Wednesday, January 02, 2013

Weight Propaganda

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.

12 Comments:

At 10:06 AM, January 02, 2013, OpenID Richard Allan said...

BMI was never intended for individual use. Applied to a population the differences between people's builds can be averaged out.

 
At 10:51 AM, January 02, 2013, Blogger Jeff Dorsai said...

these same type of "expert" critics told us to use margerine instead of butter decades ago based on a theory ... and killed millions of Americans with heart disease ...
this is theory overridng evidence once again ...

 
At 11:11 AM, January 02, 2013, Anonymous Robert Easton said...

I find the motivation strange here anyway. Seriously overweight people know they are overweight. And they know it's a bad thing. Even if they don't worry about the health issues, they know it is rarely considered attractive.

Slightly overweight people have no real health concerns from their weight. If anything people worry about their weight from an personal appearance perspective before they worry about it from a health one. So why do medical experts need to keep pointing out people's failings to them.

 
At 11:30 AM, January 02, 2013, Anonymous Robert Easton said...

Also on the absurdity of BMI, at 6'5 and 210 pounds, the world's fastest person http://i.imgur.com/xaDyq.jpg is on the boundary of being "overweight". At two inches shorter and the same weight, I am well into the "overweight" category, despite being fairly athletic. Athletic people may contribute a lot to "overweight" people living longer (if that's indeed true).

 
At 12:33 PM, January 02, 2013, Blogger Phil Birnbaum said...

I think BMI is based mass divided by height *squared*.

 
At 4:51 PM, January 02, 2013, Blogger Joseph Miller said...

Phil's right. Mass divided by height squared.

So not only does it not account for differences in shape, it also doesn't scale appropriately for big and small people of normal proportions.

 
At 10:16 PM, January 02, 2013, Anonymous Rebecca Friedman said...

It might also be related to the fact that illness, especially severe illness, tends to result in losing a lot of weight in rather too little time. Someone who is slightly overweight may handle that a lot better than someone who is not.

 
At 1:04 PM, January 03, 2013, Anonymous Greg Yarmit said...

Why do we care? If we want to lose (or gain) weight do we really need exact measurement and/or definition of obese, overweight and etc? You already know what is important for you. But measurements and definitions may be important if somebody will discriminate us based on these notions. Because we support liberty we should allow discrimination by any private company/individual. For example, airlines can define overweight based on total weight or your width (you share a row with some other passenger). Criterion (measurement) is up to discriminator to make (or not to make) money. We can foresee that health care services will discriminate by providing (or not providing) one or another treatment. If I go to one doctor that discriminates based on my obesity then I can change him. When we will have national (government policy based) healthcare (and we obviously will) then we will have problem with measurement and obesity definition.

 
At 11:26 PM, January 04, 2013, Blogger Eric Hennigan said...

I'm not sure the density measurement is sound either. I'm 6'1" and weigh 135lbs, and find it impossible to sink in a pool. According to the BMI I'm underweight, yet I still float, even when I exhale.

I would look more toward endurance tests, chemical balances, vitamin, cholesterol, sugar, and cortisol levels as more indicative of health. For you can be skinny supermodel and fail these less visible metrics.

 
At 12:17 PM, January 06, 2013, Blogger TheVidra said...

I think Rebecca is on to something. The first thing that popped in my mind when reading the brief headline (not here, but on a news site) about overweight people living longer is that the underweight people in this study most likely have their ranks swelled with seriously ill people. Actually, I know very few skinny people in the US (aside from a few hipster types), almost all others are either what I would consider "normal/average" or "overweight" based on a more global view of sizes. Therefore, I would come to the same conclusion unscientifically - if somebody is skinny here, they are either an outlier, or suffering with some affliction.

 
At 10:44 PM, January 11, 2013, Blogger David Friedman said...

The result reported was not that overweight people did better than underweight people but that overweight people did better than people with what was supposed to be the proper weight.

 
At 3:38 AM, January 14, 2013, Anonymous Anonymous said...

I'd be interested to see how BMI is related to the amount of physical exercise people get. From my own experience people who are more physically active tend to be heavier due to muscle mass and bulking up to give protection from the elements.

It wouldn't surprise me if many of the people who have a "healthy" BMI were like me: skin, bones and podge. I don't eat an awful lot because I never work up an appetite, but I don't think just eating more would be likely to make me healthier.

 

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