Validation of diet and urinary excretion derived estimates of sodium excretion against 24-h urine excretion in a worksite sample

• Validation of diet & urinary methods for sodium excretion against 24 h urine collections.
• A group of 50 manufacturing employees formed the validation sample from a recruited sample of 802.
• Neither diet nor spot urine methods provide adequate validity at the individual level.
• Group mean errors from dietary methods compare well with spot urine samples.
• 24-h urine collections remain gold standard to measure health impacts of sodium.

Background and aimsTo validate diet and urinary excretion derived estimates of sodium intake against those derived from 24-h urine collections in an Irish manufacturing workplace sample.Methods and resultsWe have compared daily sodium (Na) excretion from PABA validated 24-h urine collections with estimated daily sodium excretion derived from the following methods: a standard Food Frequency Questionnaire (FFQ), a modified 24-h dietary recall method, arithmetic extrapolations from morning and evening spot urine samples, predicted sodium excretion from morning and evening spot urine samples using Tanaka's, Kawasaki's and the INTERSALT formula. All were assessed using mean differences (SD), Bland–Altman plots, correlation coefficients and ROC Area under the Curve (AUC) for a cut off of ≥100 mmol of Na/day. The Food Choice at Work study recruited 802 participants aged 18–64 years, 50 of whom formed the validation sample. The mean measured 24-h urinary sodium (gold standard) was 138 mmol/day (8.1 g salt). At the group level, mean differences were small for both dietary methods and for the arithmetic extrapolations from morning urine samples. The Tanaka, Kawasaki and INTERSALT methods provided biased estimates of 24-h urinary sodium. R2 values for all methods ranged from 0.1 to 0.48 and AUC findings from 0.57 to 0.76.ConclusionNeither dietary nor spot urine sample methods provide adequate validity in the estimation of 24-h urinary sodium at the individual level. However, group mean errors from dietary methods are small and random and compare favourably with those from spot urine samples in this population.