Nutr Res Pract.  2020 Feb;14(1):25-31. 10.4162/nrp.2020.14.1.25.

Development of objective indicators for quantitative analysis of sodium intake: the sodium to potassium ratio of second-void urine is correlated with 24-hour urinary sodium excretion

• Kim JG
• Han SW
• Yi JH
• Park HC
• Han SY

• KMID: 2468510
• DOI: http://doi.org/10.4162/nrp.2020.14.1.25

Abstract

BACKGROUND/OBJECTIVES
To date, sodium intake has been evaluated based on spot urine instead of 24-hour (hr) urine collection. Nevertheless, the optimal method for assessing daily sodium intake remains unclear.
SUBJECTS/METHODS
Fifteen male (age 32.7 ± 6.5 years) participants were offered 3 meals with a total of 9-10 g salt over 24 hours, and 24-hr urine was collected from the second-void urine of the first day to the first-void urine of the second day. Twenty-four-hr urinary sodium (24UNa) was estimated using Tanaka's equation and the Korean formula, and spot urine Na, potassium (K), chloride (Cl), urea nitrogen (UN), creatinine (Cr), specific gravity (SG) and osmolality (Osm) were measured. The ratios of urinary Na to other parameters were calculated, and correlations with total measured 24UNa were identified.
RESULTS
Average 24-hr urine volume was 1,403 ± 475 mL, and measured 24UNa was 143.9 ± 42.1 mEq (range, 87.1-239.4 mEq). Measured 24UNa was significantly correlated with urinary Na/UN (r = 0.560, P < 0.01), urinary Na/Osm (r = 0.510, P < 0.01), urinary Na/Cr (r = 0.392, P < 0.01), urinary Na/K (r = 0.290, P < 0.01), 24UNa estimated using Tanaka's equation (r = 0.452, P < 0.01) and the Korean formula (r = 0.414, P < 0.01), age (r = 0.548, P < 0.01), weight (r = 0.497, P < 0.01), and height (r = 0.393, P < 0.01) in all spot urine samples. Estimated 24UNa based on the second-void spot urine of the first day tended to be more closely correlated with measured 24UNa than were estimates from the other spot urine samples. The significant parameters correlated with the second-void urine of the first day were urinary Na/K (r = 0.647, P < 0.01), urinary Na/Cr (r = 0.558, P < 0.05), and estimated 24UNa using Tanaka's equation (r = 0.616, P < 0.05) and the Korean formula (r = 0.588, P < 0.05).
CONCLUSIONS
Second-void urine is more reliable than first-void urine for estimating 24UNa. Urinary Na/K in the second-void urine on the first day is significantly correlated with 24UNa. Further studies are needed to establish the most reliable index and the optimal time of urine sampling for predicting 24UNa.

Keyword

Dietary sodium; sodium excretion; urine collection

MeSH Terms

Creatinine
Humans
Male
Meals
Methods
Nitrogen
Osmolar Concentration
Potassium*
Sodium*
Sodium, Dietary
Specific Gravity
Urea
Urine Specimen Collection
Creatinine
Nitrogen
Potassium
Sodium
Sodium, Dietary
Urea

Figure

• 

  Fig. 1 Correlations between measured 24UNa, and urinary Na/K in the second-void spot urine samples taken on the first day (A) and in the first-void spot urine samples taken on the second day (B).Pearson's correlation coefficients and P-values are shown for each plot. Each line represents the linear regression for the plotted plot. 24UNa, 24-hour urinary sodium; Na, sodium; K, potassium.

• 

  Fig. 2 Correlations between measured 24UNa and 24UNa estimated from Tanaka's equation (A) and the Korean formula (B) in the second-void spot urine samples taken on the first dayPearson's correlation coefficients and P-values are shown for each plot. Each line represents the linear regression for the plotted data. 24UNa: 24-hour urinary sodium.

• 

  Fig. 3 Correlations between measured 24UNa and 24UNa estimated from Tanaka's equation (A) and the Korean formula (B) in the first-void spot urine samples taken on the second day.Pearson's correlation coefficients and P-values are shown for each plot. Each line represents the linear regression for the plotted plot. 24UNa: 24-hour urinary sodium.

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