Background Current measurements of dietary and urinary sodium are fraught with methodological difficulties. Dietary methods tend to underestimate sodium consumption due to under-reporting of discretionary sources of salt (added at the table, or during cooking). The 24-hour urine collection method which is considered the ‘gold standard’ is burdensome and potentially limited by under-collection. Several methods have been used to predict 24-hour sodium excretion from spot urine samples, including arithmetic extrapolation and the application of predictive formulae based on spot sodium to creatinine ratios as a means of controlling for urinary concentration, including those of Tanaka and Kawasaki. However the use of these methods in analytical epidemiological research is controversial. The aim of this study was to validate diet and urinary excretion derived estimates of sodium intake against those derived from 24-hour urine collections in an Irish manufacturing workplace sample.
Methods We have compared daily sodium (Na) excretion from PABA (Para-aminobenzoic acid) validated 24-hour urine collections with estimated daily sodium excretion derived from the following methods: a standard Food Frequency Questionnaire (FFQ), a modified 24-hour dietary recall method, arithmetic extrapolations from morning and evening spot urine samples, predicted sodium excretion from morning and evening spot urine samples using both Tanaka’s and Kawasaki’s formula. All were assessed using mean differences (SD), Bland-Altman plots, correlation coefficients and ROC Area under the Curve (AUC) with a cut off of ≥100 mmol of Na/day, used to define high salt intake.
Results The Food Choice at Work study recruited 802 participants aged 18–64 years, 50 of whom formed the validation sample (provided a complete 24-hour urine collection). The mean dietary sodium intake was 138 mmol/day (8.1g salt) from the 24-hour urine collections. At the group level mean differences (mmol/day) were small for the 24-hour dietary recall method (3.8 (SD 69.4)), FFQ method (9.1 (SD 52.4)) and the arithmetic extrapolation from morning urine samples (3.8 (SD 77.4)). Biassed estimates of 24-hour sodium excretion were reported for Tanaka (10.9 (SD 54)), and Kawasaki (–47 (SD 61)) based estimates. The AUC findings for all methods ranged from 0.56 (arithmetic extrapolations evening spot) to 0.76 (FFQ).
Conclusion Neither dietary nor spot urine sample methods provide adequate validity in the estimation of 24-hour sodium excretion at the individual level. At the group level the errors from dietary methods are small and random and compare favourably with those from spot urine samples in this population.
- Dietary sodium
- urinary sodium
- 24-hour collection
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