Statistical issues in analyzing 24-hour dietary recall and 24-hour urine collection data for sodium and potassium intakes

Dietary recalls and urine assays provide different metrics for assessing so dium and potassium intakes. Means, variances, and correlations of data obta ined from these two modes of measurement differ. Pooling of these data is n ot straightforward, and results from studies employing the different modes may not be comparable. To explore differences between these metrics, the au thors used data from the Trial of Nonpharmacologic Intervention in the Elde rly (TONE), which included repeated standardized 24-hour dietary recalls an d 24-hour urine collections administered over 3 years of follow-up, to esti mate sodium and potassium intakes. The authors examined data from 341 contr ol participants assigned to usual care that were collected between August 1 992 and December 1995. Dietary recalls yielded estimates of sodium intake t hat averaged 22% less than those from urine assays and estimates of potassi um intake that averaged 16% greater than those from urine assays. Sodium in take estimates were less repeatable (r = 0.22 for diet; r = 0.30 for urine) than potassium intake estimates (r= 0.49 for diet; r= 0.50 for urine), mak ing relations with outcomes more difficult to characterize. Overall, the pe rformance of the two measurement modes was fairly similar across demographi c subgroups. Errors in separate estimations of long term sodium and potassi um intakes using short term data were strongly correlated, more strongly th an the underlying long term intakes of these electrolytes. Because of the c orrelated measurement error, estimated regression coefficients for linear m odels including both electrolytes as predictors may be confounded such that the separate relations between these nutrients and outcomes such as blood pressure cannot be reliably estimated by common analytical strategies.