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

## TL;DR

This study finds that most women at infertility clinics in Japan have low vitamin D levels, with higher rates in certain seasons and among those with higher BMI.

## Contribution

The study provides real-world data on vitamin D deficiency among Japanese women seeking infertility treatment and its correlates.

## Key findings

- 66.8% of women were vitamin D deficient and 89.0% insufficient.
- Vitamin D levels varied by season and were higher in older women and lower in those with higher BMI.
- Supplement users had significantly better vitamin D status than non-users.

## Abstract

To estimate the prevalence and correlates of low serum 25‐hydroxyvitamin D [25(OH)D] among women attending infertility clinics in Japan.

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.

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).

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.