Lowering salt intake has been shown to decrease blood pressure (BP). However, data from large cohort studies on salt intake and cardiovascular and renal outcomes are inconsistent. These studies have estimated long-term salt intake using 1 baseline measurement, which may be inaccurate. In this retrospective cohort study, the effect of using successive and multiple 24h samples on the relation between salt intake and long-term outcome was investigated.Design and Method:
We selected adult subjects with an eGFR > 60 mL/min and at least 1 outpatient 24h urine sample >300 mL between 1998–1999. Salt intake was estimated using 1 baseline 24h sodium excretion into urine and the average of 1, 5 and 15-year samples (Fig-A). We divided subjects in tertiles of 24h sodium excretion for Cox-regression analysis, after adjustment for cardiovascular and renal risk factors. Primary outcomes were a composite of cardiovascular events and mortality, and development of end-stage renal disease (ESRD: renal replacement therapy or >60% eGFR decline).Results:
We included 574 subjects with 9,776 24h urine samples during follow-up. Average (±SD) age was 47 ± 14 years and 46% were male. Median follow-up was 16.2 years. Average 24h salt excretion, 10 ± 4 grams, was equal among all methods (p = 0.98) (Fig-B). However, relative to 1 baseline measurement, 49% (1-year), 50% (5-year) and 48% (15-year) of the subjects had a difference of >2 grams in salt intake with new estimations (Fig-C). As a result, 45–50% of all subjects switched between tertiles of salt intake. Point estimates of risk for primary outcomes changed considerably when using single baseline vs. subsequent 24h samples, particularly for ESRD (Fig-D,E).Conclusions:
Relative to 1 baseline 24h sodium measurement, the use of multiple 24h urine samples resulted in different estimations of an individual's salt intake, while population averages remained similar. This had significant consequences for the relation between salt intake and long-term renal outcomes.