Prevalence and Determinants of Low Serum Vitamin D Among Women Attending Infertility Clinics in Japan: A Real‐World Multicenter Cross‐Sectional Study

ABSTRACT Purpose To estimate the prevalence and correlates of low serum 25‐hydroxyvitamin D [25(OH)D] among women attending infertility clinics in Japan. Methods We conducted a multicenter cross‐sectional study of 17 261 women with measured 25(OH)D. Deficiency and insufficiency were defined as < 20 and < 30 ng/mL. Monthly/seasonal variation was assessed, and multivariable logistic regression evaluated correlates of deficiency. In a complete‐case subset with vitamin D supplement and multivitamin information, 25(OH)D levels and 3‐category status (deficient/insufficient/sufficient) were compared between users and non‐users. The association between diminished ovarian reserve (DOR) and vitamin D status categories was assessed using the chi‐square test. Results Median 25(OH)D was 16.4 ng/mL; 66.8% were deficient and 89.0% insufficient. Levels varied by month and season ( p < 0.001). Higher BMI was associated with higher odds of deficiency, whereas older age was associated with lower odds. Supplement users had higher 25(OH)D and a more favorable 3‐category distribution ( p < 0.001); notably, 94.0% of supplement‐free women were insufficient (< 30 ng/mL). Vitamin D status categories were not significantly associated with DOR ( p = 0.238). Conclusions Low 25(OH)D is highly prevalent in infertility care in Japan with marked seasonal variation and adiposity‐related vulnerability. Supplement use was associated with higher 25(OH)D but did not explain the overall high burden.