Vitamin D deficiency and male erectile dysfunction
Optimal blood concentrations of vitamin D are vital for a healthy sexual life in men . Calcitriol (the active form of vitamin D) induced the production of nitric oxide (NO) in cultured endothelial cells, and NO is a potent vasodilator associated with penile erection . The outcome of a meta-analysis conducted by Crafa et al.  suggests that hypovitaminosis D (serum concentrations of 25-hydroxyvitamin D < 20 ng/mL) is associated with male erectile dysfunction (ED). Men with 25-hydroxyvitamin D deficiency had a higher incidence of ED compared to those with serum levels at least 30 ng/mL . Vitamin D deficiency may weaken erectile function by promoting endothelial dysfunction . However, vitamin D supplementation improved erectile function which was complemented by an increase in serum testosterone levels in middle-aged men . An optimal blood concentration of vitamin D may promote arterial blood supply to the cavernous bodies by improving endothelial vasodilation via mediating the bioavailability of NO, which is an effective endothelium vasodilator . The intake of vitamin D supplement was significantly associated with increased chances of survival among patients with cardiovascular diseases and diabetes, most especially in patients who are vitamin D deficient . The increase in the chances of survival among the patients was associated with improvements in endothelial function . Normal levels of vitamin D [serum concentrations of 25-hydroxyvitamin D] range between 30 and 80 ng/mL . Serum concentrations between 21 and 29 ng/mL are considered insufficient . Serum concentrations of 25-hydroxyvitamin D < 20 ng/mL are considered deficient, and it increases the risk of ED; moreover, severe deficiency may be defined as serum concentrations < 10 ng/mL . The incidence of ED declines when serum concentrations of 25-hydroxyvitamin D are over 35 ng/mL . Low serum concentrations of 25-hydroxyvitamin D are strong diagnostic criteria for ED in men with type 2 diabetes, because vitamin D deficiency affects endothelial function consequently increasing the risk of cardiovascular diseases . A considerable percentage of men with ED had lower serum concentrations of 25-hydroxyvitamin D, and it is more prevalent among patients with arteriogenic etiology, which indicates that hypovitaminosis D may weaken erectile function by increasing endothelial dysfunction . The recommended daily amount of vitamin D is 600 international units (IU) for adults aged 18–70 years and 800 IU for adults older than 70 years [7, 8]. Moreover, to sustain or improve endothelial and erectile function which is maintaining serum levels of 25-hydroxyvitamin D above 30 ng/mL, male adults need to take at least 1500–2000 IU/day of supplemental vitamin D [6,7,8].