Doctors are rethinking the commonly used weight metric
It’s the main calculation used to assess healthy body weight and helps determine eligibility for in-demand GLP-1 weight-loss drugs. It’s also imprecise at best, misleading at worst, say doctors.
Body-mass index, commonly called 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 our 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 the 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,” says Dr. Fatima Cody Stanford, an associate professor of medicine and pediatrics at Harvard Medical School and an obesity medicine physician at Massachusetts General Hospital Weight Center. “And just because someone is large doesn’t mean they’re not healthy.”
The limits of BMI
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, says Dr. George Bray, an obesity researcher and professor emeritus at Pennington Biomedical Research Center in Baton Rouge, La.
The metric was meant for population-level data, doctors and researchers say, as it is useful for tracking obesity across large groups of people.
For individuals, though, BMI is more useful when looked at in combination with other health metrics, such as blood pressure, blood sugar levels and cholesterol, doctors say.
“It’s a single number and no single number is going to encompass a person’s total health situation,” says Dr. Wajahat Mehal, a professor at Yale School of Medicine and director of the Yale Weight Loss Program. “People put too much emphasis on it.”
Other ways to measure weight health
There are a 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 varies and more studies are needed before they are widely used, doctors say.
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 it 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 BMI does, but its disadvantage is that it’s hard to measure accurately, says Dr. Mitchell Lazar, director of the Institute for Diabetes, Obesity and Metabolism at the University of Pennsylvania.
A better way to measure obesity is a calculation commonly used in England, a ratio of waist circumference to height, says Bray. 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.
Race and gender differences
BMI’s usefulness can vary depending on your race and gender, notes Dr. Gitanjali Srivastava, a professor and co-director at the Vanderbilt Weight Loss Center in Nashville, Tenn.
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, she says.
“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, sex, height and weight, because his or her [visceral fat] is significantly greater,” says Srivastava, 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, doctors say. A woman whose BMI has stayed the same after going through menopause might still be less healthy than she was earlier if more of her fat is visceral.
“It’s not just about how we look, it’s about our health,” says Stanford.
Source: Wall Street Journal
