Estimating 24-hour Urinary Excretion of Sodium and Potassium is More Reliable from 24-hour Urine than Spot Urine Sample in a Feeding Study of US Older Postmenopausal Women

Abstract

Abstract Background Assessing estimated sodium (Na) and potassium (K) intakes derived from 24-h urinary excretions versus a spot urine sample if comparable could reduce participant burden in epidemiologic and clinical studies. Objectives In a two-week controlled feeding study, Na and K excretion from a 24-h urine collection were compared with a first void spot urine sample, applying established algorithms and enhanced models to estimate 24-h excretion. Actual and estimated 24-h excretions were evaluated relative to mean daily Na and K intakes in the feeding study. Methods 153 older postmenopausal women ages 75.4 ± 3.5 years participated in a 2-week controlled feeding study with a 4-day repeating menu cycle based on their usual intake (ClinicalTrials.gov Identifier: NCT00000611). Of the 150 participants who provided both a first void spot urine sample and a 24-h urine collection on the penultimate study day, statistical methods included Pearson correlations for Na and K between intake, 24-h collections and the 24-h estimated excretions using four established algorithms; enhanced biomarker models by regressing ln-transformed intakes on ln-transformed 24-h excretions or ln-transformed 24-h estimated excretions plus participant characteristics; and sensitivity analyses for factors potentially influencing Na or K excretion, e.g., possible kidney disease estimated eGFR < 60 mL/min/1.73m). Results Pearson correlation coefficients between Na and K intakes and actual 24-h excretions were 0.57 and 0.38–0.44 for estimated 24-h excretions, depending on electrolyte and algorithm used. Enhanced biomarker model cross-validated R2 (CVR2s) for 24-h excretions were 38.5% (Na), 40.2% (K) and 42.0% (Na/K). After excluding participants with possible kidney disease, the CVR2s were 43.2% (Na), 40.2% (K) and 38.1% (Na/K). Conclusions 24-h urine excretion measurement performs better than estimated 24-h excretion from a spot urine as a biomarker for Na and K intake among a sample of primarily white postmenopausal women. Summary 24-h urine excretion measurement performs better than estimated 24-h excretion from a spot urine as a biomarker for Na and K intake among a sample of primarily white postmenopausal women.