Despite the increasing awareness of the problem of obesity in the United States, most Americans don't know whether they are gaining or losing weight, according to new research from the Institute for Health Metrics and Evaluation (IHME) at the University of Washington.

Obesity increased in the US between 2008 and 2009, but in response to the questions about year-to-year changes in weight that were included in the most widespread public health survey in the country, on average, people said that they lost weight. Men did a worse job estimating their own weight changes than women. And older adults were less attuned to their weight changes than young adults. The findings are being published in the article "In denial: misperceptions of weight change among adults in the United States" in the August edition of Preventive Medicine.

"If people aren't in touch with their weight and changes in their weight over time, they might not be motivated to lose weight," said Dr. Catherine Wetmore, the lead author on the paper. "Misreporting of weight gains and losses also has policy implications. If we had relied on the reported data about weight change between 2008 and 2009, we would have undercounted approximately 4.4 million obese adults in the US."

A range of public health campaigns in recent years have urged Americans to lose weight to lower their chances of developing heart disease, diabetes, and other chronic conditions. To understand whether people in the US are heeding this advice, Dr. Wetmore, a former Post-Graduate Fellow at IHME and now a biostatistician at Children's National Medical Center, and IHME Professor Dr. Ali Mokdad compared self-reported changes in body weight between 2008 and 2009.

They used data from the Behavioral Risk Factor Surveillance System (BRFSS), a yearly cross-sectional survey of adults in the US designed to monitor leading risk factors for morbidity and mortality nationwide. More than 775,000 people were surveyed in the years analyzed, and they were asked multiple questions about their weight, including how much they weighed on the day of their interview and how much they weighed one year prior to their interview.

The researchers found that, on average, American adults gained weight over the study period -- because the reported weights increased between the 2008 and 2009 surveys -- but the 2009 study participants told surveyors that they had lost weight during the previous year. Based on the weights they reported, the prevalence of obesity in the US would have declined from 2008 to 2009. Instead, the prevalence of obesity inched upward from 26% to 26.5%, and average weight increased by about one pound per person between 2008 and 2009.

"We all know on some level that people can be dishonest about their weight," Dr. Mokdad said. "But now we know that they can be misreporting annual changes in their weight, to the extent of more than two pounds per year among adults over the age of 50, or more than four pounds per year among those with diabetes. On average, American adults were off by about a pound, which, over time, can really add up and have a significant health impact."

Not everyone reported losing weight. The researchers found that reports of unintentional weight gain were more common in specific groups:

    men and women under the age of 40
    those identifying as black, Native American, or Hispanic
    current and former smokers
    those consuming less than five servings of fruits and vegetables per day
    those reporting no physical activity
    those with diagnosed chronic diseases, frequent poor mental health, and insufficient sleep
    those lacking health care coverage

"It's very popular right now to talk about the underlying environmental causes of obesity, whether it's too much fast food or not enough parks," Dr. Wetmore said. "While we know that the environment definitely plays a role, these results show that we need to do a better job helping people to be aware of what's going on with their own bodies."