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.