Lifestyle interventions influence relative errors in self-reported diet intake of sodium and potassium

PURPOSE: To characterize the distribution of errors in self-reported sodium and potassium dietary intakes relative to mure objective urine measures among participants receiving lifestyle interventions. METHODS: We analyzed longitudinal data from 900 individuals with hypertension who had been enrolled in a randomized controlled clinical trial to establish with ther usual care or three lifestyle interventions (weight loss, sodium reduction, and combined weight loss and sodium reduction) could effectively substitute for phamacotherapy. Repeated standardized 24-hour diet recalls and 24-hour urine collections were collected over up to three years of follow-up to estimate sodium and potassium intakes. By contrasting self-reported and urine-based sodium and potassium data collected before and during interventions, we examined the relative impact of intervention assignment on estimated intakes, repeatability, and multivariate measurement error. RESULTS: Relative to urine based measures, mean self-reported sodium intakes were biased about 10% lower among participants assigned to combined weight loss and sodium reduction, hut were unaffected by the other interventions. The repeatability of self-report measures increased slightly with time, particularly among participants assigned to sodium interventions. Errors in self-reported sodium and potassium intakes were correlated before the start of the intervention, but became uncorrelated among individuals assigned to sodium restriction interventions. CONCLUSIONS: Lifestyle interventions may influence not only diet intake, hut also the measurement of diet intake. Ann Epidemiol 2001;11:85-93. (C) 2001 Elsevier Science Inc. All rights reserved.