Crohn's disease: the hot hypothesis.

The peculiar geographic distribution of inflammatory bowel disease is a puzzle for researchers. A low vitamin D status has now been linked to several Th1-mediated autoimmune diseases, including multiple sclerosis, type 1 diabetes and rheumatoid arthritis, with the strongest evidence for the vitamin's protective role in multiple sclerosis. Sunlight and vitamin D may be potent immunomodulatory agents by down-regulating Th1-driven immune responses and inducing the synthesis of antimicrobial peptides considered as natural antibiotics of the immune system. Similarly to multiple sclerosis, we propose in CD the so-called north-south gradient may be partly explained by variations in the degree of sun exposure, with vitamin D being a "seasonal stimulus". These observations may yield a better understanding of the pathophysiology of Crohn's disease and pave the way for developing new therapeutic approaches for an incurable disease. Whether a low vitamin D status is associated with an increased risk of Crohn's disease in the general population and whether vitamin D and heliotherapy may be effective in treating Crohn's disease will require additional investigations.