This multicenter, randomized, double-blind, placebo-controlled clinical trial aims to evaluate the efficacy of Vitamin C supplementation in improving the prognosis of patients with Chronic Obstructive Pulmonary Disease (COPD) through its antioxidant effects.

The primary objectives of the study are to determine:

1. Whether Vitamin C reduces the frequency of acute COPD exacerbations.
2. Whether Vitamin C improves pulmonary function and health-related quality of life in patients with COPD.

The study will compare Vitamin C to a placebo to assess its impact on oxidative stress and its potential to enhance clinical outcomes in COPD management.

Participants will:

1. Receive either daily oral Vitamin C supplementation or a placebo for a period of 6 months. Undergo monthly clinical evaluations, including spirometry and symptom assessments.
2. Report any adverse events or health changes during the trial.

Inclusion Criteria:

1. Age 40-80 years (including 40 and 80).

2. Patients with COPD classified as GOLD stages 2-4:

   Post-bronchodilator FEV1/FVC < 70% and FEV1 < 80% of predicted value.

3. No respiratory infection or acute exacerbation of COPD within 4 weeks prior to enrollment. Acute exacerbation is defined as an acute worsening of respiratory symptoms requiring additional treatment.

4. The patient voluntarily consents to participate in the study, is capable of verbal or written communication, and can understand and sign the informed consent form. The patient must also be able to complete the necessary assessments required for the study.

Exclusion Criteria:

1. Presence of other respiratory diseases such as alpha-1 antitrypsin deficiency, primary ciliary dyskinesia, active pulmonary infection (e.g., tuberculosis), lung cancer, pulmonary fibrosis, cystic fibrosis, obesity hypoventilation syndrome, sarcoidosis, pulmonary hypertension, or clinically significant interstitial lung disease (patients with COPD as the primary disease and comorbid bronchiectasis may be included).
2. History of asthma or asthma-COPD overlap syndrome: Patients currently diagnosed with asthma, according to GINA or other recognized guidelines, will be excluded. Patients with a childhood history of asthma (defined as diagnosis before age 18 and fully resolved) may be included.
3. History of lung resection or expected need for lung volume reduction surgery during the study.
4. Clinically significant abnormalities on chest CT that are not attributable to COPD.
5. Presence of clinically significant cardiovascular, neurological, psychiatric, renal, hepatic, immunological, gastrointestinal, genitourinary, musculoskeletal, dermatological, sensory, endocrine diseases, or hematological abnormalities, as judged by the investigator, that may affect patient safety or confound the effectiveness or safety analysis during the study.
6. Unstable cardiac disease: Patients will be excluded if they have any of the following conditions: a. Myocardial infarction, unstable angina/acute coronary syndrome, coronary artery bypass graft (CABG) surgery, or percutaneous coronary intervention (PCI) within the past 6 months. b. Structural heart disease such as hypertrophic cardiomyopathy or significant valvular disease. c. Unstable or life-threatening arrhythmias requiring intervention within the past 3 months. d. NYHA class II-IV heart failure.
7. Active malignancy: Patients with active cancer will be excluded.
8. Recent Vitamin C use: Patients who have used Vitamin C supplements within the last 4 weeks will be excluded.
9. Diabetes: Patients with poorly controlled diabetes or fasting blood glucose > 10 mmol/L will be excluded.
10. History of kidney stones or diagnosis of kidney stones within the past year, as high doses of Vitamin C may increase the risk of stone formation.