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(2), 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.