Constipation/therapy; Defecation/physiology; Gastrointestinal Motility/physiology; Chronic Disease/epidemiology; Chronic Disease/therapy; Colon/diagnostic imaging; Colon/innervation; Colon/metabolism; Colon/physiopathology; Constipation/diagnosis; Constipation/epidemiology; Constipation/etiology; Defecography; Dietary Fiber/administration & dosage; Dietary Supplements; Digital Rectal Examination; Electromyography; Gastrointestinal Motility/drug effects; Humans; Intestinal Mucosa/drug effects; Intestinal Mucosa/metabolism; Intestinal Mucosa/physiopathology; Laxatives/administration & dosage; Magnetic Resonance Imaging; Manometry; Pelvic Floor/innervation; Pelvic Floor/physiopathology; Prevalence; Receptors, Opioid, mu/antagonists & inhibitors; Receptors, Opioid, mu/metabolism; Rectum/diagnostic imaging; Rectum/innervation; Rectum/metabolism; Rectum/physiopathology; Secretagogues/administration & dosage