November 2024
WHAT'S YOUR BMI?
Doctors Rethink Main Weight Metric
Critics note it doesn’t distinguish between two types of fat or identify where our fat is located.
It’s the main calculation used to assess healthy body weight and helps determine eligibility for GLP-1 weight-loss drugs. It’s also imprecise at best, misleading at worst. Body-mass index, commonly known as BMI, is a ratio of height to weight that is used to classify people as being at a healthy weight, overweight or obese. One of its biggest flaws is that it doesn’t distinguish between two types of fat (subcutaneous and visceral), or identify where your fat is located. That information is essential to understanding not just your size but your health. Visceral fat, which tends to be concentrated deep in the abdominal region around important internal organs and leads to weight gain around our belly, is the more concerning kind of fat. It influences our risk for developing problems like elevated blood sugar and blood pressure, which increase the chances of heart disease and Type 2 diabetes. Subcutaneous fat, which is the fat you can pinch just under your skin, is less detrimental to your health. BMI also doesn’t distinguish between muscle and fat, or account for differences between people of different genders, races, and ages. Just because someone is small doesn’t mean they’re healthy. And just because someone is large doesn’t mean they’re not healthy.
BMI originated in the 1800s when a Belgian scientist used data from European males to come up with average body weights. In 1972 Ancel Keys, an American physiologist who studied diet’s impact on health, evaluated the BMI and several other formulas and concluded that BMI was preferable. The metric was meant for population-level data, as it is useful for tracking obesity across large groups of people. For individuals, though, BMI is more useful in combination with other health metrics. It’s a single number and no single number is going to encompass a person’s total health situation. Too much emphasis has been placed on this health metric.
There are few sophisticated precise tests that can measure percentage of body fat and tell you how much visceral versus subcutaneous fat you have. Those tools include MRIs and DEXA scans. But such tests aren’t practical for wide use because they’re expensive, and DEXA scans expose people to small amounts of radiation. Some doctors’ offices also have bioelectrical impedance scales, which measure body composition by sending a small electric current through the body. But their reliability widely varies. A better, more realistic way to assess weight health is to use a formula that takes waist circumference into account. That gives you a better sense of visceral fat levels, because visceral fat tends to accumulate in the abdominal region. Some doctors have proposed using a ratio of waist-to-hip circumference as an alternative. That calculation may better predict metabolic health than does BMI, but its disadvantage is that it’s difficult to accurately measure. A better way to measure obesity is a calculation commonly used in England, a ratio of waist circumference to height. If that number is greater than 0.5, that means you have increased central fat and are at greater risk for health problems. Some researchers have also proposed a so-called body roundness index, which takes waist circumference into account.
BMI’s usefulness can vary depending on your race and gender. For example, Asians are born with more visceral fat. As they age, they face risks for cardiometabolic diseases at a lower BMI threshold compared with people of other races. A South Asian adult who has a BMI of 25 is not equivalent to a BMI of 25 in a Caucasian counterpart of similar gender, height and weight because of the South Asian’s visceral fat is significantly greater, making their risk for heart disease and diabetes higher. Women have about 10% more body fat than men but they have less visceral fat until they reach menopause. When women’s estrogen levels drop during menopause, their visceral fat levels increase and they tend to gain midsection weight, making their health risks equivalent to men’s. It’s not just about how we look, it’s about our health.
For more information contact your physician or Registered Dietitian.
Sumathi Reddy