IBD is caused by an abnormal immune response to the gut bacteria in people who are genetically predisposed. There has been a huge increase in the number of people diagnosed with IBD since World War II, likely due to changes in our environment. It is possible that the abundance of vitamin D in the body may be one of those environmental factors that the investigators can control to make patients with IBD better. Vitamin D acts on cells of the immune system and causes many effects, including the production of a "natural antibiotic" called cathelicidin. The investigators know that when people are supplemented with vitamin D, levels of cathelicidin produced by these immune cells increase. By supplementing children with Crohn's disease with vitamin D, the investigators may be able to alter their immune system "naturally," making their disease better. A consensus of vitamin D experts believes that vitamin D levels need to reach a level of 40-70 ng/mL in the blood in order to have effects on the immune system. Raising vitamin D levels to this range is one of the goals in the current study.

Vitamin D is an important nutrient controlling the health and development of our bones. Many patients with inflammatory bowel disease (IBD) are deficient in levels of vitamin D in their bodies. This is probably because vitamin D is lost from inflamed intestinal tissue into the stools. But while much attention has been given to studying the impact of vitamin D deficiency on the bone status of patients with IBD, our understanding of how vitamin D deficiency might affect the immune system in these patients is relatively poor. The investigators intend to study vitamin D supplementation in children with Crohn's disease, ages 8 to 18 years. At the time of enrollment, the investigators will gather data on disease activity using both a simple history and physical exam, as well as blood and stool tests. In addition, the investigators will measure the levels of cathelicidin produced by the immune cells in their blood. The investigators will then supplement 20 children with vitamin D for a total of 6 months. During the study, patients will be seen every two months, where the investigators will monitor their vitamin D levels as well as perform rigorous safety monitoring for toxicity using blood and urine tests. And at study conclusion, the investigators will again judge their disease severity and check their vitamin D levels in addition to tests of cathelicidin levels. The investigators believe that at study conclusion, the investigators will have achieved several important objectives. First, as a public health benefit, the investigators will show that large doses of supplemental vitamin D are safe in children and provide more benefit with less risk. Our patients will achieve those levels of vitamin D agreed by expert opinion that are required to cause effects on the immune system, and the investigators will see an increase in the amount of cathelicidin produced by their immune cells. As an added piece of information, the investigators would like to determine if there are any improvements in disease activity in patients supplemented with vitamin D.