Water intake mediates cross-sectional associations between urinary kidney injury biomarkers and race in emerging adults

Abstract

Purpose: Experimental hypohydration increases acute kidney injury (AKI) biomarkers, indicating potential kidney stress. Black adults are more likely than White adults to be underhydrated, yet it is unclear whether habitual hydration is associated with AKI biomarkers or whether racial differences exist, particularly in emerging adults.Methods: Fifty-four participants (24 Black/30 White, 27 M/27 F, age 21 ± 1 y, mean ± SD) provided diet diaries to assess water intake and collected 24-hour urine samples. We assessed hydration biomarkers (urine osmolality, urine specific gravity (USG)), creatinine clearance, and urinary AKI biomarkers, including kidney injury molecule 1 (KIM-1), neutrophil gelatinase–associated lipocalin (NGAL), thioredoxin-1 (TRX-1), tissue inhibitor of metalloproteinases 2 (TIMP-2), insulin-like growth factor binding protein 7 (IGFBP7), and TIMP-2*IGFBP7.Results: Black adults exhibited higher USG (p=0.043) and urine osmolality (534(506) vs. 303(541) mOsm/kg, p=0.007, median (IQR)), and reported lower water intake (p<0.001), but there was no difference in creatinine clearance (p=0.250) compared with White adults. Black adults exhibited higher urinary NGAL, TIMP-2, IGFBP7, and TIMP-2*IGFBP7 (ps≤0.034) concentrations, but not urinary KIM-1 or TRX-1 (ps≥0.096) concentrations. When indexed to urine flow rate, the racial difference persisted only for TIMP-2*IGFBP7 (319(467) vs. 215(214) [ng/mL]2/min, p=0.014). In a regression model with sex, urine osmolality, and water intake (R2=0.596, p<0.001), race was no longer associated with TIMP-2*IGFBP7 (p=0.168). Mediation analysis revealed that water intake mediated 82.6% (p=0.007) of the race difference in TIMP-2*IGFBP7 (overall p=0.021).Conclusion: Racial disparities in urinary TIMP-2*IGFBP7 excretion between emerging Black and White adults may be mediated by water intake differences.