Vitamin D: new roles and therapeutic potential in inflammatory bowel disease

Inflammatory bowel disease (IBD) encompasses 2 independent but related entities: ulcerative colitis (UC) and Crohn's disease. Crohn's disease is characterised by transmural patchy inflammation which can involve any portion of the gastrointestinal tract. UC is characterised by superficial inflammation that begins in the rectum and extends proximally along the colon. In Europe, approximately 2.2 million people have a diagnosis of IBD. The aetiology of IBD is unknown, however, immune, environmental and genetic factors are thought to be involved. Individuals with IBD are at risk of developing osteoporosis. In line with this, there are clear guidelines that recommend vitamin D supplementation for IBD patients to prevent bone disease, especially when undergoing steroid treatment. Despite an established role for vitamin D in IBD, deficiency is common. More novel effects of vitamin D beyond bone are emerging. It is now well established that vitamin D is an important regulator of the immune system which may have implications for the development, severity and management of immune related disorders such as IBD. The efficacy of vitamin D as an immune modulator in IBD remains to be proven. This review aims to evaluate the evidence implicating vitamin D deficiency in IBD pathogenesis, to examine vitamin D's anti-inflammatory mechanisms and to explore its therapeutic potential, optimal serum levels and dietary intakes which may support immune function in this disease.