Epidemiological assessment of diet: a comparison of a 7-day diary with a food frequency questionnaire using urinary markers of nitrogen, potassium and sodium

Background Validation studies of dietary instruments developed for epidemio logical studies have typically used some form of diet record as the standar d for comparison. Recent work suggests that comparison with diet record may overestimate the ability of the epidemiological instrument to measure habi tual dietary intake, due to lack of independence of the measurement errors. The degree of regression dilution in estimating diet-disease association m ay therefore have been correspondingly underestimated. Use of biochemical m easures of intake may mitigate the problem. In this paper, we report on the use of urinary measures of intakes of nitrogen, potassium and sodium to co mpare the performance of a semiquantitative food frequency questionnaire (F FQ) and a 7-day diet diary (7DD) to estimate average intake of these nutrie nts over one year. Methods In all, 179 individuals were asked to complete an FFQ and a 7DD on two occasions separated by approximately 12 months. The individuals were al so asked to provide 24-hour urine samples on six occasions over a 6-9-month period, covering the time at which the record FFQ and 7DD were completed. The urine was assayed for nitrogen, potassium and sodium. The protocol was completed by 123 individuals. The data from these individuals were analysed to estimate the covariance structure of the measurement errors of the FFQ, the 7DD and a single 24-hour urine measurement, and to estimate the degree of regression dilution associated with the FFQ and 7DD. Results The results demonstrated that: (1) the error variances for each of the three nutrients was more than twice as great with the FFQ than the 7DD; (2) there was substantial correlation (0.46-0.58) between the error of bot h the FFQ and the 7DD completed on different occasions; (3) there was moder ate correlation (0.24-0.29) between the error in the FFQ and the error in t he 7DD for each nutrient; (4) the correlation between errors in different n utrients was higher for the FFQ (0.77-0.80) than for the 7DD (0.52-0.70). Conclusions The regression dilution with the FFQ is considerably greater th an with the 7DD and also, for the nutrients considered, greater than would be inferred if validation studies were based solely on record or diary type instruments.