Take No Prisoners

The Farce that is the Body Mass Index

I was planning a post for today on why the BMI is bullshit, but Keith Devlin of NPR beat me to it.

To summarize their arguments, the BMI is a 200-year-old mathematical hack, created by a man who wasn’t even versed in the anatomical sciences of the day, let alone what we know now. It is not a meaningful measure of fitness, obesity or anything else, and is only still in use because it sounds scientific (because there’s a formula), it’s easy to calculate and it makes it easier for insurance companies to charge fit people as if they are obese.

I agree with all of these arguments except the last one. Devlin is hinting that obesity rates in the U.S. are overestimated because of BMI’s misclassification of heavy-but-fit people. This is likely true. However, body weight is not a good indicator of overall health, in the sense that, while being obese likely indicates poor health, not being obese does not indicate good health. This deserves repeating.

Having a normal body weight does not mean you are physically fit.

There are plenty of skinny people who are unhealthy because they have a high body fat percentage, no muscle tone and terrible cardio. So when BMI is used as a general indicator of health, misclassifying fit/heavy people as obese will be mirrored by misclassifying thin/unfit people as healthy. I don’t have any numbers on the extent of these two types of errors, so it’s hard to say which is more prevalent.

Regardless, the basic point holds: BMI is bullshit. Ignore any “science” based on it. Complain to any journalist who uses it.

8 Comments

  • Posted October 19, 2009 at 1:04 pm | Permalink

    Someone put together a great BMI photo set on Flickr, illustrating what people in the BMI categories actually look like: http://www.flickr.com/photos/77367764@N00/sets/72157602199008819/ Check the cat.

  • Kavan Wolfe
    Posted October 19, 2009 at 3:39 pm | Permalink

    @DDog, interesting…

  • CaptainReality
    Posted October 28, 2010 at 4:14 am | Permalink

    I’m 175cm tall, and weigh 77kg.

    According to BMI, I’m overweight.

    Never mind that I have little pinchable fat, and bicycle 25km a day.

    What a crock of shit.

  • Posted November 7, 2010 at 2:02 pm | Permalink

    body fats can really increase so much if you are not very careful with your diet and if you do not exercise regularly *

  • Grizzleeman
    Posted July 8, 2011 at 8:49 am | Permalink

    Like Captain Reality, I’m 5’10” tall, and weigh 190 more or less. I have six-pack abs, a 48 inch chest and a 33 inch waist. I guess the 18-year-old girls who whistle when I’m running the bike path don’t understand the BMI..

  • Joe L
    Posted October 11, 2011 at 12:10 pm | Permalink

    5’9″. 210 lbs. 51 year old. 48″ Chest, 34″ waist, 20″ “pythons”. I look pretty good for an old guy. After my last annual physical, some clown I’ve never met at the Doctors office I go to called me up saying I was Pre-Diabetic and I needed to schedule a follow up and I should also get an appointment with a diet counselor. It sounded serious. I am over 50 so I figured I should go.

    As it turns out someone decided from looking at my file that I was obese. My blood work shows I’m in perfect health. Same for blood pressure and all the other lovely tests you get after 50. My regular doctor was furious and I will likely have my insurance rates go up because I’ve been tagged as obese; all because some moron with a BMI chart got a hard-on. Gee Thanks.

  • David B
    Posted November 7, 2011 at 5:59 pm | Permalink

    even 20 years ago I was amazed that such a FARCE as the BMI could pass as science and at how many doctors use it. how many healthy people are disqualified from the military and other professions and how many people have higher insurance premiums because of this FARCE.

    There are many athletic types, wrestlers, weightlifters (all muscle), are considered “obese” by this farce called the BMI.

  • h
    Posted January 20, 2012 at 12:52 am | Permalink

5 Trackbacks

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