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.
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We are extremely grateful to the Nagahama City Office and the nonprofit organization Zeroji Club for their support in performing the Nagahama Study. The authors would like to thank Enago (www.enago.jp) for the English language review.
The study was supported by a university grant, The Center of Innovation Program, The Global University Project, and a Grant-in-Aid for Scientific Research (25293141, 26670313) from the Ministry of Education, Culture, Sports, Science & Technology in Japan, the Practical Research Project for Rare/Intractable Diseases (ek0109070, ek0109196), the Comprehensive Research on Aging and Health Science Research Grants for Dementia R&D (dk0207006), the Program for an Integrated Database of Clinical and Genomic Information (kk0205008) from Japan Agency for Medical Research and Development (AMED), and the Takeda Medical Research Foundation.
Conflict of interest
The authors declare that they have no conflict of interest.