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 criticizing the 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.