Renal Potassium Excretion Is Not a Fixed Percentage of Intake

Objective: Because of its blood pressure lowering effects, high potassium intake could help to reduce cardiovascular events. As a significant amount of potassium is lost via stool, the World Health Organization advices to multiply 24-hour renal potassium excretion by 1.3 for estimation of dietary intake. We analyzed whether this 1.3 factor varies over time in healthy volunteers and tested whether factors associated with tissue cation storage, namely sodium intake, aldosterone, cortisol and urine volume, affect the renal potassium excretion. Design and method: We performed a post-hoc analysis of the Mars sodium balance studies in which ten healthy male participants consumed a fixed salt (6, 9 and 12 grams/day for periods >29 days) and potassium intake (4 grams/day) for 105 (n=4) and 205 days (n=6). For the entire study period, all urine was collected in 24-hour aliquots. We calculated the percentage of potassium intake that was excreted in the corresponding 24-hour urine collection during steady-state. Using a multivariable mixed-effects model, we assessed whether this percentage was associated with sodium intake, potassium intake, urinary aldosterone, urinary cortisol and urine volume. Results: Overall, the median renal potassium excretion was 80% of potassium intake, but percentages ranged from 24% to 304%. Within participants the interquartile range of renal potassium excretion was 25% and between participants 21%. Increased sodium intake was associated with a higher percentage of renal potassium excretion (P<0.05), whereas increased potassium intake was associated with a lower percentage of renal potassium excretion (P<0.001). There was a significant positive interaction between the effects of sodium and potassium intake (P<0.001; figure A). Additionally, urinary aldosterone (figure B), urinary cortisol (figure C) and urine volume (figure D) were associated with a higher percentage of renal potassium excretion (P<0.001). Conclusions: The percentage of ingested potassium that is excreted in urine varies considerably within and among subjects. Sodium intake, potassium intake, urinary aldosterone and cortisol levels, and urine volume independently affect the fraction of the ingested potassium that is excreted by the kidneys during steady-state. Our results suggest that the WHO's standard 1.3 conversion factor may lead to inaccurate estimation of potassium intake.