The folate depletion study requires an 8-week run-in period. During this run-in period, all participants will eat an average folate diet with no high folate containing foods. There are two screening visits during this period where initial blood-work is done and participants undergo nutritional consultation. Following the 8-week run-in period, subjects in the folate depletion arm will be hospitalized for approximately 3 months at the Rockefeller University Hospital. You may leave the hospital during the day, but are required to return each evening. During this entire time, subjects are given a low folate diet. During the last month of the study, folate supplements are given to replace folate in the body. A follow-up visit is required approximately 1 month after the study period. Blood will be drawn every two weeks. Four sigmoidoscopies will be taken at approximately monthly intervals. A male is required to use an effective form of birth control throughout his participation in the study and for two months after the study ends.