C&B Notes

Body Mismeasurement Index (BMI)

All-in-one measurements are often embraced because of their simplicity and ease-of-application.  Sometimes the use of these shortcuts involves a minimal trade-off in terms of accuracy and usefulness, while other times the underlying inadequacies of the measurement are so significant that they render the shortcut useless.  Some doctors (and common sense application) argue that Body Mass Index (BMI) falls in the latter category:

People classified as overweight, which are those with a BMI greater than or equal to 25 and less than 30, had a 6% lower chance of dying than those with a BMI greater than or equal to 18.5 and less than 25, considered the normal range.  The finding, which compiled and reinforced similar ones from prior studies, says less about the health risks of being overweight, and more about BMI’s flaws, according to some researchers.  “I suspect it’s primarily an issue with BMI being unable to measure the things it’s intended to measure, or what really matters,” said Francisco Lopez-Jimenez, a cardiologist and professor of medicine at the Mayo Clinic in Rochester, Minn.

Based on work by a 19th-century Belgian scientist, BMI has been the standard way to define obesity in the U.S. since 1998 and was a clear improvement over its predecessors, researchers say. Those included reading an insurance company’s mortality tables by height and weight, which was cumbersome and didn’t boil down the numbers to a single indicator.

The problem with BMI, many scientists say, is that it lumps together all body mass, including bone, muscle and beneficial fat, rather than singling out the more dangerous abdominal fat, which most researchers see as the real threat to health.  Prof. Lopez-Jimenez and other researchers are hunting for alternatives to BMI, ranging from a simple waist-circumference measurement to a type of X-ray that measures body-fat content.  The hold of BMI is powerful, however, and even if a low-cost alternative that better reflects people’s health is devised, it could take a major investment to change practice.