28-day run-in phase during which subjects are treated with a proton pump inhibitor (omeprazole 20 mg po q day or an equivalent dose of another proton pump inhibitor). The purpose of the run-in phase is to minimize esophagitis, which can cause histologic changes that can be confused with dysplasia. After the run-in phase, subjects will undergo an upper endoscopy for Barrett's surveillance or Barrett's mapping as part of routine clinical care. At the time of endoscopy, research biopsies will be obtained for the study. Subjects eligible and continuing in the study will take vitamin D3 (Cholecalciferol) 50,000 IU capsules once weekly with or without daily metformin for a total of two or twelve weeks depending on the severity of Barrett's esophagus. After completion of vitamin D3 subjects will return for an EGD (endoscopy) and biopsies for the research study.