Body mass index (BMI) is considered to be “a fairly reliable indicator of body fatness,” according to the Centers for Disease Control and Prevention (CDC), but the measurement, calculated from a person’s weight and height, can often leave certain factors unaccounted for. Athletes, for example, may weigh more because of muscle, and their BMI may be higher because of it. In a recent editorial, two obesity researchers argue that using a generalized system to measure obesity is inaccurate and that looking at actual cause and effect could be more efficient at targeting obesity-related diseases.
BMI is measured as weight in kilograms divided by height in meters, squared — the result of this formula is your weight status. Having a weight status between 18.5 and 24.9 is considered normal, while having one between 25 and 29.9 is overweight, and 30 and over is obese.
Is There Such a Thing as Too Much Fat — Or Too Little?
The American Medical Association recognized obesity as a disease in June. Studies have shown that obesity increases a person’s risk for developing heart disease, diabetes, stroke, arthritis, and some cancers, according to the National Institutes of Health (NIH). But other recent studies have shown that being overweight, or even obese, could come with health benefits, and this has caused “a lot of sniping back and forth between different groups of researchers,” Dr. Rexford Ahima, a medical professor at the University of Pennsylvania and co-author of the editorial, told LiveScience.
For example, a study published in The Journal of the American Medical Association found that those with a BMI within the overweight range could end up having a longer lifespan. Researchers with the CDC discovered this after looking at 97 previous studies with data from almost three million adults. The reason? Fat stores energy that can be used to help heal the body when it’s sick, researchers said. The study also found that even though overweight people are more likely to develop serious health problems, those who are underweight with the same condition are more than four times as likely to die.
Because of such reasons, BMI is an inaccurate measure of health, Ahima says. When determining BMI, it doesn’t account for where on the body fat is located. Abdominal fat is more closely associated with causing heart disease, diabetes, and death, compared to fat around the hips. Add that to other factors unaccounted for, including race, age, and gender, and it becomes even more inaccurate. BMI is used because “it’s simple,” Ahima says, but while it may provide a “reasonable measure” of body fat for some people, it’s not for everyone, including athletes or older people who may lose height as they age.
A healthy weight is largely dependent on genetics, gender, and starting weight, among other things, Ahima says, adding that obesity is also just as complex because it’s determined not only by fat, but also by fat in relation to muscle content.
Possible Alternatives to BMI
Researchers at Oxford Brooks University in England found another way to measure health, which they say works better than BMI at determining health risks and life expectancy. Using a waist-to-height ratio, they said, better predicted life expectancy, with expanded waists correlating to lower life expectancies. For example, a man who is 5-feet-10-inches tall should have a waistline of no more than 35 inches, while a woman 5-feet-4-inches tall should have one no larger than 32 inches.
More than one-third of U.S. adults are currently overweight, according to the CDC, and almost 17 percent of children ages two to 19 are also considered obese. The NIH estimates that if someone who is obese loses even five to 10 percent of his weight, he can delay or prevent some diseases from developing.
Source: Ahima R, Lazar M. The Health Risk of Obesity — Better Metrics Imperative. Science. 2013.