Development of a method for estimating dietary salt intake using the overnight urinary sodium/potassium ratio_20210802

Published: 2 August 2021| Version 1 | DOI: 10.17632/nyvpw8g7dz.1
Yuta Sumikama


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. The study involved 43 healthy adults (13 males, 30 females). From 2018 to 2020, subjects consumed a salt-adjusted diet for 11 to 30 days continuously, and overnight urine was collected daily. Using the previous day’s 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 Tanaka’s equation, respectively, and they were compared with the actual salt intake. The results of the analysis showed that Model1 was the optimal model. The Pearson’s 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 the Model 1 estimates and Tanaka’s equation estimates, respectively. The percentages of errors within ±30% from the actual salt intake were 64.2% and 58.4% for Model 1 and Tanaka’s equation, respectively. The percentages of errors within ±1.0 g from the actual salt intake were 25.0% and 25.6% for Model 1 and Tanaka’s equation, respectively. The estimating equation developed is simple and may be a useful method for daily monitoring of dietary salt intake.


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This study included 43 healthy adults (13 males, 30 females) who were recruited from 2018 to 2020. Weight and blood pressure were measured daily during the study period to confirm that there were no fluctuations. This study was conducted according to the guidelines laid down in the Declaration of Helsinki, and all procedures involving research study participants were approved by the Research Ethics Committee of Nagoya University of Arts and Sciences. Written, informed consent was obtained from all study participants. The subjects consumed a salt-adjusted meal for 11-30 days. The amount of salt was adjusted every few days in the range of 2-19 g. All nutrient intakes were calculated by confirming nutrient amounts from the Standard Tables of Food Composition in Japan 2015(Seventh Revised Edition) or from food packaging. The meals were cooked with all ingredients strictly weighed, and the subjects consumed everything completely, including seasonings, sauce and soup. Additional and reduced dietary intakes were allowed only if the exact amount of the nutrient was known, and all these intakes were recorded. The overnight urine, which is the first morning voided urine, was collected daily. The Na concentration, K concentration, Na/K ratio, and Cr concentration in the collected urine samples were measured at the Nagoya Medical Cooperative Association Nagoya Clinical Laboratory Center. Specific gravity was measured using a PAL-09S (ATAGO Co., Ltd., Tokyo, Japan), and the conductivity was measured using a LAQUAtwin <EC-33B> (HORIBA Ltd., Kyoto, Japan). In addition, urine weight was measured.


Nutrition, Clinical Nutrition