Association between socioeconomic factors and urinary sodium-to-potassium ratio: the Nagahama Study

High sodium intake is a simple modifiable risk factor for hypertension. Although not confirmed, lower socioeconomic status may be a factor that increases sodium intake. We aimed to clarify the association between socioeconomic status and urinary sodium-to-potassium ratio by cross-sectional and longitudinal analyses. The study included 9410 community residents. Spot urine sodium-to-potassium ratios were measured twice with a 5-year interval. Socioeconomic status was investigated using a self-administered questionnaire. Cross-sectional analysis revealed that educational attainment was inversely associated with urinary sodium-to-potassium ratio (years of education ≤ 9: 3.0 ± 1.8, ≤ 12: 2.9 ± 1.6, ≥ 13: 2.8 ± 1.6; P < 0.001), whereas no significant association was observed with household income. Men, particularly individuals living alone, exhibited markedly high sodium-to-potassium ratios (3.6 ± 2.3). Although frequent intake of vegetables, fruits, and dairy products was also inversely associated with the ratio, the associations with educational attainment ( ≤ 9: reference, ≤ 12: β = -0.032, P = 0.026, ≥ 13: β = -0.059, P < 0.001), marital status (β = -0.040, P < 0.001), and sex*marital status interaction (β = 0.054, P = 0.001) were independent of these covariates. Educational attainment was also inversely associated with differences in the urinary sodium-to-potassium ratio during the follow-up period (odds ratio, 0.70; P < 0.001). Lower educational attainment was an independent determinant for urinary sodium-to-potassium ratio. Health literacy education, particularly in men living alone, may be a factor for reducing salt intake even in high-income countries where equal educational opportunity is assured.