URINE SODIUM-TO-POTASSIUM RATIO DOES NOT ACCURATELY REFLECT DIETARY SODIUM-TO-POTASSIUM RATIO

Abstract

Objective: High sodium intake is associated with an increased risk of cardiovascular disease, whereas a high potassium intake is associated with a reduced risk. Dietary counselling requires accurate estimation of sodium and potassium intake, but 24-hour urine sodium and potassium excretion demonstrated not to reflect actual intake. Urine sodium-to-potassium (Na/K) ratio is a promising alternative as it may be less affected by aldosterone-induced rhythmic changes in total body sodium and potassium content, is less influenced by incomplete urine collections, and incorporates both the effect of sodium and potassium. Our study assesses whether 24-hour urine Na/K ratio is superior in predicting dietary intake compared to 24-hour urine sodium or potassium excretion. Design and method: A post-hoc analysis was performed on data from the long-term sodium balance studies Mars105 and Mars520. Ten healthy participants received a diet with a known sodium and potassium content and collected 24-hour urine samples for 105 or 205 days. We calculated the log fold difference between dietary intake and urine content of sodium, potassium and Na/K ratio. We compared these estimates of accuracy using a mixed-effects model with a random intercept per subject. We analysed whether the accuracy differed among 6, 9 and 12 grams salt intake or was affected by the number of 24-hour urine collections. Results: Urine Na/K ratio underestimated dietary Na/K ratio with a median difference of -0.21 (IQR -0.47 to 0.09). Compared to sodium and potassium assessment, the Na/K ratio was significantly less accurate in predicting intake (Figure 1A). Only for the 6 gram salt phase the Na/K ratio did not perform significantly worse than sodium measurements. Increasing the number of 24-hour urine measurements to three or seven improved accuracy of the Na/K ratio, although it remained inferior to separate assessment of sodium and potassium excretion (Figure 1B). Conclusions: The 24-hour urine Na/K ratio is less accurate than 24-hour urine sodium excretion for estimation of dietary intake in a controlled setting.