More Options

Bashing the BMI: A Closer Look at the Skeptics

Ben Radford

December 31, 2012

In an effort to address America’s obesity problem, a government panel recently issued guidelines calling for routine obesity screening, including calculating their patient’s Body Mass Index (BMI). The BMI is a simple formula: (Weight in Pounds / [Height in inches x Height in inches]) x 703. A score of 18.4 or lower indicates underweight; 18.5 to 24.9 indicates normal weight; 25 to 29.9 is overweight; and a BMI of 30 or higher suggests obesity.

According to a CBS News story, “A government panel renewed a call Monday for every adult to be screened for obesity during checkups, suggesting more physicians should be routinely calculating their patients’ BMIs. And when someone crosses the line into obesity, the doctor needs to do more than mention a diet. It's time to refer those patients for intensive nutrition-and-fitness help, say the guidelines issued by the U.S. Preventive Services Task Force.”

Bashing the BMI

Yet a lot of people are not impressed; in fact the BMI had been attacked and ridiculed for years. Just how reviled is the BMI among many the public? Do a Google search for “BMI is...” and the top three returns are “bullshit,” “wrong” and “a joke.”

Why such hate for an otherwise boring, uncontroversial medical formula?

Part of the reason is that the BMI has come under attack—not so much by medical professionals (who use it on a fairly regular basis) but by fat acceptance activists who believe that the BMI unfairly discriminates against overweight people. Author and Salon.com blogger Kate Harding, for example, created a slideshow explaining why BMI is badly flawed. Many other bloggers and writers have echoed her complaints, including at Jezebel, Huffington Post, and elsewhere.

By far the chief complaint is that BMI doesn’t give accurate readings for everyone, and overemphasizes the role of weight in health. An article provided by Men’s Health magazine for the Huffington Post notes “The next time you happen to catch a Minnesota Vikings game, take a look at Adrian Peterson, the team's 6'1, 217-pound running back. Now ask yourself: what kind of physical characteristics would you attribute to him? Athletic? Lean? Fit? All of these certainly sound like valid answers to us—but his clinical classification might surprise you. By any normal standards, Peterson is one of the fittest men on the planet. But by our country's system of measuring body fat, he’s overweight.”

For many social activists and feminists the issue is not really about a height-weight ratio formula but instead what they believe the BMI represents: judgment against overweight women (and men), and an attempt to impose high beauty standards on women. As a blogger for Change.org asks in a piece typical of anti-BMI pieces, “If the BMI doesn’t...take into account any distinctions of age, race, and sex, than perhaps its at best an outmoded, ineffective, and ultimately demoralizing standard by which to judge a woman’s health?”

There are several problems with this characterization. The questions of whether the BMI is outmoded and ineffective will be addressed shortly, but suggesting that the measure is a “demoralizing standard by which to judge a woman’s health” is bizarre: The BMI is not a “standard [of] health” (for men or women); it is a measure of adiposity (fatness). Though there is a strong positive correlation between excess weight and poor health, it is quite possible to be overweight (or even obese) and healthy. Thus the suggestion that the BMI is a measure of health is patently false. And any medical measure can be “demoralizing” in some way if it does not give its user the desired results, from a blood sugar strip to a bathroom scale. The purpose of any weight scale or measure is to be accurate, not to provide comforting assurances (like the magical mirror in “Snow White,” proclaiming that its user is the fairest—or thinnest—of them all).

BMI Limitations

It is absolutely true that the BMI overestimates the amount of body fat (mistaking muscle for fat) in people like football running backs. However the critics gloss over the fact that most of us are not professional football running backs; the vast majority of people are not in categories where BMI’s validity is skewed. The limitations of the BMI are well-known to doctors; the measurement was developed as a general guideline. No medical professional would classify a patient as underweight, normal, or obese based only on a BMI score; it’s a starting point, a general guideline, not a strict rule that correctly predicts overweight in everyone.

To use another medical example, doctors issue guidelines to the public about warning signs of a stroke (including arm weakness, speech difficulty, and face drooping). These guidelines are not perfect (people experience these symptoms without having a stroke), and are not a substitute for a doctor’s medical diagnosis. Like the BMI, the guidelines were not designed to be—and never claimed to be—the best diagnostic tool out there, but instead a general rule of thumb to give the layperson a guide to their health status. Nobody would suggest getting rid of the simple stroke checklist because it does not correctly diagnose stroke in everyone (and has the potential for false positives), yet many suggest getting ride of the BMI for exactly the same reason.

Dr. Steven Novella, a Yale physician, has examined the BMI “controversy” in-depth. Writing on his Science-Based Medicine blog, Novella notes that “It is widely recognized and admitted that BMI is problematic as applied to individuals. Muscular and athletic people may have a high BMI and not have excess adiposity, for example. Also at the extremes of height the BMI becomes harder to interpret. But this does not mean the BMI is useless. In fact, for most people BMI correlates quite well with adiposity. In one study researchers compared BMI to a more direct measure of body fat percentage using skin-fold thickness. They found that when subjects met the criterion for obesity based upon BMI, they were truly obese by skin-fold thickness 50-80% of the time (depending on gender and ethnicity). When they were not obese by BMI they were not obese by skin-fold 85-99% of the time. So BMI is a rough but useful estimate, good for large epidemiological studies where more elaborate fat percentage measurements are not practical.”

Flawed Logic

For those who reject the BMI because it is imperfect, using that logic nobody should take any medical drugs or undergo any medical procedures or surgeries, since they are imperfect and have varying success rates. The usefulness of any drug or medical procedure does not depend on whether it is completely appropriate or useful for everyone. For example a drug prescribed to treat high blood pressure is not completely effective for every single patient; for most patients it will significantly improve their symptoms—but individuals vary, and it works better for some people than others. This is not a logical reason not to use them.

Another criticism is that the BMI was developed over 150 years ago, and its longevity somehow discredits it. What would become the BMI was developed around 1850 by a Belgian statistician named Adolphe Quatelet, and has been used more or less since then. This is, of course, flawed logic: if anything the fact that the BMI has been widely used for so long is actually evidence that it works—not that it doesn’t work. To use only one example of many, germ theory has been around since at least 1815 (when Agostino Bassi did experiments showing that the etiology of disease could be traced to germs), and no one suggests that germ theory is “outmoded” or incorrect merely because it’s been around for nearly 200 years.

Ironically, efforts by critics (including many feminists) to challenge the BMI’s validity often undermine the very research they promote. By challenging the validity of the BMI, they are also indirectly (but significantly) challenging the instrument validity of research that uses BMI in its methodology—often research they themselves cite in support of their claims. Hundreds of studies related to women’s body image, self-esteem, and media exposure dating back decades have used the BMI as a measure of weight in the study’s subjects. Some of those studies are widely-cited and referenced, offered as evidence, for example, that exposure to images of thin women on television encourages eating disorders (“Eating behaviours and attitudes following prolonged exposure to television among ethnic Fijian adolescent girls.” 2002. Becker A.E., Burwell R.A., Gilman S.E., Herzog D.B., and Hamburg P. British Journal of Psychiatry. June; 180:509-14.).

If the BMI really is an invalid measure as often claimed, the validity of countless studies that using that measure must be questioned. This problem has received little if any attention, likely because most people promoting social agendas don’t actually read the original studies they cite in support of their arguments and claims. They don’t examine the research for strengths, flaws, and important caveats but instead skim the abstract, or rely on news stories to tell them what the research means.

One reason that BMI is useful is that many people do not notice weight gain. Contrary to the popular idea that most people (especially women) weigh themselves constantly in fear of gaining an ounce, studies show that that a significant number of women evaluated at six-month intervals did not recognize recent gains in weight. Nearly one-third of women did not notice a weight gain of approximately 4.5 pounds over a six-month time, and one-quarter of women did not notice a weight gain of nearly 9 pounds over the same period. Such findings concern researchers because if people don’t realize they are overweight they won’t make effort to lose weight.

The BMI is also useful in that it is a calculation that anyone can make, and does not require medical knowledge nor a medical office visit. Not everyone has affordable access to medical care and private physicians, and the BMI helps poor and underprivileged people determine whether or not they are at a healthy weight.

Many of the BMI critics’ complaints are straw man arguments stemming from a fundamental misunderstanding of what the BMI is and what it claims to do. It is not a tool used by the patriarchic medical establishment for oppressing women, nor pressuring them to conform to impossible standards of beauty. It is instead a generally useful, accurate guide to helping average men and women determine their weight status.

.(JavaScript must be enabled to view this email address)

Ben Radford's photo

Benjamin Radford is a scientific paranormal investigator, a research fellow at the Committee for Skeptical Inquiry, deputy editor of the Skeptical Inquirer, and author or co-author of six books and over a thousand articles on skepticism, critical thinking, and science literacy. His newest book is The Martians Have Landed: A History of Media Panics and Hoaxes. Radford is also a columnist for Discovery News and LiveScience.com.