**Background:** Iron depletion is common in chronic diseases including COPD, and oxidative stress is known to drive muscle dysfunction in these patients. However, whether non-anemic iron deficiency independently exacerbates oxidative/nitrosative stress and alters muscle phenotype in severe COPD had not been thoroughly investigated. The authors hypothesized that iron-deficient COPD patients would show elevated oxidative/nitrosative stress markers and reduced antioxidants in both systemic and muscle compartments.
**Methods:** This cross-sectional study recruited 40 severe COPD patients (GOLD classification) from Hospital del Mar, Barcelona (2018–2021). Patients were divided into two groups of 20 each: those with non-anemic iron deficiency (hemoglobin >12 g/dL women, >13 g/dL men; ferritin <100 ng/mL or ferritin 100–299 ng/mL with transferrin saturation <20%) and those with normal iron content. Exclusion criteria included recent exacerbations, cardiovascular disorders, musculoskeletal disease, obesity (BMI >30), and oral corticosteroid use. Clinical assessments included lung function (spirometry), anthropometry (BMI, FFMI), exercise capacity (6-minute walk test), and limb muscle strength (handgrip dynamometry, quadriceps MVC). Blood samples were analyzed for iron metabolism parameters (ferritin, transferrin saturation, hepcidin, soluble transferrin receptor) and systemic redox markers (protein tyrosine nitration via 3-nitrotyrosine ELISA, protein carbonyls, MDA-protein adducts, SOD activity, catalase activity, GSH, TEAC). Vastus lateralis muscle biopsies were obtained via open biopsy and analyzed for redox markers (immunoblotting for protein carbonyls, MDA adducts, 3-nitrotyrosine, SOD-1, SOD-2, catalase), muscle fiber phenotype (immunohistochemistry for MyHC-I and -II isoforms), and structural abnormalities (lipofuscin, internal nuclei, inflammatory cells, TUNEL for apoptosis). Sample size calculation used hepcidin as the target variable (α=0.05, β=0.2, minimum 16 patients/group).
**Key Results:** The two groups were similar in anthropometry, smoking history, lung function, and GOLD classification. Iron-deficient patients had significantly lower serum ferritin, transferrin saturation, and hepcidin, and higher soluble transferrin receptor and transferrin. The 6-minute walk test distance was reduced in iron-deficient patients (absolute and % reference values), but upper and lower limb muscle strength did not differ between groups. Protein tyrosine nitration (3-nitrotyrosine) was significantly higher in both muscle and blood of iron-deficient vs. non-iron-deficient patients. No significant differences were found in protein carbonylation or MDA-protein adducts in either compartment. Mitochondrial SOD (SOD-2) protein levels were significantly lower in vastus lateralis of iron-deficient patients, while SOD-1 and catalase showed no differences. Serum TEAC was significantly reduced in the iron-deficient group, but serum GSH did not differ. Muscle fiber analysis revealed a significantly higher proportion of fast-twitch fibers (approximately 80%) and lower proportion of slow-twitch fibers in iron-deficient patients. Cross-sectional area did not differ. Lipofuscin inclusions were significantly increased in iron-deficient muscles. No differences were seen in total abnormal fraction, internal nuclei, inflammatory cells, necrotic cells, or apoptotic nuclei. Positive correlations were found between SOD-2 protein levels and slow-twitch fiber proportions across all patients (and nearly significant in iron-deficient group alone, r=0.481, p=0.059), while fast-twitch fiber proportions negatively correlated with SOD-2 levels in all patients and in the iron-deficient group.
**Clinical Implications:** This study demonstrates that non-anemic iron deficiency in severe COPD is associated with increased nitrosative stress and reduced antioxidant capacity in both muscle and systemic compartments, along with a more pronounced shift toward fast-twitch (less fatigue-resistant) muscle fibers. These changes occurred independently of quadriceps muscle strength, suggesting iron status directly influences muscle redox balance and phenotype. The findings support routine assessment of iron metabolism parameters in severe COPD patients, as iron deficiency may contribute to exercise intolerance and muscle dysfunction through oxidative mechanisms. Therapeutic strategies targeting iron repletion could potentially improve redox balance and muscle function in this population, though longitudinal interventional studies are needed.