Self-estimated BMI, but not self-perceived body size, accurately identifies unhealthy weight in US adults.

Background: Self-perceptions of health and disease can be a major driver of health behaviors. Improving accuracy of self-ascertainment of obesity may prompt uptake of weight-control behaviors in those with obesity.

Methods: We assess performance of self-perceived body size ('too small', 'about right' or 'too large'), self-estimated BMI in kg/m, and sociodemographics in detecting measured BMI category (under-, normal-, overweight and obese; BMI cutpoints 18.5, 25 and 30) in first bivariate and then multivariable models.

Results: Of 37,281 adults in the US from NHANES, 2, 34, 33 and 32% were under-, normal-, overweight and obese. Respectively 56, 73, 60 and 91% self-perceived as 'too small', 'about right', 'too large' and 'too large.' Of those who self-perceived as 'too small', 22% were underweight and 10% were overweight or obese. 99.7% of obese participants self-estimated a BMI in the overweight/obese range, including many who did not self-perceive as 'too large'. Among obese participants, self-perception as either 'about right' or 'too small' was more likely for those who were younger (OR for perception as 'too large' 1.01 per year, 95% confidence interval 1.00-1.01) male (OR 0.33, (0.28-0.39)) nonwhite (ORs 0.36-0.79 for different ethnicities), low-income (ORs 0.61 and 1.8 for the lowest and highest of six categories, vs. the third) or measured recently (OR 0.98 (0.96-1.0) per year since 1999). Misperception was less common, but still existed, for participants with moderate or severe obesity (ORs 2.9 (2.3-3.5) and 7.9 (5.4-12), vs. 'mild.') (all p < 0.01.) CONCLUSIONS: A tenth of adults in the US with obesity, especially those from overweight peer groups, self-perceive as normal or underweight and thus may not be motivated to control their weight. However, virtually all self-estimate an overweight or obese BMI. If measured BMI is not available, self-estimates are sufficiently accurate that interventions may rely on it to identify obesity.