Does Vitamin C Impact Urinary pH When Taken with Methenamine Hippurate?

Objectives: Urinary tract infections (UTIs) affect 150 million people worldwide annually and specifically impact the aging female population. Methenamine hippurate (MH) is a non-antibiotic suppressive agent to prevent recurrent UTIs (rUTIs). It is most effective when the urine is acidic (pH < 6) and can be converted to formaldehyde, a bacteriostatic agent. To accomplish this, providers commonly instruct patients to acidify their urine with vitamin C once or twice daily. This can be a cost burden and cause unnecessary side effects to the patient.  We aim to investigate whether vitamin C affects the urinary pH in the clinical setting.  We hypothesize that adding daily vitamin C supplements with MH does not affect the urinary pH.

Methods: This is a retrospective, IRB-approved study. Patients diagnosed with RUTI by two urogynecologists between 2014 and 2023 were screened for the study. Patients were excluded with active urinary stone disease, cancer, fistula, bladder mesh erosions, current suprapubic catheter, and lack of urinalysis collected at the initial or follow-up visit. Baseline demographics and data regarding urinary pH, use of vitamin C, and subsequent UTIs were collected. The primary outcome was the change in urinary pH for those taking vitamin C and those who did not.

Results: 295 patients were screened, and 78 patients taking MH were included in the final analysis. 40 patients used vitamin C with MH, and 38 did not. The median follow-up time was 3 months for both groups. 14 out of 38 (32.5%) patients using vitamin C experienced a UTI compared to 13 out of 40 (36.8%) with no vitamin C (p = 0.687). The average daily dose of vitamin C was 700 mg. The mean urinary pH was 6.0 for patients taking vitamin C and 6.1 for those who did not (p = 0.513). The mean pH change from the initial visit to the follow-up visit was -0.1 in those who took vitamin C and 0.0 in those who did not (p = 0.442). Patients experiencing one or more UTIs while on MH suppression were found to have a higher urinary pH (pH 6.3) compared to those who did not have a UTI (pH 5.9) (p = 0.002).

Conclusions: The benefit of adding vitamin C when taking MH is unclear. The urinary pH was not affected by adding vitamin C to MH. Those who did and did not take vitamin C had nearly identical urinary pH at follow-up (6.0 versus 6.1), with vitamin C only decreasing the follow-up urinary pH by -0.1. Additionally, there was a similar rate of UTI following MH suppression with or without vitamin C. However, those who did experience a UTI in the follow-up period did have a statistically significantly higher pH than those who did not.