Sodium intake and kidney function in the general population: an observational, population-based study

Background. The relationships of sodium intake to kidney function within the population have been poorly investigated and are the objective of the study. Methods. This observational, population-based, cross-sectional and longitudinal study targeted 4595 adult participants of the Gubbio study with complete data at baseline exam. Of these participants, 3016 participated in the 15-year follow-up (mortality-corrected response rate 78.4%). Baseline measures included sodium:creatinine ratio in timed overnight urine collection, used as an index of sodium intake, together with serum creatinine, sex, age and other variables. Follow-up measures included serum creatinine and other variables. Estimated glomerular filtration rate (eGFR, mL/min/1.73 m(2)) was calculated using serum creatinine, sex and age and was taken as an index of kidney function. Results. The study cohort was stratified in sex- and age-controlled quintiles of baseline urine sodium:creatinine ratio. A higher quintile associated with higher baseline eGFR (P<0.001). In multivariable analysis, the odds ratio (OR) of Stage1 kidney function (eGFR >= 90 mL/min/1.73 m(2)) was 1.98 times higher in Quintile 5 compared with Quintile 1 [95% confidence interval (CI) 1.50-2.59, P<0.001]. The time from baseline to follow-up was 14.12.5years. Baseline to follow-up, the eGFR change was more negative along quintiles (P<0.001). In multivariable analysis, the OR in Quintile 5 compared with Quintile 1 was 2.21 for eGFR decline >= 30% (1.18-4.13, P=0.001) and 1.38 for worsened stage of kidney function (1.05-1.82, P=0.006). Findings were consistent within subgroups. Conclusions. Within the general population, an index of higher sodium intake associated cross-sectionally with higher kidney function but longitudinally with greater kidney function decline.