Lower follicular fluid vitamin D concentration is related to a higher number of large ovarian follicles.

Vitamin D receptor-knockout mice fail to produce mature oocytes, indicating vitamin D is crucial for folliculogenesis in mice. However, the actions of vitamin D during folliculogenesis remain unknown. This prospective study aimed to assess whether follicular fluid (FF) vitamin D (25OHD3) concentrations are related to specific responses to ovarian stimulation. Women undergoing ovarian stimulation for IVF participated in the study. FF 25OHD3 concentrations were assessed in the first follicle aspirate on oocyte retrieval day. Oestradiol and progesterone concentrations were assessed on the trigger day. K-means grouping analysis showed that 25OHD3 FF concentrations clustered into a higher and lower group (mean ± SEM 17.4 ± 6.61 ng/ml and 35.5 ± 7.17 ng/ml, respectively, P < 0.001). The clusters were analysed according to the oestradiol and progesterone concentrations, follicle number and size and resulting oocyte number and maturity. The FF 25OHD3 concentrations were no different among the infertility diagnoses. The lower 25OHD3 group had more follicles (≥16.0 mm, P = 0.009) and higher serum oestradiol concentrations (P < 0.03) on the day of HCG administration. In this study, lower follicular 25OHD3 concentrations predicted a better response to ovarian stimulation shown by a greater production of larger follicles and higher serum oestradiol concentrations.