Factors influencing vitamin D levels in women attending the fertility clinic and the effect on assisted fertility outcomes.

BACKGROUND Several factors affect vitamin D levels, such as sunshine, region, diet, obesity and so on. The primary objective of this study is to determine the prevalence of vitamin D deficiency (VDD) among infertile women in China and the contribution of VDD to the risk of infertility in women. METHODS This single-center, retrospective case-control study included 2,456 infertile women. We investigated the vitamin D levels in all patients in the different seasons and across different ages. The clinical data of 411 patients who were in the assisted fertilization programs [in vitro fertilization (IVF) and ICSI] were also analyzed, as well as the correlation between vitamin D status and IVF clinical outcome. RESULTS There were significant differences in 25-hydroxyvitamin D (25(OH)D) concentrations in different seasons (P<0.01). The proportions of severe VDD in spring, winter, summer, and autumn were 18.5%, 18.6%, 7.8%, and 8.8%, respectively. The normal levels of 25(OH)D concentration (50-74.9 nmol/L) in summer, autumn, spring, and winter were 28.5%, 26.4%, 13.5%, and 18.3%, respectively. The 25(OH)D concentrations in 3 months (July, August, and September) had the highest levels, with levels over 40 nmol/L in these months. Compared with winter, the risk of severe VDD was lower in summer and autumn (P<0.01). Serum 25(OH)D concentration significantly correlated with female infertility. CONCLUSIONS Inference from these results shows that vitamin D may minimize the risk of female infertility and may be related to the seasons and age.