Development of a Method for Estimating Dietary Salt Intake Using the Overnight Urinary Sodium/Potassium Ratio

Abstract

Background: There are many patients who need to restrict and assess salt in their diet. However, it is difficult to estimate daily salt intake accurately and easily. Therefore, a method for estimating dietary salt intake using the overnight urinary sodium (Na)/potassium (K) ratio was developed.

Methods: The study involved 43 healthy adults (13 males, 30 females). From 2018 to 2020, subjects consumed a salt-adjusted diet for 11 - 30 days continuously, and overnight urine was collected daily. Using the previous days salt intake as the objective variable, an equation for estimating the salt intake was developed using a general linear model. To verify the accuracy of the estimating equation, the estimated salt intake of the previous day was calculated using our equation and Tanakas equation, which is an estimating equation from spot urine widely used in clinical practice and epidemiological studies to estimate dietary salt intake, and they were compared with the actual salt intake.

Results: The results of the analysis showed that model 1 (previous days salt intake (g) = 3.62 + 0.64 urinary Na/K ratio + 0.18 conductivity (mS/cm) - 0.43 sex (male 0, female 1)) was the optimal model. Then, salt intake was estimated using model 1 and Tanakas equation, and compared with actual salt intake. The Pearsons product-moment correlation coefficient between the actual and estimated salt intake was r = 0.618 (P < 0.001) and r = 0.573 (P < 0.001) for model 1 estimates and Tanakas equation estimates, respectively. The percentages of errors within 30% from the actual salt intake were 64.2% and 58.4% for model 1 and Tanakas equation, respectively.

Conclusion: An equation for estimating salt intake was developed using the Na/K ratio and conductivity of overnight urine. Although the applicability of this method to hypertensive patients and the elderly has not been studied and needs to be clarified in the future, the estimating equation developed is simple and may be a useful method for daily monitoring of dietary salt intake.

J Clin Med Res. 2021;13(9):479-486
doi: https://doi.org/10.14740/jocmr4596