People Underestimate Weight, Research Suggests

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The scale doesn't lie. But when it comes to admitting how much they weigh, many people tell a big white fib, finds a new study. On the other hand, the same people are a little more honest about sharing their height.

Researchers at the University College of Cork in Ireland found that the gap between obesity levels that are calculated based on self-reports of weight and height, and obesity levels recorded by measured height and weight are increasing. Both methods are used to calculate the rates of overweight and obese people. However, self-reports are routinely used for large-scale epidemiological studies and researchers debate their validity.

The Irish researchers found that obese men tend to underestimate their weight more so than overweight men, whereas both overweight and obese women underestimate their weight.

People also tend to overestimate their height, though this trend has been stable over the past 10 years, according to the study. Still, older women overestimate their height more so than younger women. And men overestimate their height regardless of their age.

Both height and weight are used to calculate Body Mass Index, or BMI, which estimates the amount of body fat a person has. More than one third of Americans (35.7 percent) are obese, meaning they have a BMI of 30 or more.  And more than one-third of Americans are overweight (33.3 percent) as well, meaning they have a BMI between 25 and 29.9.

The data in the new analysis was obtained from The Survey of Lifestyle Attitudes and Nutrition (SLAN), three national health and lifestyle surveys done in Ireland in 1998, 2002 and 2007 that questioned nearly 23,000 people 18 and older.

Between 1998 and 2007, the Irish researchers reported a decline in the sensitivity score in the overweight and obese categories, especially among obese people. The sensitivity score refers to the percentage of people with a condition who are correctly identified. According to the researchers, the sensitivity score for identifying overweight and obese people decreased from 80 percent in 1998 to 64 percent in 2002 to 53 percent in 2007, meaning fewer heavy people were accurately categorized.

There are several reasons why people may underestimate their weight. One 2011 study speculated that social norms about what constitutes an ideal weight may influence how a person answers questionnaires about how much she weighs. According to those researchers, the greater the average "ideal" weight is, the less likely people will underestimate their weight.

Researchers for the current study suggest that people simply don’t know how much they weigh or how tall they are.

It is also possible that people are in "denial of their unhealthy weight, or don't want to be labeled as obese," according to the researchers. A "more plausible explanation," the researchers wrote, is that "increases in the adiposity levels of the general population may have normalized obesity. Recent literature suggests that there is a shift in the social norm of what is regarded as overweight or obese."  Height and weight errors, they continued, "may be a result of a cognitive distortion affecting the individuals' perception of their own body shape."

 Experts have long believed that both an overestimation of height and underestimation of self-reported weight contribute equally to inaccurate estimates of BMI, according to the researchers. The new findings suggest that the self-reporting of weight is the main reason BMI is underestimated. Knowing why self-reported BMI scores are declining while clinically measured BMI scores are not "brings us one step closer to accurately estimating true obesity levels in the population," the authors wrote.

That's important in order to diagnose, prevent and treat overweight and obese people. "There is little doubt that access to reliable information is crucial to health policy makers dealing with the obesity epidemic in order that they can make appropriate public health policy responses," the researchers said.

The study is published today (Jan. 23) in the journal PLOS One.

Pass it on: Underestimating weight may skew the actual rates of overweight and obese people.

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