Thursday, January 3, 2013

Obesity studies

You may have read yesterday about a study published in the Journal of the American Medical Association concluding that, although obesity overall was associated with higher mortality rates, overweight (identified by Body Mass Index, or BMI) was associated with statistically significant lower mortality rates. This conclusion has been trumpeted as vindication for the view that we are, perhaps, overfocused on the importance of weight loss for good health. See, for example, here, here, and here.

This conclusion depends, of course, on BMI being a good proxy for mortality risk. There's some evidence that it's not. One drawback, according to Well-Being Wire, is that BMI fails to account for an individual's exercise habits - you can be fit but fit with a high BMI, and slender but sedentary with a low BMI. In those cases, BMI won't tell you who is healthier.

In any case, the study is not an endorsement of heaviness or weight gain. The authors conclude: 
Relative to normal weight, obesity (all grades) and grades 2 and 3 obesity were both associated with significantly higher all-cause mortality. Grade 1 obesity was not associated with higher mortality, suggesting that the excess mortality in obesity may predominantly be due to elevated mortality at higher BMI levels.
And even the next sentence, "Overweight was associated with significantly lower all-cause mortality," needs to be read with some skepticism. As Lindsay Abrams writes at TheAtlantic's web site:
the study fails to take into account any of the various other measures used to assess health. It ignores blood pressure, blood sugar, and cholesterol -- high levels of all are directly associated with a variety of chronic conditions and diseases -- not to mention mental health and life satisfaction scores.  
She provides some interesting pictures that make the point about BMI, and concludes:
While in the most basic of ways, it makes sense to pay attention to the number on the scale, it only gives us one metric of health that, if not understood in context, is basically useless. If we could get used to looking at weight more holistically, in terms of overall health, the link between BMI and longevity wouldn't be so shocking.
So read the study. Think about it. And then continue to work out.

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