P175 ESTIMATION OF SODIUM INTAKE CAN BE IMPROVED BY CORRECTION OF 24H SODIUM EXCRETION FOR 24H URINE VOLUME

Background and Objective: Long-term sodium (Na) balance studies demonstrated that 24 h sodium excretion does not reflect 24 h sodium intake, which complicates the use of 24 h urine sodium measurements for sodium intake estimation. This discrepancy has been linked to infradian rhythms of aldosterone and cortisol, which are responsible for adaptive responses targeted to (renal) water conservation. We hypothesize that correction for these parameters will improve the accuracy of sodium intake estimates. Methods: We used data from two sodium balance studies that controlled dietary salt intake for 105 (4 men) and 205 days (6 men) at levels of 6, 9 and 12 g/day. We tested whether correction of 24 h urine sodium for 24 h urine volume, 24 h urine potassium, 24 h urine urea, 24 h urine aldosterone, 24 h urine cortisol, 24 h urine cortisol/cortisone and/or urine osmolality, increased the accuracy of 24 h sodium intake estimates using forward selection linear mixed-effects analysis with a random intercept per participant. Results: The median difference between 24 h sodium intake and excretion was 26 mmol (IQR 13 - 45 mmol). 24 h urine sodium explained only 49% of the variance in 24 h sodium intake. In 7 out of 10 subjects, 24 h urine volume was of most added value to the model, after 24 h urine sodium, increasing R 2 with 0.08±0.03. Including 24 h urine volume in the model significantly improved the model fit (P<0.001, marginal R 2 0.55). The addition of urine urea, urine aldosterone and urine osmolality also contributed significantly but the marginal R 2 only further increased to 0.56. After correction of 24 h urine sodium for 24 h urine volume, sodium intake could be estimated more accurately: the 50-mmol discrepancy between 24 h sodium intake and excretion was reduced from 18% to 10% and the 25-mmol discrepancy from 52% to 43% (P<0.001, Figure 1). Conclusions: In healthy young males, estimation of sodium intake can be improved by correcting 24 h urine sodium for 24 h urine volume.