GDNF and Parkinson disease: In the context of PD, potential neuroprotective effects exerted by vitamin D include the notion that 1,25(OH)2D indirectly inhibits the synthesis of nitric oxide, a free radical that can damage cells [6]; secondly, it indirectly stimulates the synthesis of the antioxidant glutathione [6], and; thirdly, vitamin D may act as a neurotrophic factor, through the stimulation of nerve growth factor (NGF), glial cell line-derived neurotrophic factor (GDNF) and neurotrophin 3 (NT3) [135,136,137].