Abstract MP039: Association Between Estimated 24-hour Urinary Sodium and Potassium Excretion and Blood Pressure in a Japanese Cohort: a Repeated Measure Analysis

Abstract

Background: While measurement of sodium (Na) and potassium (K) from the collection of urine over a 24-hour period are associated with blood pressure, relationships with 24-hour urinary excretion estimates from more conveniently collected spot urine samples are less clear. Objectives: The purpose of this report is to examine the association of estimated 24-hour excretion of Na, K and their ratio from annually repeated spot urine samples with systolic (SBP) and diastolic (DBP) blood pressures. Methods: Data include 4,958 normotensive Japanese aged 19 to 55 years who participated in annual physical examinations over a 5-year period in the High-Risk and Population Strategy for Occupational Health Promotion Study (1999-2004). At each examination, spot urine samples were collected in the morning and blood pressure was measured following standardized procedures. 24-hour urinary excretion of Na and K were estimated from spot urine specimens using the formula of Tanaka. Adjustments were made for potential confounding from demographic, lifestyle education, body mass index, smoking, alcohol consumption and physical activity. Mixed-effects regression models were used to examine the association of the estimated 24-hour excretion of Na, K, and the sodium-to-potassium (Na:K) ratio with SBP and DBP over the 5-year period of study. Results: A one standard deviation (SD) increase in the estimated 24-hour excretion of Na (36.6 mmol/day) was associated with a 1.3 mm Hg higher SBP and a 0.8 mm Hg higher DBP (P<.001). The association between Na and SBP was greater in men (1.4 mm Hg per SD) than in women (0.9 mm Hg per SD, P=.003 for interaction) and increased with age (1.0 mm Hg per SD at <35 years of age, 1.2 mm Hg per SD at 35 to 45 years of age, and 1.8 mm Hg per SD at >45 years of age, P<.001 for interaction). Estimated 24-hour excretion of K was inversely associated with blood pressure where a one SD increase (8.9 mmol/day) was associated with a 1.1 mm Hg lower SBP and 0.7 mm Hg lower DBP (P<.001). Unlike the estimated 24-hour excretion of Na, interactions between K and age or sex were absent. Estimated 24-hour Na:K ratio were positively associated with SBP and DBP, with greater effects in men than in women and increasing positive associations with age (P<.01 for each interaction). Conclusions: Estimated 24-hour excretion of Na, K, and the Na:K ratio from spot urine samples have significant associations with blood pressure. Findings suggest that the less costly and more convenient collection of spot urine samples for estimating excretion of Na, K, and their ratio over a 24-hour period may be an efficient way of monitoring these electrolytes in large population-based samples.