Blood pressure and diurnal variation in sodium, potassium, and water excretion

Abstract

The objective of this study was to examine associations of blood pressure (BP) with ratios of overnight to 24-h urinary excretion of sodium, potassium, and water. Each of 125 men 27–64 years of age, not taking diuretics, had BP measured during the day on a Monday. Beginning Monday evening, each participant provided three carefully timed 24-h urine collections, divided into daytime and overnight (bedtime to awakening) specimens. Proportion of total 24-h excretion of sodium, potassium, and water in the overnight specimen, standardised for creatinine excretion, was determined for each 24-h period. Associations of systolic and diastolic BP (SBP/DBP) with these proportions were examined with control for age, body mass index, alcohol intake, and heart rate. Mean BP was 116/71 mm Hg; 15 men were on non-diuretic anti-hypertensive therapy. Mean 24-h urinary excretion was 168 mmol for sodium, 68 mmol for potassium, and 16 mmol for creatinine. Mean overnight to 24-h proportions averaged over the 3 days were 30.7% for sodium, 22.0% for potassium, 32.1% for urinary volume, and 33.2% for creatinine. Partial correlations of SBP and DBP with the 3-day averages were 0.257 (P < 0.01) and 0.210 (P < 0.05) for sodium; 0.223 (P < 0.05) and 0.222 (P < 0.05) for potassium; 0.127 and 0.091 for urinary volume; and −0.033 and 0.014 for creatinine. correlations for sodium proportions were larger for the first 24-h period, compared to the second or third 24-h period. these results indicate that higher bp was associated with a relatively greater proportion of sodium and potassium excretion at night. further work is needed to clarify temporal sequence, ie, whether a relatively greater sodium and potassium excretion at night is a risk factor for higher bp (eg, via renal mechanisms), or whether higher bp results in relatively greater sodium and potassium excretion at night, or both.