This study is aiming at explore the characteristics of intestinal microbiome during the early progression of COPD, the correlation between the changes of intestinal microbiome and the severity and risk of acute exacerbation of COPD, the correlation between microbial metabolites SCFA and immune function of COPD. Then reveal the influence of intestinal microecology on the development of COPD and the possible mechanism of intestinal microecology in the pathogenesis of COPD.

1. Invite participants according to inclusion criteria and exclusion criteria and divide them into 4 groups, including healthy control (HC), high-risk COPD group (HG), early COPD group (EG), mild and moderate COPD group (MG). Research contents will be explained detailedly to the participants, and the healthy participants and COPD patients who volunteer to participate in this study will sign the informed consent form (ICF) under the premise of adequate understanding.
2. Collect clinical data of the participants and asses the severity of symptoms and the risk of acute exacerbation of COPD patients. Clinical data include general condition, history of past illness, history of present illness, personal history, family history and the examination results of blood routine, pulmonary function and compatible computed tomography. Breathlessness measurement adopt the modified British Medical Reseach Council (mMRC); symptoms measurement adopt COPD assessment test (CAT); quality of life measurement adopt St. George's Respiratory Questionnaire (SGRQ); risk of acute exacerbation measurement adopt dyspnea，degree of airflow obstruction，smoking status and the number of exacerbation (DOSE) scoring system.
3. Collect fecal specimens from the participants on the morning of the same day. During the first three days of collection, they should keep their daily dietary habits and avoid sudden changes in dietary habits. Considerations: first remove the urine, excrement into a clean dry container, do not mix with urine and other sundries; the part of the feces that do not contact the air and container is taken from the specimen; women who are menstruating cannot be sampled. Each participant collect 3 fecal samples with a sterile spoon in a sterile enzyme-free cryopreservation tube, label the sample name and date, quickly placed in a -20℃ refrigerator, and transported to the hospital within 2 hours, where they were stored at -80℃. Fecal microbiome are detected by 16S rRNA gene sequencing and metabolite short chain fatty acid (SCFA) are detected by Gaschromatography (GC).
4. Serum of participants are collected at the clinical laboratory and detect indicators related to immune function by enzyme-linked immunosorbent assay (ELISA).
5. Explore the characteristics of intestinal microbiome during the early progression of COPD, the correlation between the changes of intestinal microbiome and the severity and risk of acute exacerbation of COPD, the correlation between microbial metabolites SCFA and immune function of COPD.

Inclusion Criteria:

1. ≥10 pack-years smoking history;
2. Examination of pulmonary function and compatible computed tomography meeting group requirements (as shown in Groups and Interventions).

Exclusion Criteria:

1. Take antibiotics, probiotics, prebiotics, synbiotics and other drugs that obviously interfere with intestinal microbiome within 2 months;
2. Suffer from other chronic respiratory diseases other than COPD (such as bronchial asthma, allergic rhinitis, pulmonary interstitial fibrosis, bronchiectasis, lung cancer, etc.);
3. Suffer from severe intestinal diseases (such as inflammatory bowel disease, intestinal infections, colorectal cancer, etc.);
4. Suffer from serious hematopoietic system diseases, and the brain, heart, liver, kidney and other important organs are damaged;
5. Suffer from severe hypertension, coronary heart disease, diabetes and other chronic diseases and taking drugs for long-term maintenance;
6. Suffer from active infectious diseases (hepatitis B, tuberculosis, etc.);
7. Pregnant or lactating women;
8. Patients with obvious anxiety, depression and other psychiatric symptoms and patients with schizophrenia.

Participants are from Physical Examination Department and Respiratory and Critical Care Medicine Department of the Second Affiliated Hospital of Xi 'an Jiaotong University.

We plan to share with others the deidentified individual-patient data (IPD) underlying the results presented in the article (including tables, figures, and appendices or supplementary material).

Immediately following publication; no end date

We plan to share with anyone who wishes to access the data for any purpose of analyses and data are available indefinitely.