Obesity as a major determinant of underreporting in a self-administered food frequency questionnaire: results from the EPIC-Potsdam Study.

The phenomenon of underreporting of dietary intake has been observed previously in many epidemiologic studies. In this study it was investigated whether dependencies exist between energy intake obtained by a semi-quantitative, self-administered food frequency questionnaire and lifestyle or anthropometric factors, particularly obesity. The study population consisted of 2,531 subjects, men aged 40 to 64 years and women aged 35 to 64 years from the general population of Potsdam and the surrounding areas. First, subjects were allocated into quintiles of the ratio 'reported energy intake (EI)' to 'calculated basal metabolic rate (BMR)' as a measure of age and weight adjusted energy intake. No apparent dependencies between socio-economic variables and the ratio EI/BMR were observed. Among anthropometric variables, BMI and related measures of obesity were inversely related to the ratio EI/BMR in men and women. While dietary intake was directly related to the ratio EI/BMR in absolute quantities, energy adjusted intake of fat, protein, carbohydrate, and alcohol was found to be independent of this ratio. Energy adjusted food group consumption was also found to be independent of the ratio EI/BMR, showing only slightly increasing trends across quintiles of EI/BMR for cereals and fats, and a slightly decreasing trend for sweet foods in women. When subjects were classified into three categories of BMI, reported energy intake decreased across categories. Estimated energy expenditure based on BMR was increasing with BMI categories. A close direct relationship was observed between BMI categories and the difference between reported energy intake and estimated energy expenditure. It is concluded that obesity is a major determinant of under-reporting. Energy adjusted dietary variables were found to be largely independent of such methodological influences.