Does this state make my butt look big?

A study published this month in the journal Obesity reports that the largest percentage of obese people in the United States live in the Great Plains, not in the South, as surveys have long indicated.

Researchers found 41 percent obesity in a census region that includes Kansas, Minnesota, Missouri, Iowa, Nebraska, and the Dakotas, while the East Central South region—Alabama, Mississippi, Tennessee, and Kentucky – weighed in with 31 percent obesity. Mississippi and Alabama have long ranked first and second as the most obese states in the nation, according to data compiled by the CDC.

The study suggests the dubious honor of being the fattest region in the U.S. should go to the nation’s breadbasket, not to the buckle of its BBQ belt.

“That’s a pretty big difference,” said senior author George Howard, Dr.P.H., chair of biostatistics at the University of Alabama at Birmingham. “Don’t get me wrong, 31 percent obesity is not good, but it’s not the worst.”

The difference is important, too, because these kinds of rankings often help determine which states get federal public health dollars for research and anti-obesity campaigns.

How could this happen? Blame a problem that bedevils all kinds of research: self-reported data.

When you rely on people to tell you about themselves, all kinds of factors, conscious and unconscious, can distort the answers.  We’re particularly bad at answering truthfully when self-image or values or are at stake.  Feelings about body size tend to be influenced by both.

A 2007 review of studies that have compared self-reported to measured heights and weights found that people tend to overestimate their height and underestimate their weight. Those reports are further skewed by gender.

“Women tend to apparently under report their weight by more than men do. But men tend to over report their height more than women do. And both of those have the impact of reducing your BMI,” Howard said.

All this fibbing might be fine as long as everybody stuck to the same story everywhere. At least then, self-reported estimates of weight would be skewed in a reliably biased way. But Howard said that’s not what seems to be happening across the United States.

He thinks southerners may have been unfairly branded the biggest simply because they feel less ashamed of their size, and are thus more likely to be truthful about it.

“What we were thinking is that perhaps it’s not as much of a social stigma to be obese in the South, so when you’re asked how much you weigh, you get pretty close to telling truth, but when you’re from Minnesota or California or something it’s not as socially acceptable to be obese,” he said.

Howard discovered the discrepancy as he tried to reconcile data from a long-running national study of stroke with numbers from the CDC’s Behavioral Risk Factor Surveillance System (BRFSS).

In the REGARDS study, which stands for Reasons for Geographic and Racial Differences in Stroke, participants are weighed and measured in their homes. The BRFSS is a telephone survey that relies solely on self-reported information.

Both are respected large studies. REGARDS has enrolled about 30,000 people across the country. BRFSS surveys roughly 300,000 people in the United States each year. What’s more, they both start with the same list of people to recruit new participants, a national database called GENESYS which is maintained by a company called Marketing Systems Group.

Howard said the differences in the regional obesity rates reported in the two studies had him scratching his head.

“We spent about a year thinking that we’d messed up somehow and trying to figure out how,” he said. “I kept asking myself, ‘what have we done to differentially sample thin people in the South and fat people elsewhere?’”

Then he came up with the idea of using a third study as a tiebreaker. He picked the National Health and Nutritional Examination Survey. Like REGARDS, NHANES relies on scales, not self reporting, to measure weight and height.

“NHANES lines up almost perfectly with us,” he said. “If we’re wrong, NHANES is wrong, too.”

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