Vitamin D is mainly known for the role it plays in calcium absorption to ensure optimal well-being of bones and teeth. In reality, its function is much broader and relates to cardiovascular health, the immune response, diabetes. Furthermore, along with sex hormones, vitamin D modulates reproductive processes in both women and men.
L’ 80-90% of vitamin D is produced by the skin due to UVB rays, while a smaller quantity derives from diet and / or supplementation. Vitamin D from the skin and diet is metabolised in the liver to 25-hydroxyvitamin D, a form used to evaluate the state of vitamin D and classify it in a sufficient value if it is higher than 30 ng / ml, an insufficient value if it is between 20 and 29 ng / ml, deficiency if less than 20 ng / ml. The 25-hydroxyvitamin D form is converted at the renal level, but also at the level of many other tissues, into 1,25-dihydroxyvitamin D, which is the active form that acts through specific receptors for vitamin D distributed in various tissues such as the skeleton, parathyroid glands and reproductive tissues.
VITAMIN D AND FERTILITY
The correlation between seasons and exposure to sunlight with reproduction has been extensively studied: the circulating levels of 25-hydroxyvitamin D show seasonal variability with high levels in summer and autumn and low levels in winter and spring. In northern countries, where there is a strong contrast of brightness depending on the season, conception rates decrease during the dark winter months, while there is a peak of conception during the summer and consequently a peak of births during the spring.Many studies have shown effects of vitamin D on the results of in vitro fertilization pathways, on pathological conditions such as polycystic ovary syndrome (PCOS), endometriosis and on the quality of seminal fluid (sperm count, motility and morphology ). Vitamin D stimulates the production of steroid hormones: it is essential for the synthesis of estrogens in both sexes. Men with low levels of vitamin D show low testosterone levels and low percentage of motile and morphologically normal spermatozoa. Endometriosis has a pathogenesis related to inflammatory and immune-mediated processes: vitamin D is involved in the regulation of the immune system. A vitamin D deficiency is frequently present in women with polycystic ovary syndrome, the most common female endocrine disorder with a prevalence 5-10% in women of reproductive age. Many authors have highlighted a correlation between obesity and vitamin D deficiency, but it is unclear whether vitamin D insufficiency derives from obesity or vice versa. In these patients, increases in circulating vitamin D levels after careful therapy were related to normalization or improvement of menstrual cyclicity, acne and insulin resistance. A particularly interesting finding emerges from a study conducted among 101 young women in which it was highlighted an inverse correlation between vitamin D levels and progesterone and estradiol levels: high levels of vitamin D are associated with low risk of breast cancer due to the potential effect of reducing levels of estradiol and progesterone. Vitamin D deficiency is common in women during pregnancy due to an increased maternal and fetal consumption: this deficit is associated with an increased risk of gestational diabetes and other pregnancy complications such as gestational hypertension and preeclampsia, bacterial vaginosis, intrauterine growth retardation, preterm delivery. For these reasons, vitamin D supplementation is safe and effective from pregnancy to delivery.