A recent article
in the Journal of the American Medical Association reports that being
overweight, as defined by
body mass index, may be good for you—that people in the recommended BMI
range are more likely to die ("all cause mortality") than people whose
weight classifies them as overweight but not obese. What I found most
interesting about the
news coverage of the article was the reaction reported—people quoted as
article without offering any good reason to think it was wrong. From the
USA Today story
Walter Willett, head of the department of nutrition at the Harvard
School of Public Health, says the findings are "complete rubbish"
because the methodology used in the analysis seriously underestimates
"the hazards of being overweight and obese."
Steven Heymsfield, one of the authors on the accompanying editorial in
the journal and the executive director of the Pennington Biomedical
Research Center in Baton Rouge. "We don't really know the ideal weight
for a long life and optimal health. Science is still working that out.
But falling in the normal, healthy weight range is still the safest
place to be."
Other people offered possible ways of
explaining away the result, such as the suggestion that overweight
people got more medical attention, but no actual evidence. My impression
was that in this case, as in the case of evidence in favor of the
moderate consumption of alcohol that I discussed
some time back,
there is an official truth and a tendency to discount evidence against
it. The Heymsfield quote is a nice example of one way of doing so, since
it appears to endorse the orthodox view of what people should do while
actually saying nothing: Falling in the healthy weight range is the
safest place to be—and we don't know what the healthy range is.
A related point is the popularity of
the body mass index, along with the use of objective sounding terms
("overweight," "obese," "obesity grade 1," ...) for arbitrary
categories. The nice thing about BMI is that it is easy for anyone to
calculate, since it is merely weight divided by height. The problem is
that it is a poor measure, since people differ in other relevant
dimensions, most notably in how wide they are. If your objective to
produce accurate information relevant to health, you would want to take
that into account. But doing so makes it harder to pressure other people
into losing weight, since the more complicated the measure, the easier
it is for people who don't want to diet to explain away their weight.
The friend I am currently visiting with tells me that he was offered, by both a doctor and a physical trainer, a simple rule for telling whether you are overweight: Take a deep breath and jump in the water. If you float you weigh too much. The theory, presumably, is that the test measures your average density, that fat is less dense than muscle which is (I'm guessing) less dense than bone, so the test is measuring the relative amount of each in your body. It doesn't work for women, since even women who are not overweight are likely to float.