Effects of Ergothioneine Supplementation on Glomerular Filtration and Patient-Reported Urological Symptoms in Adults with Early Renal Function Decline: A Single-Center, Open-Label, Self-Controlled Trial

Background: Early renal function decline is often accompanied by bothersome urological symptoms, yet effective early-stage nutritional interventions remain limited. L-Ergothioneine (EGT), a diet-derived antioxidant concentrated in renal tissue via the OCTN1 transporter, has shown renoprotective potential preclinically, but human interventional data are sparse. Methods: In this single-center, open-label, self-controlled trial, 31 adults (aged 45-70 years) with early renal function decline and persistent urological symptoms ([&ge;]3 months) received oral EGT (120 mg/day) for 90 days; 27 completed the study. Participants served as their own controls. The primary outcome was the within-subject change in eGFR (CKD-EPI 2021 creatinine); secondary outcomes included cystatin C-based eGFR, serum creatinine, UACR, a 10-item voiding diary, and a low-back-pain visual analogue scale (VAS). Within-subject changes were assessed by paired t-test or Wilcoxon signed-rank test. Results: Creatinine-based eGFR increased from 86.04 {+/-} 17.89 to 93.25 {+/-} 19.00 mL/min/1.73 m2 (+8.4%; p = 0.0016) and serum creatinine fell by 7.0% (p = 0.015). However, cystatin C-based eGFR and serum cystatin C were unchanged (p = 0.31 and p = 0.99), so the filtration signal was not corroborated by an independent, muscle-mass-independent marker. UACR showed a non-significant downward trend. Patient-reported outcomes improved most robustly: the total voiding diary score decreased by 57.2% (p < 0.0001) and low-back-pain VAS by 67.2% (p = 0.0002), with significant relief of urgency, frequency, and voiding difficulty. No product-related adverse events occurred. Conclusions: In this uncontrolled study, 90-day EGT supplementation was associated with marked improvement in urological symptoms and in creatinine-based eGFR, although the latter was not confirmed by cystatin C. These changes cannot be attributed to EGT alone and may substantially reflect placebo and natural-history effects. The findings are hypothesis-generating and warrant confirmation in a randomized, placebo-controlled trial using validated symptom instruments. Trial Registration: ChiCTR2500108897; Prospectively registered on 2025-09-08.