Saliva cell-free mitochondrial DNA (cf-mtDNA) as a dynamic biomarker of stress and emotion in daily life: Evidence from two independent repeated-measures studies.

The present study analyzes the impact of naturalistic stress and emotions on saliva cell-free mitochondrial DNA (cf-mtDNA) in daily life across two independent cohorts with different temporal resolutions. Study 1 examined the interaction between daily stress and major depressive disorder (MDD) on cf-mtDNA in young adults (n= 18, 8 MDD, 10 controls) across four days. For individuals with MDD, stress exposure was associated with a 68% reduction in cf-mtDNA. A higher number or greater severity of stressors also reduced cf-mtDNA by 24 to 27%. Study 2 extended this framework by implementing a finer temporal resolution, measuring saliva and affective states every hour, up to 20 times per day for 2 days (n = 25). Negative emotions, including stress and frustration, were associated with reductions in cf-mtDNA of 15%, whereas positive emotions, such as happiness and calm, predicted increases of up to 28%. The strength and direction of the effects were person- and context-dependent. These findings suggest that cf-mtDNA does not exhibit a uniform stress response in daily life. Instead, it reflects dynamic signaling shaped by timing, emotional context, and diagnostic status. This work demonstrates that interpreting cf-mtDNA as a stress biomarker in real-world settings requires modeling timing and heterogeneity of effects.