Validity of self-reported weight and height in Austrian adults: sociodemographic determinants and consequences for the classification of BMI categories.

OBJECTIVE Epidemiological studies have shown that adults tend to underestimate their weight and overestimate their height. This may lead to a misclassification of their BMI in studies based on self-reported data. The aim of the present study was to assess the validity of self-reported weight and height in Austrian adults. DESIGN Data on weight, height, health behaviour and sociodemographic characteristics of adults were collected in a standardized procedure via a self-filling questionnaire and a medical examination including measurements of weight and height. SETTING A publicly accessible out-patient clinic in southern Austria. SUBJECTS Austrian residents (n 473) aged 18 years and older who attended a health check participated in the study. RESULTS The mean difference between reported and measured BMI was not significant in younger adults (<35 years: mean difference -0·21 kg/m2; P < 0·08) but increased significantly with age (≥55 years: mean difference -0·68 kg/m2; P < 0·001). The prevalence of normal weight (BMI = 18·5-24·9 kg/m2) and overweight (BMI = 25·0-29·9 kg/m2) was overestimated based on the self-reported data on BMI, while that for underweight (BMI < 18·5 kg/m2) and obesity (BMI ≥ 30·0 kg/m2) was underestimated (P < 0·001). The self-reported data showed an obesity prevalence of 12·5 %, while measurement showed a prevalence of 15·4 % (P < 0·001). CONCLUSIONS Our results indicate that prevalence rates of obesity are probably underestimated for Austrian adults when using self-reported weight and height information. The deviations from the measured data clearly increased with age. Analyses based on self-reported data should therefore be adjusted for the age dependency of the validity.