The Role of Vitamin D in Reproductive Health-A Trojan Horse or the Golden Fleece?

In the last decade, vitamin D was in the spotlight in many fields of research. Despite numerous publications, its influence on reproductive health remains ambiguous. This paper presents an up-to-date review of current knowledge concerning the role of cholecalciferol in human reproduction. It covers various infertility issues, such as polycystic ovary syndrome, endometriosis, myoma-induced infertility, male infertility, premature ovary failure and in vitro fertilization techniques. Vitamin D deficiency, defined as serum concentration of 25-hydroxycalciferol of less than 50 nmol/L, is commonly noted more frequently than only in fertility clinic patients. It is a global trend that is observed in all age groups. The results of original publications dated up to 2015 have been summarized and discussed in a critical manner. Most experts agree that vitamin D supplementation is a necessity, particularly in women suffering from obesity, insulin resistance or small ovarian reserve, as well as in men with oligo- and asthenozoospermia if serum concentration should fall below 50 nmol/L (normal range up to 125 nmol/L). High concentration of vitamin D and its metabolites in decidua during the 1st trimester suggests its important role in the implantation process and a local immunological embryo-protection. On the other hand, evidence-based research did not prove a significant difference so far in ovulation stimulation or embryo development depending on vitamin D level. In one of the publications, it was also found that vitamin D binding protein (VDBP) has a molecular similarity to anti-sperm antibodies, and another one concluded that both low (<50 nmol/L) and high (>125 nmol/L) concentration of vitamin D are associated with decreased number and quality of spermatozoa in semen. Vitamin D is definitely not a Trojan Horse in reproductive health, since there were no adverse effects reported for vitamin D intake of up to 10,000 IU/day, but to proclaim it the Golden Fleece, more evidence is needed.