Vitamin C and Kidney Safety
Vitamin C supplementation should be strictly limited to 100 mg/day in adults with chronic kidney disease or on dialysis to prevent dangerous oxalate accumulation and kidney damage, while those with a history of calcium-oxalate kidney stones should avoid supplementation above this threshold due to increased crystallization risk. 1
Dosing Recommendations by Clinical Scenario
Chronic Kidney Disease (Not on Dialysis)
- Limit total vitamin C intake to 100 mg/day from all sources (diet plus supplements) for CKD Stage 1-4 patients to prevent oxalate accumulation in plasma and soft tissues 1
- This conservative limit is necessary because impaired kidney function cannot adequately clear oxalate, the metabolic end-product of vitamin C 1
Hemodialysis Patients
- The safe upper limit is 100 mg/day for routine supplementation in chronic hemodialysis patients 1
- Hemodialysis patients lose approximately 68 mg of vitamin C daily in dialysate effluent, creating a deficiency risk that must be balanced against toxicity 1
- Water-soluble vitamin monitoring and replacement is essential due to dialysis losses and restricted fruit intake 1, 2
- Never exceed 100 mg/day in chronic hemodialysis to prevent oxalate accumulation 1
Continuous Renal Replacement Therapy (CRRT)
- During critical illness requiring CRRT, higher doses of 2-3 g/day IV may be necessary in the acute phase due to increased utilization and effluent losses 1
- This exception applies only during acute critical illness, not chronic management 1
Calcium-Oxalate Stone Formers
- Restrict vitamin C intake to approximately 100 mg/day in patients with recurrent calcium oxalate stones 3
- Supplementation with 1-2 g/day significantly increases urinary oxalate excretion (from 31-34 mg/24h to 48-50 mg/24h) and calcium oxalate crystallization risk (Tiselius index increases from 0.92-0.99 to 1.43-1.61) 4
- Even in healthy subjects, high-dose vitamin C increases oxalate excretion and crystallization risk 4
Critical Safety Considerations
Oxalate Toxicity Risk
- Excessive vitamin C intake results in increased oxalate concentrations in plasma and soft tissues, particularly dangerous in renal impairment 1
- A recent case report documented renal oxalosis and progressive kidney disease in a patient consuming 3 g daily of vitamin C during COVID-19, demonstrating that high doses can cause irreversible kidney damage even in previously stable CKD patients 5
- The risk is real and clinically significant—prompt recognition and cessation is critical to prevent permanent damage 5
Monitoring Requirements
- Monitor serum vitamin C levels and adjust supplementation based on individual dialysis losses and clinical status 1
- Closely monitor patients on prolonged renal replacement therapy for signs of oxalate accumulation 1
- Electrolyte abnormalities must be monitored in patients with AKI or CKD receiving kidney replacement therapy 1
Common Pitfalls to Avoid
- Do not rely on older studies suggesting safety of high-dose vitamin C—older assays favored in vitro conversion of ascorbic acid to oxalate during sample processing, artificially inflating oxalate measurements 3
- Do not assume that because vitamin C is water-soluble it is automatically safe in kidney disease—the metabolic conversion to oxalate creates a lipid-soluble toxic metabolite that accumulates 1
- Do not extrapolate general population recommendations to CKD patients—the 90 mg/day recommendation for healthy men and 75 mg/day for women does not apply to those with impaired renal function 2
- Beware of over-the-counter supplements marketed for immune support—patients often dramatically increase intake during illness (as seen during COVID-19 pandemic) without understanding kidney risks 5
Special Population Considerations
Pediatric Dialysis Patients
- Lower supplemental doses are required—infants on automated peritoneal dialysis receiving 140% of RDA from diet need only 15 mg/day supplement to reach 180% of RDA 1