Association between Salivary Mitochondrial DNA Copy Number and Chronic Fatigue according to Combined Symptoms in Korean Adults

BACKGROUND We examined the association between salivary mitochondrial DNA (mtDNA) copy number and chronic fatigue combined with depression and insomnia. METHODS This cross-sectional study included 58 healthy adults with moderate to severe fatigue (Brief Fatigue Inventory [BFI] ≥4) for longer than 6 months. Subjects were classified as those without combined symptoms, with either depression (Beck Depression Inventory [BDI] ≥13) or insomnia (Pittsburgh Sleep Quality Index [PSQI] ≥5), or with both depression and insomnia. Salivary mtDNA copy number was measured by real-time quantitative polymerase chain reaction. The association was evaluated using a general linear model. RESULTS About 76% of participants had either depression or insomnia as additional symptoms. These subjects were predominately female, drank more alcohol, and exercised less than those without combined symptoms (P<0.05). The group with both depression and insomnia exhibited significantly higher BFI and lower mtDNA copy number than those without combined symptoms (P<0.05). After adjusting for confounding factors, significant negative associations between mtDNA copy number and usual fatigue were found in the group without combined symptoms, whereas the negative associations in the group with combined symptoms were attenuated. BDI and PSQI were not associated with mtDNA copy number. CONCLUSION Chronic fatigue is negatively associated with salivary mtDNA copy number. Salivary mtDNA copy number may be a biological marker of fatigue with or without combined symptoms, indicating that a separate approach is necessary.