<qa>  What is the name of the study that the researchers are involved in?The researchers are involved in a research program to understand how vitamin D and calcium in the diet or as supplements may lower the risk of colorectal polyps and cancer. To that end, the researchers are conducting a study of vitamin D supplementation in which volunteer subjects are provided a Western style diet for 2 separate 4 week periods at the Rockefeller University Hospital. During one of these 4 week inpatient periods, subjects receive calcium supplements and during the other 4 week inpatient period they receive a placebo tablet. The researchers determine changes within the colon as a result of supplementing the vitamin D in the presence of high and low calcium. A more detailed description of the study is provided below. 