• Fri, Feb 1 2013

Obesity Research Skewed Because We Lie And Underestimate Our Weights

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People lie about weight in everyday situations for all sorts of reasons, but do they lie when participating in obesity research surveys? Probably — though maybe not intentionally. A new study from Ireland suggests many people are just bad at estimating their weight, height or BMI.

Nonetheless, “it may be profoundly affecting what we know about obesity epidemics,” writes Jeff Nisbet at U.S. News & World Report. Researchers use body mass index to assess whether an individual is categorized as being a healthy weight, overweight or obese in clinical data and obesity statistics.

Some BMI measurements actually take place in clinics, but much of the population’s BMI data is self-reported. So when researchers use that data to calculate obesity prevalence, it matters whether that self-reporting is somewhat accurate. Previous studies have shown people tend to overestimate their own height and underestimate their weight, and researchers have factored this a bit into their statistical analyses about rates of obesity.

But new research from the University College of Cork in Ireland shows nearly half of self-reported BMI scores for men and women in the obese and overweight categories may be wrong.

The researchers used data from three national surveys conducted in Ireland in 1998, 2002 and 2007. In 2007, 47% of people gave an incorrect weight, up from around 20% in 1998. In the course of a decade, people got significantly worse at reporting correct weight.

Based largely on such self-reports, studies routinely indicate that more than a third of Americans are considered obese and another third are considered overweight—which has all sorts of healthcare and disease prevention implications. … But if the Irish research is any indication, the situation could be considerably worse because people routinely underestimate their own weight.

The bias towards reporting a lower weight was most notable in people whose BMIs put them in the obese category.

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