Inclusion Criteria:  Aged 40-80 years without diabetes; CRAs removed without recurrence before recruitment; Must sign the consent form after being fully informed and understanding the purpose and procedure of this study.  Exclusion Criteria:  Familial adenomatous polyposis (FAP) or hereditary non-polyposis colorectal cancer (HNPCC, Lynch syndrome); Regularly taking aspirin, nonsteroidal anti-inflammatory drugs (NSAIDs), cyclo-oxygenase 2 (COX2) inhibitors, calcium or vitamin D; Have a history of gastrointestinal surgery; Have severe heart, liver or kidney disease ; Have cancer history; Women with pregnant, during breast-feeding period, or with expect pregnancy; Diabetes（taking diabetes medicines or HbA1c>6.5%); Inflammatory bowel disease; Mental illness; Intolerant to metformin; Cannot tolerate colonoscopy; Staffs in this clinical trial; Poor bowel preparation for colonoscopy or the examination time is shorter than 6 minutes; Unsuitable for inclusion by the investigator