Vitamin D and infectious diseases.

In the early 1920s the antirachitic effect of food irradiated with ultraviolet light and cod liver oil has been recognized. The antirachitic substance was identified and called "vitamin D". Since then the key role of vitamin D in calcium and bone homeostasis has been investigated. Moreover, it has been recognized that vitamin D is able to modulate a variety of processes and regulatory systems such as host defense, inflammation, immunity, and repair. According to recent studies, vitamin D deficiency is likely to be an important etiological factor in the pathogenesis of many chronic diseases, as well as it has been associated with higher mortality rate for respiratory disease. In this regard, either observational studies aimed to verify an association between low vitamin D level and the incidence of respiratory tract infections (RTIs) or clinical trials on the effect of vitamin D as a supplementary treatment in RTIs patients have been presented in the emerging clinical literature. Conflicting results have been demonstrated in several randomized, double-blind, placebo controlled trials concerning the vitamin D treatment in tuberculosis. Some studies suggest a beneficial effect by vitamin D but it could not be reproduced in larger studies so far. In conclusion, although basic science research suggests that vitamin D may play an important role in modulating immune functions, no strong evidence exists whether correction of vitamin D depletion may be useful in the prevention or treatment of infections. Further and larger studies may clarify the role of vitamin D in infection.