Background and objectives: High dietary acid load may accelerate kidney function decline. We prospectively investigated whether dietary acid load is associated with graft outcomes in kidney transplant recipients and whether venous bicarbonate (HCO3−) mediates this association.
Design, setting, participants and measurements: We used data from 642 kidney transplant recipients with a functioning graft ≥1 year after transplantation. Net endogenous acid production (NEAP) was estimated using food frequency questionnaires (FFQ) and, alternatively, 24-hour urinary urea and potassium excretion to estimate NEAPUrine. We defined composite kidney endpoint as doubling of plasma creatinine or graft failure. Multivariable Cox regression analyses, adjusted for potential confounders, were used to study the associations of dietary acid load with kidney endpoint. We evaluated potential mediation effects of venous HCO3− , urinary HCO3− excretion, urinary ammonium (NH4+) excretion, titratable acid excretion, and net acid excretion on the association between NEAP and kidney endpoint.
Results: Median NEAPFFQ and NEAPUrine were 40 (Interquartile range [IQR] 35-45) and 54 (IQR 44-66) mEq/day, respectively. During a median follow-up time of 5.3 (IQR 4.1-6.0) years, 121 (19%) participants reached kidney endpoint. After multivariable adjustment, NEAPFFQ and NEAPUrine (per SD higher) were independently associated with higher risk for kidney endpoint (hazard ratio [HR] 1.33; 95% confidence interval [CI] 1.12-1.57, P=0.001 and HR 95%CI, 1.44 [1.24-1.69], P<0.001 resp.). Baseline venous HCO3− mediated 20% of the association between NEAPFFQ and kidney endpoint. Baseline venous HCO3−, urinary NH4+ excretion and net acid excretion mediated 25%, -14% and -18% resp. of the association between NEAPUrine and kidney endpoint.
Conclusion: Higher dietary acid load was associated with a higher risk of doubling of plasma creatinine or graft failure, and this association was partly mediated by venous HCO3−, urinary NH4+ and net acid excretion.