The problem with BMI: Why this simple system for measuring obesity is so controversial

Watch the CBS Reports documentary, "Speaking Frankly | Fat Shaming," in the video player above. 

It's a simple little tool that's been used by doctors for decades, but the system of measuring Body Mass Index, or BMI, is increasingly coming under fire. Critics have denounced BMI as not just unreliable but sexist and racist, and even many clinicians who see value in it also recognize its flaws. 

BMI is a formula that uses a person's height and weight ratio to assess whether they're underweight, normal weight, overweight or obese, although it is not a direct measure of body fat. First invented nearly 200 years ago by the Belgian mathematician, astronomer and statistician Adolphe Quetelet, it was originally known as the Quetelet Index until American physiologist Ancel Keys came up with the term Body Mass Index in 1972. 

According to the Centers for Disease Control and Prevention, BMI is an "inexpensive and easy screening method" that is "strongly correlated" with weight-related medical conditions — serious health problems including heart disease, stroke, type 2 diabetes, sleep apnea and breathing problems, and numerous types of cancer. About 40% of U.S. adults are considered obese.

But in recent years, doctors, scientists and dietitians have expressed concerns over the shortcomings of BMI. Though it's used to diagnose obesity — defined as having a BMI of 30 or  above — it does not provide any information about how weight actually impacts an individual's health. 

"The problem with the BMI, while it does adjust for height, it does not adjust for what makes up the weight," said Dr. David Katz, an expert in preventive medicine and founding director of the Yale-Griffin Prevention Research Center. "So somebody who's extremely muscular could have a high BMI and yet have a very low percent body fat, which is what you're really concerned with. Somebody who has very little musculature could actually register as lean on the BMI scale, and still have an excess body fat."

Many professional athletes are considered clinically obese based on the BMI scale, because it cannot distinguish the weight of muscle from the weight of fat. 

Dr. Lindo Bacon, a physiologist based in California, says people considered "normal weight" based on their BMI can also suffer from diseases like diabetes and heart disease, which are typically associated with excess weight. 

"I think that this has been one of the most damaging tools in medicine today because what we know is that it's a very poor predictor of health and that, for example, there are many people that are in the overweight and obese categories that live long disease-free lives," Bacon told CBS Reports. "But yet, they're put into these categories and told that they're unhealthy and told to do a lot of damaging things that then get in the way of their health."

A group of Canadian doctors published new guidelines this month urging physicians to rethink the way they care for patients with obesity. Among other things, they suggested that BMI should not be used alone as a tool to evaluate health. "Although BMI is widely used to assess and classify obesity (adiposity)," they write, "it is not an accurate tool for identifying adiposity-related complications."

Some researchers are advocating for a new kind of classification system based on the concept of Adiposity-Based Chronic Disease (ABCD) — focusing more on the health issues associated with obesity rather than body size alone. It would incorporate BMI as just one of several considerations to help determine a more specific and individualized course of action for patients.

Other researchers cite studies over the years that indicate it is in fact a useful predictor of health outcomes, especially for those with very low or high BMIs. Harvard's T.H. Chan School of Public Health acknowledges that while "BMI is not a perfect measure," and tends to overstate levels of fat in people who are muscular or athletic, "most people are not athletes, and for most people, BMI is a very good gauge of their level of body fat." Tracking BMI over a person's life can provide important insight into whether the person is at a healthy weight.

Despite its shortcomings, Katz says BMI still has some value when looking at obesity trends more broadly.

"The BMI is a crude but useful tool," he explained. "Here's why the BMI is a useful epidemiologic ally: because we do not have an epidemic of muscularity in the United States … we do have an epidemic of obesity. So when you look at populations and you look at trends in BMI, does it tell you what's happening to the obesity epidemic? Absolutely. At the population level, the highly muscular group is a rounding error."
 
But on an individual level, Bacon argues categorizing people based on their BMI does them a disservice, and that we shouldn't rely on it. 

"We really need a much more sophisticated, nuanced discussion that's going to help people to be healthier, and it's really quite damaging when we make assumptions based on BMI," said Bacon. "There are a lot of other ways to help to figure out whether or not people are healthy that give us much more valuable information than BMI ever can about health."