Urinary Flow Rates and Body Mass Index in NHANES: Potential Confounding in Assessments of Obesity and Other Health Outcomes Related to Urinary Biomarker Concentrations

摘要

Urinary analyte concentrations are affected both by exposure level and by urinary flow rate (UFR). Systematic variations in UFR with body mass index (BMI) could confound associations between BMI or other health outcomes and biomarker concentrations. We assessed patterns of UFR across age, sex, race/ethnicity, and BMI category in the NHANES 2009-2012 datasets. Geometric mean (GM) UFRs (ml/hr-kg BW) were compared across age-stratified (6-11, 12-19, and 20+ years) demographic subgroups (sex, race/ethnicity categories, and BMI category). Patterns across BMI category or race/ethnicity based on either urinary concentration or mass excretion rates (ng/hr-kg BW) were assessed in example age groups for three case study chemicals: bisphenol A (BPA), 2,5-dichlorophenol (2,5-DCP), and methyl paraben (MP). GM UFR varied significantly across age groups, from 1.01 ml/hr-kg in ages 6 to 11 to 0.62 ml/hr-kg in adults ages 20+ (p＜0.0001). UFR differed significantly among categories of race/ethnicity but not by sex, and varied inversely with BMI category in all age groups by up to three-fold, and also varied inversely with waist circumference. BPA and 2,5-DCP concentrations were positively associated with BMI category; associations were null or reversed based on analyte mass excretion rate. Observed differences in MP concentrations among race/ethnicity were reduced, but not nullified, using urinary excretion rate. Conventional hydration status adjustments did not fully address the effect of flow rate variations. UFR varies systematically with age and BMI category and can confound assessments of exposure-health outcome associations based on urinary analyte concentration or creatinine-corrected concentration.