Dr. Andy

Reflections on medicine and biology among other things

Sunday, April 24, 2005

Weight and Mortality III: BMI and "normal weight"

I know I'm blogging a lot about a single study that isn't even in an area of my expertise, but I think this a huge deal whose import has yet to be recognized.

I think both BMI and the definition of "normal weight" as a BMI of 18.5-25 need to be revisited.

BMI (weight in kg/ height in m^2) is a rather crude measure of obesity. I'm 6'2" and 210 right now, and while I'd like to lose 10 or 15 lbs, I don't think many people would say I'm fat. Someone else might be significantly overweight with the same statistics. On the other hand, there are professional athletes who are 6'2" 225 and have 4% body fat.

BMI may be a useful crude measure for epidemiologic studies, but I don't think it really captures how fat someone is. I speculate that people have gotten more muscular over the last 30 years as weight work and strength training have come into vogue for everyone, not just football players and weight lifters, but I have exactly zero data to back that up.

At this point I think it is past time to readjust what is considered "normal weight." According to the discussion in the JAMA study:
In many studies, a plot of the relative risk of mortality against BMI follows a U-shaped curve, with the minimum mortality close to a BMI of 25; mortality increases both as BMI increases above 25 and as BMI decreases below 25,
While, I don't agree with those who think this has all been a big conspiracy, you have to wonder why the "normal weight" category starts at the low point of mortality and goes down, not both ways.

I'd suggest the categories be recalibrated as follows:

<18.5 severely underweight
18.5-22 underweight
23-28 normal
28-33 overweight
>34 obese

Even better would be to go back and use actual data to draw lines, but these categories would probably correspond much better to actual mortality risk. And actual size as well. 57% of the population in the most recent survey were above average weight.

UPDATED reformatted new categories


At 10:39 AM, Blogger Primer said...

Thanks for the mini-series of posts on this JAMA article.

It appears that the news media is overinterpreting this study. It would be a shame if the take home message, "that it's okay and perhaps beneficial to be overweight," was promoted.

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