Dose-Effect Response in Acupuncture for Functional Constipation

Inclusion Criteria: * Meet Rome IV diagnostic criteria for functional constipation * Symptoms present for ≥6 months, meeting diagnostic criteria in the last 3 months * Mean weekly complete spontaneous bowel movements (CSBMs) ≤2 during 14-day baseline period * Age 18-75 years * No use of constipation medications for at least 2 weeks prior to treatment (except rescue medication) * No acupuncture treatment for constipation in the past 3 months * Not currently participating in another clinical trial * Willing and able to provide written informed consent Exclusion Criteria: * Constipation secondary to other causes (IBS, organic diseases, medications, endocrine disorders, metabolic disorders, neurological disorders, or gastrointestinal surgery) * Loose or watery stools (Bristol type 6 or 7) \>1 time during baseline without laxative use * History of pelvic floor dysfunction * Use of probiotics, fiber supplements, or laxatives within 2 weeks prior to treatment (2-week washout required) * Severe hemorrhoids or anal fissures * Severe or uncontrolled heart, liver, or kidney disease; abdominal aortic aneurysm; hepatosplenomegaly; cognitive impairment; or psychiatric disorders * Dependence on opioids or anticholinergic drugs * Red flags: unexplained weight loss \>10% in 3 months, hematochezia or positive fecal occult blood, family history of colon cancer (first-degree relative diagnosed \<50 years), anemia (Hb \<110 g/L), or elevated inflammatory markers * Contraindications to acupuncture: coagulation disorders or use of anticoagulants * Pregnancy or breastfeeding * Unable to comply with follow-up or contraindications to MRI (e.g., cardiac pacemaker, non-titanium aneurysm clips, metallic implants, claustrophobia)