The present invention provides preventive approaches and treatments for an inflammatory bowel disorder (e.g., Crohn's disease or ulcerative colitis) or a gastrointestinal tumor (e.g., a colorectal cancer) comprising administering an O-glycan composition (e.g., mucins) to a subject, such as a human patient. In addition, the present invention also provides transgenic mice that fail to synthesize core 1-derived or core 3-derived O-glycans.