Low levels of vitamin D are associated with two major forms of human inflammatory bowel disease: Crohn's disease and ulcerative colitis.  Deficiencies in vitamin D have been linked to the severity of the case of inflammatory bowel disease, however, whether vitamin D deficiency causes inflammatory bowel disease or is a symptom of the disease is not clear.
There is some evidence that vitamin D supplementation therapy for people with inflammatory bowel disease may be associated with improvements in scores for clinical inflammatory bowel disease activity and biochemical markers. Vitamin D treatment may be associated with less frequent relapse of symptoms in IBD. It is not clear if this treatment improves the person's quality of life or what the clinical response to vitamin D treatment. The ideal treatment regime and dose of vitamin D therapy has not been well enough studied.