Study of Folic Acid, Calcium and Vitamin D in Preventing Colorectal Polyps and Colorectal Cancer

Inclusion Criteria: * individuals aged 50-80 years * Patients had undergone complete colonoscopy with no adenoma found * Those who voluntarily sign the consent form after being fully informed and understanding the purpose and procedures of the study, characters of the disease, effect of medications, methods of related examinations, and potential risk/benefits of the study Exclusion Criteria: * Patients who are hypersensitive or intolerant to the drugs * Patients who are intolerant to another colonoscopy examination * Pregnant women, woman during breast-feeding period, or women with expect pregnancy * Patients with diabetes mellitus, severe heart or renal disease，or cancer history * Patients with malignant neoplasm, or suspicious colorectal cancer * Patients who are not able to cooperate * Individuals who are involved in designing, planning or performing this experiment * Patients with medical conditions who are not appropriate to participate the study * Patients who are take aspirin or cyclooxygenase 2 inhibitor （COX2）.