Does Vitamin C Cause Alkalization of Urine?
No, oral vitamin C does not alkalinize urine—it either has no effect on urinary pH or causes minimal acidification that is clinically insignificant.
Effect on Urinary pH
Vitamin C supplementation does not change urinary pH in a clinically meaningful way. In calcium stone-forming patients receiving either 1 g or 2 g of vitamin C daily for 3 days, fasting urinary pH remained unchanged (5.8 before and after supplementation at both doses) 1. A controlled study in healthy volunteers and patients with infected urine showed that even with 6 g daily (2 g three times daily), the mean pH decrease was only -0.16 to -0.18 pH units—a change so small that vitamin C cannot be recommended to lower the pH of alkaline urines 2.
Why Vitamin C Does Not Effectively Acidify Urine
The minimal effect occurs because:
- Gastrointestinal absorption of ascorbic acid is a saturable process, meaning excess vitamin C is not absorbed once physiological limits are reached 3
- Renal tubular reabsorption is also saturable, with excess vitamin C excreted unchanged in urine rather than being metabolized to acidic compounds 3
- Urinary ascorbate excretion plateaus within 24 hours of supplementation, after which ingested vitamin C is simply excreted unmetabolized without affecting pH 4
Clinical Implications for UTI Management
Current guidelines explicitly state that urinary acidification with vitamin C is not supported for UTI prevention or treatment. The British Journal of Sports Medicine guidelines for spinal cord injured athletes recommend that cranberries, methenamine salts, acidifying or alkalinizing agents cannot be recommended for UTI prevention based on current literature 5. If alkalinization is used during acute UTI, it should be only for symptomatic relief and must not interfere with appropriate antibiotic selection 6.
Important Caveat: Oxalate Formation vs. pH Effect
While vitamin C does not alkalinize urine, it does increase urinary oxalate excretion by approximately 22% in stone-formers 1. This metabolic effect is separate from any pH change. Men consuming ≥1000 mg/day had a 40% higher risk of calcium oxalate stone formation 7, and the American College of Physicians recommends that calcium stone formers with hyperoxaluria should discontinue vitamin C supplements entirely 7.
Common Pitfall to Avoid
Do not confuse vitamin C's lack of urinary alkalinization with its well-documented effect on increasing urinary oxalate. The two are separate mechanisms: vitamin C is metabolized to oxalate (increasing stone risk in susceptible individuals) but does not meaningfully change urinary pH 1, 2.