Q: I finished reading your Skeptical Inquirer article1 on BMI (“Bashing the BMI: A Closer Look at the Skeptics”). It’s a good piece; however, I feel compelled to point out a logical flaw. You wrote, “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.” Data show something works and is useful, not time. The same could be said about homeopathy, mega dosing, acupuncture, and so on.
—G. Moffitt
A: The Body Mass Index (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. My article explained why the BMI has been ridiculed for years—not by medical professionals, who use it on a regular basis, but by others.
By far the chief complaint is that BMI doesn’t give accurate readings for everyone, and overemphasizes the role of weight in health. … 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, and no one suggests that germ theory is “outmoded” or incorrect merely because it’s been around for nearly 200 years. (Radford 2012)
As I wrote that, it occurred to me that someone might take me to task (and I appreciate readers not letting me down), but I think it’s valid. On the surface it might seem like an argument from antiquity logical fallacy (also known as the appeal to tradition), which suggests that something is good or proven useful merely because it is old, traditional, or has “always has been done.” The implicit argument is that, all else being equal, people wouldn’t keep doing something if it didn’t work.
But the context here is important: the BMI is not used among soothsayers, acupuncturists, or homeopaths; it is used in the medical establishment. It is true that data, not time, determine whether something is useful, but data and time are not independent factors. They are inextricably linked.
It is true that now and then a drug or medical procedure is, in retrospect, found to be no more effective than a placebo. But when it is, medicine addresses it, and the drugs and treatments fall by the wayside and are discredited—usually within a few years. Because of this, medical practices (such as the BMI) are likely to be valid if implemented for over a century. I did not suggest that the BMI’s longevity by itself proves its efficacy or validity—just that it lends some support to that assumption.
The examples offered, such as homeopathy and acupuncture, are a bit different because the argument from antiquity is typically offered in place of—not in addition to—published research demonstrating the efficacy of the treatments (patients hear, “Of course this works; it’s been used for centuries!” not “Of course this works; it’s been scientifically tested for centuries. Look at these studies!”). Not only have they not been rigorously tested throughout their use, but they aren’t as old as their proponents often claim.
There’s another important difference between homeopathy and acupuncture and the BMI: The BMI is a measurement, while the others are interventional therapies. You can do randomized, blinded, controlled clinical trials on acupuncture, vitamin megadoses, homeopathy, etc.; either the outcomes of interest (such as blood pressure, pain, or disease) clinically improve, or they don’t.
The BMI is different; it’s a straightforward calculation, so there’s really nothing to test or “disprove” about the BMI itself. One can of course challenge whether being overweight or obese is a significant health risk, but that’s a different question and not specific to the BMI (as opposed to other ways of measuring body fat such as calipers, hydrostatic weighing, bioelectrical impedance analysis, etc.). Dozens of studies, and several meta-analyses, of the association between BMI number and health have consistently demonstrated that excess weight is indeed unhealthy. For example, “This meta-analysis provides further evidence on the impact of obesity on both aspects of health-related quality of life” (Ul-Haq et al. 2013); “We document, once again, excess mortality associated with obesity” (McGee 2005); and “The associations of both overweight and obesity with higher all-cause mortality were broadly consistent in four continents” (Global BMI Mortality Collaboration 2016).
As I noted in my article, the limitations of the BMI are well known to doctors. No competent doctor 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. The BMI’s long-term use—coupled with a raft of peer-reviewed data, of course—suggest it’s a useful measure of health.
Note
1. This article was an online only exclusive for skepticalinquirer.org.
References
Global BMI Mortality Collaboration. 2016. Body-mass index and all-cause mortality: Individual-participant-data meta-analysis of 239 prospective studies in four continents. Lancet 388: 10046.
McGee, D.L., Diverse Populations Collaboration. 2005. Body mass index and mortality: A meta-analysis based on person-level data from twenty-six observational studies. Annals of Epidemiology 15(2): 87–97. DOI: 10.1016/j.annepidem.2004.05.012. PMID: 15652713.
Radford, Benjamin. 2012. Bashing the BMI: A closer look at the skeptics. Skeptical Inquirer Online (December 31). Available online at /exclusive/bashing-the-bmi/.
Ul-Haq, Z., D.F. Mackay, E. Fenwick, et al. 2013. Meta-analysis of the association between body mass index and health-related quality of life among adults, assessed by the SF-36. Obesity 21(3): E322–327.