Between the models who are too skinny by health standards and the many American women whose expanding waistlines put them in the obese category, how can you reliably judge your own weight? 

Maybe you can't.

In a new study, nearly 25 percent of overweight and obese women rated themselves as normal or even underweight, while a good chunk of female participants who were normal or underweight reported practicing dieting behaviors, some of them  unhealthy, to peel off the pounds. [7 Diet Tricks That Really Work]

Researchers from the University of Texas Medical Branch (UTMB) at Galveston said that both body-size skews — people oblivious to their excess weight, and people who mistakenly think they're fat — are cause for concern.

"Overweight individuals who do not recognize that they are overweight are far less likely to eat healthfully and exercise," said lead author Dr. Abbey Berenson, professor in the department of obstetrics and gynecology, who is also director of the Center for Interdisciplinary Research in Women's Health. "These patients are at risk for cardiovascular disease,  type 2 diabetes and other serious problems."

One of the reasons for the gap between perception and actual poundage: "If more people are overweight who are around you, you're more likely to perceive it as normal," Berenson told LiveScience.

"I'm not fat"

Berenson and UTMB colleague Mahbubur Rahman analyzed survey results from more than 2,200 U.S. women ages 18 to 25, who answered questions about self-perceived weight, actual height and weight, and socio-demographic variables. For instance, the survey asked, "How would you describe your weight?" and participants were given the choice of the following answers: "very underweight, slightly underweight, about the right weight, slightly overweight and very overweight."

Participants also reported the number of days over the previous week they had exercised for at least 30 minutes continuously, as well as their unhealthy weight-related behaviors over that stint, including using diet pills, diet powder or diet liquids; laxatives or diuretics; inducing vomiting; skipping meals; dieting/eating less or differently; smoking more cigarettes (which suppresses the appetite); or avoiding carbohydrates.

The participants were categorized according to their body-mass index as normal weight. Overall, 52 percent of the study participants had BMIs they classified them as overweight or obese.

The shares of overweight Hispanics and African-Americans who thought of themselves as normal-weight (nearly 25 percent and 30 percent, respectively) were significantly greater than for white respondents. Fifteen percent of the overweight white women thought they were normal or underweight.

At the other end of the spectrum, 16 percent of the white women and 20 percent of the Hispanic women who were either normal or underweight thought they were overweight.

Individuals who didn't know they were fat were significantly less likely than others to engage in healthy or unhealthy weight-related behaviors. Meanwhile, those who mistakenly thought they were overweight were more than twice as likely as women who recognized their normal weight to diet, skip meals and smoke more cigarettes.

Guys, gals and weight

The study focused on young women partly because those were the clientele at the family planning clinics where the researchers sought volunteers. "In addition this age population has been shown to be at increased risk for behaviors that put their health at risk," Berenson said.

Even so, guys aren't immune. Based on research published in 2009 in the journal Epidemiology, men are even more prone to thinking their weight is fine when in fact it meets criteria for overweight or obese. For instance, nearly half the overweight men over 20 years old said they were "underweight" or "about the right weight," compared with less than a quarter (22 percent) of women. For obese men, 12 percent reported being underweight or normal weight, compared with 4.5 percent of women in that study.

Berenson suggests media could be partly to blame for the misperceptions. You'd be much more likely to see a fat man in a leading role than a pudgy woman, she said during a telephone interview, adding that being an overweight man seems to be more acceptable than being an overweight woman.

Adults don't just misjudge their own weight, they also point their blurry lenses at their kids. In a study published in June in the journal Clinical Pediatrics, researchers found 71 percent of participating parents with overweight or obese toddlers misperceived their child's weight as either healthy or lighter than healthy.

To keep people straight regarding their body weights, Berenson suggested doctors treat BMI just as they would another routine screening, like blood pressure. Just as a doctor might let you know your blood pressure reading is elevated, he or she would also let you know whether your body-mass index is in the overweight/obese range, normal or underweight.

The research is detailed in the December issue of the journal Obstetrics and Gynecology.

This article was reprinted with permission from LiveScience.

Related on LiveScience: