Blood Mitochondrial Biomarker Reveals Sex-Specific Links Between Fitness and Brain Aging

Alzheimer's disease (AD) risk is shaped by modifiable factors like physical activity and cardiorespiratory fitness (CRF), yet the cellular mechanisms connecting exercise capacity to brain health remain poorly defined. Women are twice as likely as men to develop AD and consistently show lower CRF — a convergence that makes understanding sex-specific mitochondrial biology particularly urgent.

This cross-sectional study drew on 34 cognitively healthy, sedentary older adults (ages 65–80; 18 male, 16 female) enrolled in the COMbined Exercise Trial (COMET). Participants underwent graded maximal exercise testing using a Modified Balke protocol to capture VO₂peak and oxygen uptake efficiency slope (OUES). Body composition was assessed by dual-energy X-ray absorptiometry (DXA). Blood lymphocytes were isolated from ACD tubes within 6 hours of collection and stained with four fluorescent probes — MitoTracker (mitochondrial mass), MitoSOX (superoxide/ROS), TMRE (membrane potential), Annexin V (apoptosis), and Hoechst (nuclear) — then analyzed by flow cytometry. These measures were combined into a validated composite mitochondrial function index (MFI).

At the group level, males and females were matched for age, BMI, APOE4 carrier status, and resting/peak respiratory exchange ratio. However, males had significantly higher lean mass, VO₂peak, OUES, and MFI (all p ≤ 0.01), while females had higher body fat percentage. Plasma pTau217 did not differ between sexes. When analyzed within each sex, distinct metabolic patterns emerged. In males, MFI declined significantly with advancing age (R² = 0.382, p = 0.01), suggesting progressive mitochondrial deterioration across the male aging trajectory. In females, no such age-MFI relationship was found; instead, mitochondrial superoxide levels (MitoSOX signal) were negatively associated with both lean mass (R² = 0.648, p < 0.01) and OUES (R² = 0.271, p = 0.04). Higher oxidative stress from mitochondria predicted lower fitness capacity and lower muscle mass specifically in women.

These findings suggest the mitochondrial-fitness axis operates through different biological routes depending on sex. In males, the dominant signal is an age-related decline in overall mitochondrial competence. In females, mitochondrial reactive oxygen species (ROS) — not overall function — are the stronger correlate of fitness and body composition. This may reflect hormonal differences, particularly post-menopausal loss of estrogen's antioxidant and mitochondrial-protective effects. Given that both poor fitness and mitochondrial dysfunction are independently linked to AD pathology, sex-divergent mitochondrial profiles could help explain why women carry greater AD burden.

The study is preliminary and limited by a small, convenience sample from a single trial. Longitudinal and interventional designs are needed to determine whether exercise training shifts these mitochondrial biomarkers and whether such shifts differ by sex.