Depending on whether someone has already been randomized, participants of the parent study will be invited to take part in the adjunct study in one of two ways: 'Aim 1' only, or 'All Aims'. First, patients who have already been randomized will be asked to allow biopsies to be made of their rectal tissue during their 3- or 5-year follow-up colonoscopy (Aim 1). Biopsies, which will be used for our biomarker measurements, are very tiny pieces of tissue that can be examined under the microscope. Second, patients who have not yet been randomized will be invited to participate more fully (All Aims) in the adjunct study. This involves having outpatient rectal biopsies taken immediately after their first phone call, their 1-year follow-up visit, and 7 - 21 days before their 3- or 5-year follow-up colonoscopy. Finally, during their 3- or 5-year colonoscopy, biopsies will be taken from three areas of the colon: the rectum (same area as the outpatient biopsies), the sigmoid colon, and the ascending colon. From all of the biopsies taken from all of the visits and colon sites, biomarker measurements will be of normal proteins that occur in the surface cells lining the colon. Study researchers will then analyze whether calcium and/or vitamin D affect these biomarkers and whether the effects predict who gets new polyps.