Mechanisms of action of vitamin D in delaying aging and preventing disease by inhibiting oxidative stress.

Although several recent studies have shown that vitamin D supplementation beneficially decreases oxidative stress parameters, there is no consensus on this subject in humans. Thus the role of vitamin D supplementation has recently become a controversial topic because large intervention studies in humans have not shown significant benefits. These studies have indicated that supplementation with precursor forms of active vitamin D has no effect on all-cause mortality, cannot reduce the fracture risk of the elderly, cannot reduce the incidence of cancer or cardiovascular disease in the elderly, and cannot significantly reduce the incidence risk of diabetes in the elderly. However, a link between several age-related diseases and enhanced oxidative stress has been found in mice with insufficient or deficient 1,25-dihydroxyvitamin D (1,25(OH)D), the active form of vitamin D, which indicates that reduced active vitamin D accelerates aging and age-related diseases by increasing oxidative stress. Furthermore, supplementation of exogenous 1,25(OH)D, or antioxidants, could dramatically postpone aging, prevent osteoporosis and spontaneous tumor development induced by 1,25(OH)D insufficiency or deficiency, by inhibiting oxidative stress. Mechanistically, the antioxidative effects of 1,25(OH)D are carried out via the vitamin D receptor (VDR) by activation of the Nrf2 oxidative stress response pathway though transcriptional or posttranscriptional activation of Nrf2 or transcriptional upregulation of Sirt1 and Bmi1 expression. Whether discrepancies between studies in humans and in mice reflect the different forms of vitamin D examined remains to be determined.