Sedentarism Exhibits a Distinct Mitochondrial Bioenergetic Phenotype Detectable by Cardiopulmonary Exercise and Lactate Testing (CPELT)

Background: Physical inactivity is a major contributor to cardiometabolic disease and mortality. Although mitochondrial dysfunction characterizes overt pathology, whether sedentarism constitutes a distinct and measurable bioenergetic disease state, rather than simply reduced fitness, has not been established. Methods: Nine sedentary (SED) and ten physically active (AC) healthy males (42 ± 14 yr) were studied. Skeletal muscle bioenergetics were assessed using high-resolution respirometry, fluxomics, metabolomics, and protein expression analyses. Whole-body physiology was evaluated using cardiopulmonary exercise and lactate testing (CPELT). Results: At rest, SED exhibited marked reductions in mitochondrial capacity, including Complex I (−36%), Complex II (−28%), electron transport system capacity (−34%), and ATP-synthase-coupled respiration (−30%, all p < 0.01). The most pronounced alteration was a 49% reduction in mitochondrial pyruvate carrier (MPC1) expression, which closely correlated with reduced pyruvate oxidation (−37%, p = 0.006) and lower TCA intermediates. SED also showed reduced MCT1 abundance, impaired fatty-acid oxidation capacity (−32% to −35%), decreased CPT1 activity (−51%), altered cardiolipin composition, and elevated ROS/O2 flux ratios. During exercise, SED demonstrated lower VO2max (−38%), reduced fat oxidation (−35%), and higher blood lactate accumulation (>60%, p < 0.001). Mitochondrial function was strongly associated with exercise performance (r = 0.57–0.78, p < 0.01). Conclusions: Healthy sedentary adults displayed a coordinated reduction in tissue-level mitochondrial oxidative capacity, substrate-handling markers, cardiolipin abundance, and metabolic flexibility. These findings should be interpreted as an integrated per-mg skeletal-muscle bioenergetic phenotype in which lower mitochondrial density may account for much of the observed reduction. Within this phenotype, the 49% reduction in MPC1 alongside preserved GLUT4, LDHA, and LDHB abundance represents an outstanding differential observation that future studies with direct mitochondrial-content normalization should test. CPELT-derived fat oxidation and blood lactate responses reflected this tissue-level bioenergetic phenotype, providing candidate noninvasive physiological markers for future longitudinal and interventional studies.