**Background:** Obstructive sleep apnea (OSA) is highly prevalent (5–15% general population, 50–80% in T2DM and obesity) and is an independent risk factor for glucose intolerance, insulin resistance, and type 2 diabetes. Elevated circulating free fatty acids (FFAs) have been proposed as a mechanistic link between intermittent hypoxia and impaired glucose metabolism, as FFAs can induce insulin resistance in muscle and liver and impair β-cell function. While adipose tissue lipolysis is known to be upregulated in OSA, whether reduced skeletal muscle FFA oxidation contributes to elevated plasma FFAs and metabolic dysregulation had not been clarified. This study aimed to determine whether severe OSA modifies skeletal muscle lipid oxidation in nondiabetic and T2DM individuals.
**Methods:** Forty-four participants were recruited into four groups: nondiabetic controls without OSA (n=14), nondiabetic with severe OSA (n=9), T2DM without OSA (n=10), and T2DM with severe OSA (n=11). Groups were matched for age, BMI, and adiposity. OSA severity was assessed by home type III sleep study (AHI ≥ 30 defined severe). After an overnight fast, participants underwent anthropometric assessment, biochemical analysis, and a frequent-sampling intravenous glucose tolerance test (IVGTT) with minimal model analysis. Muscle biopsies from the vastus lateralis were obtained two weeks later. High-resolution respirometry was used to measure oxygen consumption rate (OCR) in biopsy homogenates under basal conditions (malate+ADP), after palmitoyl carnitine (0.04 mM) to assess lipid oxidation, and after succinate (10 mM) for maximal mitochondrial respiration. Protein expression of FATP4, CD36, and CPT1 was quantified by Western blot; gene expression by qPCR.
**Key Results:** The groups were well-matched for age (62.2–64.0 years), BMI (32.7–35.2 kg/m²), and body fat percentage (28.1–30.9%). AHI was 4.3±0.7 (control), 47.2±4.0 (OSA), 5.9±0.9 (T2DM), and 47.8±4.9 (T2DM+OSA). Time spent with SpO₂ <90% (T90) was 1.8±0.7%, 24.2±7.6%, 10.1±4.1%, and 34.6±9.2%, respectively. The disposition index (DI) was reduced by 39% in OSA vs. control (652.1±112.9 vs. 1077.1±88.5) and by 51% in T2DM+OSA vs. T2DM (174.2±33.4 vs. 355.2±109.5). Fasting plasma FFA levels did not differ between groups (530±40, 530±50, 500±30, 560±40 µmol/L). Basal OCR did not differ between groups. Palmitate addition stimulated respiration significantly within each group (52%, 38%, 30%, and 56% increase in control, OSA, T2DM, and T2DM+OSA, respectively; all p<0.05), but no differences in palmitate-induced OCR were observed between groups (ANOVA p>0.05). Similarly, succinate-stimulated respiration did not differ between groups. Protein expression of FATP4, CD36, and CPT1 was stable across all groups (all NS). Gene expression of FATP4, CD36, and CPT1 also showed no group differences. Hypoxia severity (T90, T85) was positively associated with CD36 protein expression (r=0.328 and r=0.321, p<0.05). FATP4 protein was negatively associated with HOMA-IR (r=−0.335, p<0.05). CPT1 protein correlated with FATP4 protein (r=0.519, p<0.001).
**Clinical Implications:** This study provides strong evidence that severe OSA does not impair skeletal muscle lipid oxidation capacity or alter the expression of key fatty acid transport and oxidation proteins (FATP4, CD36, CPT1) in either nondiabetic or T2DM individuals. The findings suggest that elevated plasma FFAs previously reported in OSA are likely due to increased adipose tissue lipolysis rather than reduced muscle FFA clearance. This has important therapeutic implications: pharmacological interventions targeting adipose tissue lipolysis may be more effective than those aimed at muscle lipid oxidation for managing OSA-related metabolic complications. The study is limited by its ex vivo design, small sample size, and measurements taken after awakening (not during sleep), which may allow recovery from acute hypoxic effects. Nonetheless, the results indicate that daytime muscle lipid oxidation is not a significant determinant of OSA-associated glucose dysregulation.