Jan 05 2011
It seems that for every established science there is an ideological group who is motivated to deny it. Denialism is a thriving pseudoscience and affects any issue with the slightest political or social implications. Sometimes, even easily verifiable facts can be denied, as people seem willing to make up their own facts as needed.
Denialists have an easy job – to spread doubt and confusion. It is far easier to muddy the waters with subtle distortions and logical fallacies than it is to set the record straight. Even when every bit of misinformation is countered, the general public is often left with the sense that the topic is controversial or uncertain. If denial is in line with a group’s ideology, then even the suggestion of doubt may be enough to reject solid science.
We see this when it comes to the effectiveness of vaccines, the evolution of life on earth, and anthropogenic global warming. A recent Pew poll shows that the campaign of global warming denial has been fairly successful – while the science becomes more solid around the consensus that the earth is warming and humans are contributing to this, the public is becoming less convinced.
I often encounter denial even when it comes to simple things, like body weight. You would think that the question of how many Americans are overweight or obese would be fairly straightforward, but no data is so straightforward that it cannot be distorted by dedicated ideologues. For example, in my recent post on diet a commenter made this argument:
One important fact in this discussion is that in the 1990′s the government revised the way they determine obesity and being overweight and overnight the number of people who were obese doubled. Those who want to use this as “proof” that obesity is rampant do so fully aware that they are misrepresenting the facts. In fact obesity has not statistically increased in the dozen or so years since that change and for men has decreased slightly.
There is so much wrong with this one paragraph that it will take me the rest of the post to deal with it. It is a horrendous straw man – the conclusion that overweight and obesity in this country, and generally in industrialized nations, has been on the increase is not at all based upon this one-time redefinition of these categories.
But let’s back up a bit and give some background.
Body Mass Index
The terms overweight and obesity have had various definitions in the past, but in recent years the various health organizations have settled on consensus operational definitions (for obvious practical reasons). Their definition relates to body mass index, which is a person’s weight in kilograms (kg) divided by their height in meters (m) squared.
It should be noted that BMI is a measure of weight, not fat (adiposity). BMI is used for convenience, as height and weight data are often available but more direct measures of body fat are not. It is widely recognized and admitted that BMI is problematic as applied to individuals. Muscular and athletic people may have a high BMI and not have excess adiposity, for example. Also at the extremes of height the BMI becomes harder to interpret.
But this does not mean the BMI is useless. In fact, for most people BMI correlates quite well with adiposity. In one study researchers compared BMI to a more direct measure of body fat percentage using skin-fold thickness. They found that when subjects met the criterion for obesity based upon BMI, they were truly obese by skin-fold thickness 50-80% of the time (depending on gender and ethnicity). When they were not obese by BMI they were not obese by skin-fold 85-99% of the time.
So BMI is a rough but useful estimate, good for large epidemiological studies where more elaborate fat percentage measurements are not practical. However, those who wish to deny the “obesity epidemic” have found BMI to be a convenient target for sowing doubt.
There is ongoing research into the utility of supplementing BMI with other easy measures, like waist circumference. This seems to be a more accurate measure of adiposity, and specifically risk from being overweight.
Overweight and Obese
Because BMI is a convenient measure, it has become the measure of choice in defining overweight and obesity. For children and adolescents overweight is defined as a BMI in the 85-95% percentile by age and gender, while obesity is >95% percentile BMI. For adults overweight is defined as a BMI of >=25.0 but <30.0, obese is defined by BMI >=30.0 and < 40.0, and extremely obese is defined as BMI >=40.0.
These cutoffs, like all such cutoffs for medical definitions, are partly arbitrary (they constitute drawing a line to demarcate a spectrum) but are evidence- based. This is similar to definitions for hypertension, for example. Researchers typically will set the cutoff to capture most people who are at risk for medical complications.
This is where the controversy comes into play with overweight and obesity. In 1998 the NIH decided to lower the cutoff for BMI for overweight, from 28 for men and 27 for women to 25 for both sexes. This was based upon an expert panel review of hundreds of studies. It also brought the NIH definition in line with the World Health Organization and other health organizations. The BMI 25 cutoff has now become generally accepted. The cutoff for obesity was not changed – it was and remains a BMI of 30.
Of course this means that any estimates of overweight (but not obesity) based upon the newer lower cutoff of BMI 25 would be greater than estimates based upon the previous criteria. This raised a bit of a kurfuffle, as it always seems to do when medical definitions are altered. This happened with the lowering of the cholesterol cutoff, blood pressure for hypertension, and blood sugar for diabetes.
This event in 1998 now has become a central argument in the arsenal of obesity deniers, and is evidenced by the commenter above. If you search on “obesity statistics”, on the first page you will get this apparent libertarian site which quotes a “food industry spokesman” as saying:
In 1998, the U.S. Government changed the standards by which body mass index is measured. As a result, close to 30 million Americans were shifted from a government-approved weight to the overweight and obese category, without gaining an ounce, Burrita said.
This is slightly misleading, as the obese category was not changed. But the main point is that this 1998 redefinition is being used to argue that the obesity epidemic is all smoke and mirrors. The article goes on to quote this gem from William Quick:
According to an American Medical Association report, 14.5 % of Americans in 1980 were obese, a total of 32,700,000 (based on a population of 226,000,000). If, as the above article states, the numbers of obese Americans have “doubled” in the past twenty years, this would mean there are now about 66 million of them. But thirty million of those fatties were created by a change in definition, so by the standards of 1980 [we would calculate an] obesity percentage of 12.85 percent, an actual decrease in obesity percentage since 1980.
That’s some massively flawed reasoning. Again we see the confusion of the overweight and obese categories (as with the commenter’s statement above). But also there are many false assumptions in that back-of-the-envelope calculation. Quick is mixing statistics from different sources and contexts, and the result is a mess.
What we really need is a look at the numbers over time using the same definition. Fortunately, most epidemiologists are not dolts and they get this very basic concept. In fact, it doesn’t get much more basic than this, and it would take some pretty naive incompetence to use inconsistent definitions over time.
The CDC has crunched the numbers for us, and using the modern cutoffs for overweight, obese, and extremely obese applied to BMI data for the last few decades they document a pretty steady increase in American fatness over time. Take a look at the video on the site to see this data presented graphically. Also, it is summarized in the graph here.
As you can see, the lines go steadily up – with the exception that the overweight category has decreased in the last decade. However, it seems that this is due to the shifting of people from the overweight category to the obese category, not to the normal weight category.
Of course, you could cherry pick by just looking at the overweight category. Looking at all the data, however, tells the real story.
The bottom line is that there is an obesity epidemic. The data is clear. There is always complexity to exploit, if one is motivated to sow confusion, and the obesity data is no exception.
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