Body Mass Index: Bogus

22:05 Tue 07 Jul 2009
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I’ve never paid much attention to things like body mass index, and always had an idea that it might not be completely reliable as an indicator of individual health, but apparently it really shouldn’t be trusted at all.

NPR has a ten-point demolition of the measure. Apparently the whole thing is essentially a statistical hack—and its originator explicitly stated that it should not be used for individuals. Yet I’ve definitely seen plenty of suggestions to self-check with it, and furthermore insurance companies here apparently use it to justify (what else) higher premiums for certain individuals… despite the fact that highly fit people with very low body fat will end up having allegedly “unhealthy” BMI numbers.

Bottom line: don’t use it; try to use one of the other measures (including waistline) instead.

The only problem I have with the piece is that it doesn’t address whether or not the index really is unreliable when dealing with millions of individuals. It might be statistically fair to assume that the highly fit are a tiny percentage of the population, and therefore the BMI measurement of American obesity might be almost as accurate as more expensive measurements.

2 Responses to “Body Mass Index: Bogus”

  1. Mollydot Says:

    It seems to work for me. Perhaps I am an average man…

    Another two things to watch out for – I’ve heard, more than once, that life expectancy is actually longer in the “overweight” group than in the “ideal” group. This maybe due to bias in researchers – knocking out data points from people who are thin maybe because they’re sick, but refusing to believe that people can get eat from being sick or injured.

    The category definitions have changed. So when you see than the number of obese have gone up massively since year X, that could be because of people whose weight remained the same yet went up a category between those years. I don’t remember the year, but I’m sure some googling can tell you.

  2. Tadhg Says:

    Yes, it’s not clear just to what extent obesity is rising, for the reasons you cite, but it seems clear that it is rising. As for the increased life expectancy for the “overweight” group, I had read about that, but haven’t seen a compelling explanation.

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