VALIDITY OF INDIVIDUAL PORTION SIZE ESTIMATES IN A FOOD FREQUENCY QUESTIONNAIRE

Background. Dietary surveys often rely upon subjects' ability to estim ate their portion sizes instead of weighing them, but information on t he validity of such estimates is lacking. This study investigated the validity of estimated portion sizes from a self-administered food freq uency questionnaire including photos. Methods. Validity was evaluated by comparison with portion sizes from 14-day weighed food records. Eig ht sets of photos were included in the study (three meat dishes, one f ish dish, three vegetables, potatoes), each set with four options. Sub jects (59 men, 85 women) were 40-64 years. Results. The majority of su bjects (85-95%) selected the most correct photo or a neighbouring phot o. However, regression analyses showed that the relationship between t he estimated and the measured portion sizes was relatively weak for mo st of the foods tested, and correlations were only significant for thr ee of the eight foods. This apparent contradiction was partly due to t he pattern of the error: subjects selecting small portions had underes timated their actual portions while subjects selecting targe portions had overestimated them. This error pattern could not be explained by t he theoretical error resulting from the use of four discrete portion s izes instead of continuous weights, neither was it a simple consequenc e of the unidirectional errors at the extreme ends. Actual portion siz es varied from day to day, with intra-individual coefficients of varia tion of 34-40%. Conclusions. The photos used in the present study were of limited value for ranking individuals correctly according to their actual portion sizes. Whether this is a common feature of photos used in food frequency questionnaires remains to be answered.