DISCREPANCIES BETWEEN DIETARY AND URINE NA/K RATIO CAN BE EXPLAINED BY ALDOSTERONE, CORTISOL AND URINE VOLUME

Abstract

Objective: Increased urine sodium-to-potassium (Na/K) ratio is associated with hypertension and cardiovascular disease. However, we demonstrated that urine Na/K ratio does not accurately reflect dietary Na/K ratio. As rhythmic patterns of sodium storage and release are associated with aldosterone and cortisol levels and previous studies linked tissue sodium accumulation to water conservation, we investigated whether the discrepancy between urine and dietary Na/K ratio could be explained by these factors. Design and method: We performed a post-hoc analysis of the long-term sodium balance studies Mars105 and Mars520. Ten healthy participants consumed a diet with known sodium and potassium content. For 105 or 205 days 24-hour urine samples were collected. We calculated the difference between urine and dietary Na/K ratio. A linear mixed-effect model was used to determine whether this difference varied across salt intake levels. Furthermore, we fitted two linear mixed-effect models to explain discrepancies between urine and dietary Na/K ratio. In the first, sodium intake, potassium intake, urine aldosterone and urine cortisol were fixed effects and for each factor random slopes per participant were allowed. In the second, sodium intake, potassium intake and urine volume were fixed effects and for each factor random slopes per participant were allowed. We analysed the salt intake levels 6, 9 and 12 grams separately. Results: Urine Na/K ratio underestimated dietary Na/K ratio (median difference -0.21, IQR -0.47 to 0.09). The discrepancy was significantly larger at higher salt intake levels (mean difference -0.11, -0.22 and -0.36 for 6, 9 and 12 grams salt intake, respectively, P < 0.001). At all salt intake levels, higher aldosterone levels were associated with underestimation of the dietary Na/K ratio (Figure 1A). In contrast, higher urine cortisol and higher urine volume were associated with overestimation of the dietary Na/K ratio (Figure 1B and C respectively). Conclusions: Sodium intake, aldosterone, cortisol and urine volume can explain the discrepancy between dietary and urine Na/K ratio. These data suggest that tissue sodium accumulation contributes to the inability to estimate dietary intake using the Na/K ratio in a single 24-hour urine collection.