Dietary intake of oxalic acid affects plasma oxalic acid concentration in patients with kidney failure.

BACKGROUND & AIMS: Hyperoxaluria may cause kidney stones, kidney function decline and eventually kidney (transplant) failure. High plasma concentrations may lead to systemic oxalosis. Plasma oxalic acid concentrations increase when kidney function deteriorates. Apart from kidney function, the contribution of dietary intake on the oxalic acid plasma concentration is unknown.

METHODS

Average and last 24 h dietary oxalic acid intake was assessed by a self-developed food frequency questionnaire. Based on frequency and portion size, the FFQ calculates daily average oxalic acid intake and last 24 h intake. Plasma oxalic acid concentration was determined immediately before kidney transplantation.

RESULTS

We included 418 patients, median age 62 years (51-69), 60 % male, 66 % on dialysis, median dialysis vintage 13 months (0-27). Average oxalic acid intake was 199 mg/day (range 4-1599); in the last 24 h before transplantation it was 138 mg/day (range 0-3906). Median plasma oxalic acid concentration was 32.2 μmol/L (range 4.6-243.2), while 98.3 % of patients had concentrations above the upper normal value (7.2 μmol/L). Multivariable linear regression analysis showed that plasma oxalic acid concentration was associated with average and last 24-h intake, body mass index, dialysis vintage, age, residual diuresis, and dialysis versus pre-dialysis status. The associations found are clinically relevant, with the lowest and highest intakes translating to plasma oxalic acid changes of 23.5 and 49.5 μmol/L, respectively.

CONCLUSION

In nearly all patients with kidney failure plasma oxalic acid concentration was above the upper normal concentration and dietary oxalic acid intake affects plasma oxalic acid concentration.