In an adult with calcium‑oxalate kidney stones and impaired renal function, does taking vitamin C tablets lower urine pH (make the urine more acidic)?

Does Vitamin C Make Urine More Acidic?

No, vitamin C supplementation does not significantly lower urine pH or make urine more acidic in calcium oxalate stone formers. 1

Evidence on Urinary pH Effects

The most direct evidence comes from a controlled study in calcium stone-forming patients that measured fasting urinary pH before and after vitamin C supplementation:

• Urinary pH remained unchanged at 5.8 after both 1 gram and 2 grams daily of vitamin C supplementation for 3 days 1
• This finding was consistent across both stone-forming patients and healthy controls 1

The Real Risk: Increased Urinary Oxalate, Not Acidification

While vitamin C does not acidify urine, it poses a different and more clinically significant risk for patients with calcium oxalate stones and impaired renal function:

Oxalate Metabolism Mechanism

• Vitamin C is metabolized to oxalate, which directly increases urinary oxalate excretion 2, 3
• Supplementation with 1-2 grams daily increases urinary oxalate by approximately 22-60% 2, 1
• This elevation in urinary oxalate increases the calcium oxalate supersaturation index (Tiselius index) by approximately 55-63%, substantially raising stone formation risk 1

Evidence-Based Risk Thresholds

• Men consuming ≥1,000 mg/day of vitamin C have a 40% higher risk of stone formation compared to those consuming <90 mg/day 2, 3
• The risk relationship was observed only after adjusting for dietary potassium intake, suggesting the effect is independent of other dietary factors 2

Clinical Recommendations for Your Patient

For an adult with calcium oxalate stones, hyperoxaluria, and impaired renal function, vitamin C supplements should be discontinued entirely. 2, 3

Specific Management Algorithm

1. Discontinue all vitamin C supplements immediately if the patient has documented hyperoxaluria 2

2. Limit total vitamin C intake to <100 mg/day from all sources in patients with:

   • Recurrent calcium oxalate stones 4
   • Renal impairment 5
   • History of oxalate kidney stones 5

3. Do not restrict dietary vitamin C from food sources, as foods high in vitamin C also contain inhibitory factors like potassium 2

4. Monitor for oxalate nephropathy in high-risk patients:

   • Patients with renal impairment are at increased risk for acute and chronic oxalate nephropathy from vitamin C 5
   • The FDA label specifically warns that renal disease and history of oxalate stones increase nephropathy risk 5

Important Clinical Pitfalls

• Do not confuse the lack of pH effect with safety: The absence of urinary acidification does not mean vitamin C is safe for stone formers—the oxalate pathway is the mechanism of harm 1

• Beware of in vitro artifact: Older studies showing massive oxalate increases may have been confounded by in vitro conversion of ascorbate to oxalate during sample processing without EDTA preservative 4, 1

• Recognize individual metabolic variation: Some individuals demonstrate exceptionally high conversion rates of ascorbate to oxalate (up to 350% increase), manifesting as crystalluria and even hematuria 6

Contradictory Evidence Acknowledgment

One older study 7 found no significant increase in oxalate excretion with 4 grams daily vitamin C supplementation, but this conflicts with multiple higher-quality studies 2, 1 and current guideline recommendations. The weight of evidence, including prospective observational data showing 40% increased stone risk 2, supports restriction in your patient population.