Abstract Mitochondrial dysfunction and stem cell exhaustion contribute to age-related immune decline, yet clinical interventions targeting immune aging are lacking. Recently, we demonstrated that urolithin A (UA), a mitophagy inducer, expands T memory stem cells (TSCM) and naive T cells in mice. In this randomized, double-blind, placebo-controlled trial, 50 healthy middle-aged adults received oral UA (1,000 mg day−1) or placebo for 4 weeks; time points of analysis were baseline and day 28.