Urolithin A, Spermidine, and NAD+ Precursors in Human Healthy Aging: A Comparative Narrative Review of Recent Clinical Evidence

Background: Nutritional geroscience has increasingly focused on compounds that target conserved cellular maintenance pathways implicated in aging. Urolithin A, spermidine, and NAD+ precursors, particularly nicotinamide riboside and nicotinamide mononucleotide, are frequently grouped as “longevity supplements,” although they differ substantially in mechanism, clinical evidence, and translational maturity. Methods: This focused narrative review evaluated peer-reviewed evidence published between January 2016 and April 2026, prioritizing systematic reviews, meta-analyses, and human randomized controlled trials. Outcomes of interest included muscle performance, cognitive function, cardiometabolic markers, inflammation, immune aging, mitochondrial biomarkers, quality of life, and safety. Results: Urolithin A: 1 peer-reviewed systematic review and 10 human randomized studies were identified, including 9 placebo-controlled efficacy/safety trials with approximately 392 randomized participants; 1 randomized bioavailability crossover study increased the total to approximately 492 participants. Spermidine: 0 full peer-reviewed systematic reviews/meta-analyses, 1 structured mini-review, and 6 unique peer-reviewed randomized trials were identified, involving approximately 360 - 370 participants. Oral NAD+ precursors: 6 peer-reviewed systematic reviews/meta-analyses and 25 unique peer-reviewed randomized NR/NMN trials were identified, involving approximately 1,070 randomized participants. Conclusion: Current human evidence does not support treating urolithin A, spermidine, and NAD+ precursors as equivalent longevity interventions. Urolithin A currently shows the most coherent translational profile, with aligned mechanistic, biomarker, and early functional evidence, particularly for mitochondrial health, muscle endurance, and immune aging. Spermidine remains biologically plausible but clinically unvalidated. Oral NR/NMN reliably increases NAD+-related biomarkers and exhibits robust pharmacodynamic activity, but current evidence for broad anti-aging efficacy remains promising yet inconsistent across clinical outcomes. Longer, adequately powered, head-to-head trials using standardized geroscience outcomes are needed.