Excessive sodium intake has long been recognized as a risk factor for hypertension. The first step to preventing hypertension is to accurately calculate one’s sodium intake. Because approximately 93% of dietary sodium is excreted in the urine [1], 24-h urine collections can be used to determine real-time sodium intake. However, due to the substantial day-to-day variation in dietary sodium intake, it is difficult to obtain an accurate estimate of the usual sodium intake from a single 24-h urine collection. Therefore, repetitive sampling may be necessary to accurately estimate sodium intake [2], but it is inefficient and time-consuming for primary prevention.
Recently, great emphasis has been given to the urinary sodium/potassium ratio (Na/K ratio) as another indicator of the burden of sodium on blood pressure (BP). Because potassium lowers BP, the Na/K ratio is more closely correlated with concurrently measured BP than sodium alone or dietary sodium intake [3], as estimated by the Tanaka formula [4]. Furthermore, since the Na/K ratio does not require a creatinine value, it can be easily determined with simple equipment [5] that enables the recording of several measurements over the course of a day. According to a previous study [6], measurements of six random daytime casual urine samples on separate days are necessary to determine the usual Na/K level. However, it is unclear how many measurements are required to achieve Na/K values that can be relied upon for assessing hypertension risk. According to a study published in this issue of Hypertension Research, Kogure et al. [7] reported the result of an association analysis between multiple daily measured morning urinary Na/K ratios and home BP levels that helped to resolve this unanswered question. From an analysis of the 1-day to 10-day mean values of the urinary Na/K ratios and home-measured BP levels, they suggested that a measuring period of more than three days would be beneficial. These valuable findings contribute to the practical application of the spot urine Na/K ratio as a marker of hypertension risk.
Another issue that makes the practical application of the Na/K ratio difficult is the lack of an exact cutoff value. Due to the linear correlation between the urinary Na/K ratio and home BP level, Kogure et al. [6] were unable to determine a cutoff value using the same dataset. However, as shown in Table 2 of Kogure’s publication, a urinary Na/K ratio ≥6.0 was consistently associated with the home BP level with an odds ratio greater than 2.0, regardless of the number of measurement days, and approximately 20% of the subjects in their study had a 10-day mean urinary Na/K ratio level ≥6.0 (Fig. 1). Therefore, the spot urine Na/K ratio may be beneficial in identifying groups at high risk of developing hypertension. According to a general population-based longitudinal study by Nagahama [3], approximately 8% of the subjects had Na/K ratios ≥6.0 in their spot urine collected between 0900 and 1700 h at the baseline. Many variables, including fasting time, renal function, and testing season, affect the spot urine Na/K value [3], and therefore, the measurement timing is important when employing the spot urine Na/K ratio as a risk indicator of hypertension. Biologically and practically, the best way to standardize measurement settings is to use morning spot urine, which was used in the study by Kogure et al. However, it should be noted that elderly patients may underestimate their morning urine Na/K ratio levels because of their frequent nocturnal urination [8] and elderly people are more salt-sensitive, which increases their nocturnal sodium excretion during the night [9, 10].
Urinary Na excretion can be reduced by restricting sodium intake. Furthermore, the urinary Na/K ratio can also be lowered by increasing the consumption of potassium-rich foods such as leafy greens, fruits, and dairy products [11]. The correlation between the Na/K ratio and BP level remains consistent regardless of the Na and K concentrations [12]. However, increasing potassium intake may be more helpful in lowering BP levels in middle-aged people with preserved renal function, while sodium restriction may be more effective in older people [12].
References
Lucko AM, Doktorchik C, Woodward M, Cogswell M, Neal B, Rabi D, et al. Percentage of ingested sodium excreted in 24-hour urine collections: a systematic review and meta-analysis. J Clin Hypertens. 2018;20:1220–9. https://doi.org/10.1111/jch.13353
Judge C, Narula S, Mente A, Smyth A, Yusuf S, O’Donnell MJ. Measuring sodium intake: research and clinical applications. J Hypertens. 2021;39:2344–52. https://doi.org/10.1097/HJH.0000000000002951
Tabara Y, Takahashi Y, Kumagai K, Setoh K, Kawaguchi T, Takahashi M, et al. Descriptive epidemiology of spot urine sodium-to-potassium ratio clarified close relationship with blood pressure level: the Nagahama study. J Hypertens. 2015;33:2407–13. https://doi.org/10.1097/HJH.0000000000000734
Tanaka T, Okamura T, Miura K, Kadowaki T, Ueshima H, Nakagawa H, et al. A simple method to estimate populational 24-h urinary sodium and potassium excretion using a casual urine specimen. J Hum Hypertens. 2002;16:97–103. https://doi.org/10.1038/sj.jhh.1001307
Iwahori T, Ueshima H, Ohgami N, Yamashita H, Miyagawa N, Kondo K, et al. Effectiveness of a self-monitoring device for urinary sodium-to-potassium ratio on dietary improvement in free-living adults: a randomized controlled trial. J Epidemiol. 2018;28:41–7. https://doi.org/10.2188/jea.JE20160144
Iwahori T, Ueshima H, Miyagawa N, Ohgami N, Yamashita H, Ohkubo T, et al. Six random specimens of daytime casual urine on different days are sufficient to estimate daily sodium/potassium ratio in comparison to 7-day 24-h urine collections. Hypertens Res. 2014;37:765–71. https://doi.org/10.1038/hr.2014.76
Kogure M, Nakamura T, Tsuchiya N, Hirata T, Nochioka K, Narita A, et al. Consideration of the reference value and number of measurements of urinary sodium-to-potassium ratio based on the prevalence of untreated home hypertension: TMM Cohort Study. Hypertens Res. 2022. https://doi.org/10.1038/s41440-021-00843-7
Tabara Y, Matsumoto T, Murase K, Setoh K, Kawaguchi T, Nagashima S, et al. Lifestyle habits associated with nocturnal urination frequency: the Nagahama study. Neurourol Urodyn. 2019;38:2359–67. https://doi.org/10.1002/nau.24156
Fujii T, Uzu T, Nishimura M, Takeji M, Kuroda S, Nakamura S, et al. Circadian rhythm of natriuresis is disturbed in nondipper type of essential hypertension. Am J Kidney Dis. 1999;33:29–35. https://doi.org/10.1016/s0272-6386(99)70254-4
Uzu T, Kazembe FS, Ishikawa K, Nakamura S, Inenaga T, Kimura G. High sodium sensitivity implicates nocturnal hypertension in essential hypertension. Hypertension. 1996;28:139–42. https://doi.org/10.1161/01.hyp.28.1.139
Yamashita M, Tabara Y, Higo Y, Setoh K, Kawaguchi T, Takahashi Y, et al. Association between socioeconomic factors and urinary sodium-to-potassium ratio: the Nagahama Study. Hypertens Res. 2018;41:973–80. https://doi.org/10.1038/s41440-018-0101-x
Higo Y, Nagashima S, Tabara Y, Setoh K, Kawaguchi T, Takahashi Y, et al. Association of the spot urine sodium-to-potassium ratio with blood pressure is independent of urinary Na and K levels: the Nagahama study. Hypertens Res. 2019;42:1624–30. https://doi.org/10.1038/s41440-019-0276-9
Author information
Authors and Affiliations
Corresponding author
Ethics declarations
Conflict of interest
The author declares no competing interests.
Additional information
Publisher’s note Springer Nature remains neutral with regard to jurisdictional claims in published maps and institutional affiliations.
Rights and permissions
About this article
Cite this article
Tabara, Y. The spot urine sodium-to-potassium ratio as a marker of hypertension risk. Hypertens Res 45, 924–925 (2022). https://doi.org/10.1038/s41440-022-00879-3
Received:
Accepted:
Published:
Issue Date:
DOI: https://doi.org/10.1038/s41440-022-00879-3