Sperm mitochondrial DNA copy number exhibits higher longitudinal consistency than conventional semen parameters in a clinical cohort

RESEARCH QUESTION: What is the temporal stability of sperm mitochondrial DNA copy number (mtDNAcn) relative to conventional semen parameters and sperm DNA integrity measures? DESIGN: Longitudinal observational study with repeated semen samples in which conventional semen parameters, sperm mitochondrial DNA copy number and sperm DNA integrity measures were assessed. Ninety-five men participating in the Folic Acid and Zinc Supplementation Trial (FAZST) who provided paired semen samples at baseline and at 2, 4 or 6 months. Temporal stability of sperm mtDNAcn, conventional semen parameters, high DNA stainability (HDS) and DNA fragmentation index (DFI) was assessed using intraclass correlation coefficients (ICCs). RESULTS: Sperm mtDNAcn and HDS had the highest temporal stability (ICCs >0.90), exceeding that of all conventional semen parameters and DFI (ICC range 0.37‒0.76). The temporal stability of sperm mtDNAcn was consistent across sample intervals, with no significant differences in ICCs between time points. Sperm mtDNAcn was inversely correlated with sperm concentration, total sperm count, motility and morphology, and positively correlated with DFI and HDS. CONCLUSION: Sperm mtDNAcn had greater temporal stability than conventional semen parameters, suggesting it may be less susceptible to short-term ejaculate variability and supporting further investigation of its potential utility as a biomarker of male reproductive health.