Does degree of obesity influence the validity of reported energy and protein intake?: Results from the SOS Dietary Questionnaire

Objective: To test the validity of a dietary questionnaire which was developed with the particular goal of measuring dietary intake in obese subjects. Design: Reported energy intake was compared with 24 h energy expenditure measured in a chamber for indirect calorimetry (24 EE) and reported nitrogen intake with nitrogen in urine collected during the 24 h in the chamber. Subjects: Twenty-nine overweight men and women, body mass index (BMI) ranging from 25.5-49.5 kg/m(2). Results: Reported energy intake correlated significantly with 24 EE (r = 0.50, P = 0.006) and reported urinary nitrogen correlated significantly with urinary nitrogen excretion (r = 0.56, P = 0.0015). Mean reported energy intake +/- s.d. was 10.2 +/- 3.6 MJ and mean 24 EE +/- s.d. was 10.3 +/- 1.9 MJ. Although this difference was small and non significant, it indicates some underreporting if one can assume that these overweight subjects are less physically active in the chamber than in free-living conditions. Reported nitrogen intake also suggested underreporting at the group level. However, when the data were analysed at the individual level it was clear that the underreporting errors did not increase with increasing degree of obesity. Conclusions: Previous studies with the SOS dietary questionnaire have demonstrated that it is possible to obtain plausible energy intakes from both obese and nonobese subjects. This present analysis further demonstrates that the questionnaire discriminates overweight subjects with high and low intakes of energy and protein, using unbiased biomarkers to judge validity. These data provide additional support for the usefulness of the SOS dietary questionnaire.