrics of good health.
(Send your questions to askthedoctors@mednet.ucla.edu, or write: Ask the Doctors, c/o UCLA Health Sciences Media Relations, 10960 Wilshire Blvd., Suite 1955, Los Angeles, CA, 90024. Owing to the volume of mail, personal replies cannot be provided.)
