Vitamin D levels and prolonged menstrual cycle in women with polycystic ovary syndrome: a cross-sectional study

Objective This study evaluated serum vitamin D (VD) levels in women with polycystic ovary syndrome (PCOS) and assessed their association with menstrual cycle characteristics. Methods In this cross-sectional study, 449 women diagnosed with PCOS were stratified based on 25-hydroxyvitamin D 25(OH)D levels: low VD group (20 ng/mL) and normal VD group (≥20 ng/mL). Menstrual cycles of 26–35 days were defined as normal; cycles exceeding 35 days were classified as prolonged. All participants underwent measurements of body mass index (BMI), fasting plasma glucose (FPG), fasting insulin (FINS), and sex hormones. Results The prevalence of prolonged menstrual cycles was significantly higher in the low VD group (87.2%) compared to the normal VD group (70%) ( p 0.001). Regression analysis demonstrated an inverse association between VD levels and the risk of prolonged cycles ( p 0.001). This association remained significant after adjusting for age, BMI, HOMA-IR, and total testosterone. Each 1 ng/mL increase in VD was associated with an 9% reduction in the risk of prolonged cycles. Fitted curves indicated a non-linear relationship between VD levels and the prevalence of prolonged cycles. Inflection point analysis identified VD = 27.76 ng/mL (approximately 28 ng/mL) as the inflection point. Below this level, the prevalence of prolonged cycles decreased as VD increased; above 28 ng/mL, the prevalence plateaued. Conclusion In this cross-sectional study, lower vitamin D levels were associated with a higher prevalence of prolonged menstrual cycles among women with PCOS. This association suggests that further investigation into whether maintaining vitamin D levels at or above 28 ng/mL could influence menstrual cycle regularity in this population is warranted.