The history of BMI and why we still use it

22nd July 2009

The history of BMI and why we still use it has one of the best histories of BMI I’ve ever seen and why, despite it’s demonstrated ineffectiveness, doctors nonetheless persist in using it.  The story, while succinct, is still two pages long, so I’ll paraphrase the highlights here, supplemented with some of my own commentary:

Belgian polymath Adolphe Quetelet devised what we now know as the BMI equation in 1832 as a way to define the “normal man.”  He never intended for the equation (weight equals height squared) to be used to determine body fat — his project was intended to describe the standard proportions of the human build.  The equation was largely ignored by the medical community even though insurance companies began using somewhat vague comparisons of height and weight among policyholders beginning in the early twentieth century.   Slate writer Jeremy Singer-Vine doesn’t go in much detail about these tables, but I’ve written on their history here.

Medical researchers searched for an accurate, uniform way to measure fatness for decades when in 1972, physiology professor and obesity researcher Ancel Keys published his “Indices of Relative Weight and Obesity,” a landmark study of more than 7,400 men in five countries.  Keys considered the various height-weight formulas in existance and found Quetelet’s equation to be the best marker of body-fat percentage.  He renamed this number the body mass index or BMI.  As Singer-Vine reports, the new number caught on among researchers who had previously relied on slower and more expensive measures of body fat or on the broad and ambiguous categories defined by the insurance companies.  The number also sowed the seeds for the later and continuing bombardment of anti-obesity research.  “The cheap and easy BMI test allowed [researchers] to plan and execute ambitious new studies involving hundreds of thousands of participants and to go back through troves of historical height and weight data and estimate levels of obesity in previous decades,” writes Singer-Vine.

At first BMI was used by epidemiologists in studies of population health, but was quickly adopted by doctors who wanted a quick and easy way to measure body fat in their patients.  By 1985, the National Institutes of Health began defining obesity according to body mass index.  At first, the thresholds were established at 27.8 for men and 27.3 for women.  Then in 1998, the NIH consolidated the threshold for men and women — even though the relationship between BMI and body fat is different by sex — and added the category of overweight.  The new, drastically lowered thresholds were now 25 for overweight and 30 for obesity.  It’s worth adding here that many who were on the “independent” board making the recommendations for the new lower cutoffs had ties to the commercial weight-loss industry and stood to profit financially should more people be considered overweight and obese.

Here’s the kicker: Like Quetelet, Keys never intended for BMI to be used in this way.  In fact, his original paper warned against using BMI for individual diagnoses, since the equation ignores variables like a person’s age or gender, and I would also add, also their ethnicity, frame size and muscle mass ratio. Writes Singer-Vine:

It’s one thing to estimate the average percent body fat for large groups with diverse builds, Keys argued, but quite another to slap a number and label on someone without regard for these factors…  Now Keys’ misgivings are gaining traction across the world of medicine: BMI simply doesn’t work when it comes to individual measurements.

No matter how attentive they might be, health professionals have increasingly used body mass index to justify lifestyle recommendations for their patients. And online BMI calculators—there’s even one hosted by the NIH—invite people to diagnose themselves without any medical supervision whatsoever. Faulty readings could promote a negative self-image among healthy people and lead them to pursue unnecessary diets. Or the opposite problem: People with a little too much body fat might be lulled into a false sense of complacency by a misleading BMI.

Singer-Vine points out (as I’ve noted before) that waist-to-hip ratios are a much more accurate way of determining the kinds of body fat that might actually pose health risks. And WHR, as it’s called, is just as easy and quick to record as BMI yet few doctors have made the switch.  Why?  WHR require slightly more time and training than it takes to record BMI and they don’t come with any official cutoffs that can be used to make easy assessments. “The body mass index is cheap and easy, and it has the incumbent advantage,” concludes Singer-Vine.  “In short, BMI is here to stay—despite, but also because of, its flaws.”

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This entry was posted on Wednesday, July 22nd, 2009 at 10:26 am and is filed under Gender and Sexuality, Health, Nutrition & Fitness, New Research. You can follow any responses to this entry through the RSS 2.0 feed. Both comments and pings are currently closed.

There are currently 17 responses to “The history of BMI and why we still use it”

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  1. 1 On July 22nd, 2009, elizabethpatch said:

    thanks for posting this clear synopsis. The BMI seems so official, and its quite clear that between 1832 when it was devised and 1985, when the NIH began using it as a measure of “health” that much was lost in translation.

  2. 2 On July 22nd, 2009, Liza said:

    I had a doctor get very snippy and condescending with me when I told him I didn’t believe BMI was a good measure of weight or health (while he was telling me mine was too high). I kinda wish I had his email address so I could forward this on!

  3. 3 On July 22nd, 2009, Miss Laura Mars said:

    I just wanted to note that, even if WHR became the new standard, it still reduces people to simple formulas which allow them to be broadly categorized. These kinds of formulas discourage the medical community from taking an overall view of each person’s health, weighing all their various health indicators (including those that can be quantified like blood pressure or cholesterol, but also those that cannot, like sense of well-being and happiness).

  4. 4 On July 22nd, 2009, Rachel said:

    @Miss Laura Mars: True, but I don’t think that you can convince doctors to give up all formulas by which they measure body fat and WHR, at least, is a more reliable and accurate measure of the kinds of fat that may pose health problems. And while WHR is easy to measure, the additional time and training it requires may lead doctors to notice or become aware of those other health indicators so that they can better see one’s overall health profile.

  5. 5 On July 22nd, 2009, LonieMc. said:

    Has anyone else noticed that the validity of BMI is being questioned somewhat by the powers that be now that there are so many studies showing that overweight is the healthiest place to be?

  6. 6 On July 22nd, 2009, Susan said:

    Sandy over at Junkfood Science has quite a few articles about how waist measurment is basically a load of crap as well.

  7. 7 On July 22nd, 2009, Literate Shrew said:

    Good point, LonieMc. It’s like they’re saying “Oh, well, since overweight can’t = healthy, we should rethink what ‘overweight’ means.” Reminds me of the BMI down-shift of the 90s, in which millions were made ‘overweight’ overnight.

  8. 8 On July 22nd, 2009, Miss Laura Mars said:

    @Rachel: You make good points. I guess I just worry about the possible resultant privileging of certain fat bodies over others (hourglass/pear shape = good fat person, apple/rectangle shape = bad fat person).

  9. 9 On July 22nd, 2009, Katja said:

    Waist to hip ratio is not so good either. Please read this summary… It may be another way to use a physical measure, rather than take the time to take a history of behavior- exercise habits, exercise capacity, dietary habits, family history, smoking etc… check it out! As a former doctor, we look for something easy and quick, we should be careful…

  10. 10 On July 23rd, 2009, Rachel said:

    I’m sure WHR has its flaws — for example, if they take my WHR, they’d have to take into account the 20 pounds of loose skin I have on my abdomen — but I do think WHR is a more accurate measure of visceral, abdominal fat than is BMI. And keep in mind that WHR does not only target fat people — even thin women can have high WHR ratios. For example, I have the same WHR as Megan Fox. And while I think Sandy does a very good job at dissecting the science, I always remember to read what she writes with a grain of salt given her professional affiliations and their agendas.

  11. 11 On July 23rd, 2009, Rachel_in_WY said:

    I was a certified personal trainer in high school and as an undergrad, so I was in the industry and taking continuing education courses on these topics during much of the transition from percentage of body fat to BMI. Almost everyone I knew in the industry was mystified by the transition. It was bizarre because you would have very muscular people turning up in the overweight and obese categories while skinny-flabby people who couldn’t run half a mile without resting would get a great measurement. Before that transition, most gyms would have a checklist that included resting heart rate, how quickly your heart rate recovered after moderate exercise, and body fat percentage as well as WHR. This often revealed that people who fall into the overweight category were actually in pretty good shape, and it made it more difficult to motivate the skinny-flabby clients to develop new habits and focus on their cardio-vascular health. My sense is that now most gyms incorporate BMI into their diagnostic process but don’t use it exclusively. Too bad medical professionals can’t take a similar pragmatic approach.

  12. 12 On July 23rd, 2009, Katja said:

    Can someone tell me what Sandy’s professional affiliations and biases are? (Junkfood Science) I certainly don’t like her health care policy bias, but don’t know the background there…

  13. 13 On August 1st, 2009, Leanne said:

    My favorite is when people try to defend bmi as being “better than just looking at height and weight alone”. It seems a huge number of people fail to realize that’s all bmi is. I’ve seen so-called medical professionals make this mistake – not just average lay-people. Some of them even seem to think that bmi is somehow related to your body fat percentage. I suppose I’m particularly frustrated with it since the army relies on it so much – I’ve seen people max the army physical fitness test, but have their careers end because they struggle to meet the weight regulations, based on the bmi calculation which was never meant to apply to individual athletes.

  14. 14 On August 3rd, 2009, Liza said:

    Katja, Sandy is a registered nurse, it says so right on her bio on the blog. As well as BSN and CCP, but I’m not sure what those mean.

  15. 15 On May 6th, 2010, emmanuel said:

    I belive BMI should be used to dictate not diagnos obesity, and the only thing I found contrarily debatable was Singer’s writing about false readings having negative outcomes. Healthy people feeling otherwise, or unhealthy people becoming complacent. All in all, doctors should only diagnos idealistic health or the adverse based on the some necessary test assessments, not BMI alone.

  16. 16 On May 6th, 2010, emmanuel said:

    Also, the original article on slate is better versed. Wish I had read it first.

  17. 17 On April 18th, 2011, Titan Raines Blog » Linear Sugar said:

    [...] generations may see obesity was not about weight but a line (called body mass index) we drew in the sand and dared people to cross. And when people did, we blamed them. 0 [...]

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