Vitamin D and in vitro fertilization: a systematic review.

PURPOSE: Vitamin D (VD) action on calcium is well known, but its other properties, particularly in the human reproductive system, are not completely understood. This review aims at assessing the relationship between serum VD levels and IVF results. METHODS: A systematic review was carried out using MEDLINE, EMBASE, LILACS, Google scholar, the CAPES journal portal, and the Cochrane Library, as well as the descriptors "vitamin D" and "fertilization in vitro." The review was conducted by two authors following the PRISMA recommendations between September 2021 and February 2022. RESULTS: Eighteen articles were selected. Five of them showed a positive correlation between serum VD levels and IVF results, 12 lacked any association, and 1 exhibited a negative correlation. The 3 studies that assessed VD in the follicular fluid found a positive correlation between the serum and follicular levels. Non-Hispanic White patients seemed to suffer the consequence of vitamin D deficiency more than Asian patients. In one study alone in the VD-deficient group, there was a larger number of natural killer (NK) cells, B cells, a greater proportion of helper T cells/cytotoxic T cells (Th/Tc), and an association with a smaller number of mature oocytes. CONCLUSION: The association between serum VD levels and the post-IVF pregnancy rate is uncertain. However, VD levels might be more relevant in the White than in the Asian ethnicity and in the number of aspiration follicles, and they could act in the immune system having an impact both on embryo implantation and on pregnancy.