Vitamin D and PCOS
Roughly two-thirds of women with polycystic ovary syndrome are vitamin D deficient — a much higher rate than the general population. The overlap is not coincidence: vitamin D influences insulin sensitivity and ovarian steroidogenesis, both central to PCOS pathophysiology.
Why PCOS and low 25(OH)D co-occur
Higher BMI (common in PCOS) sequesters cholecalciferol in adipose tissue, lowering serum 25(OH)D. Insulin resistance further impairs downstream vitamin D signalling. And women with PCOS report less outdoor time and more sedentary behaviour on average, compounding cutaneous synthesis loss. The result is a bidirectional loop where each condition worsens the other.
Insulin sensitivity
Meta-analyses of PCOS supplementation trials show modest but consistent improvements in HOMA-IR (typically 10–20% reduction) and fasting insulin with 2,000–4,000 IU/day cholecalciferol over 8–12 weeks in deficient women. Effects on HbA1c are smaller but also positive. This is one of the more replicated benefits of vitamin D repletion in PCOS.
Menstrual regularity and ovulation
Trials combining vitamin D (up to 50,000 IU weekly) with metformin have shown improved cycle regularity and higher spontaneous ovulation rates compared to metformin alone. Vitamin D monotherapy also modestly improves cycle regularity in deficient women. Neither intervention matches the ovulation-induction efficacy of letrozole or clomiphene, but vitamin D is a safe adjunct.
Androgens (testosterone, DHEAS)
Small trials show 15–25% reductions in free testosterone and DHEAS after vitamin D correction, likely mediated through improved insulin sensitivity (which lowers ovarian androgen production). Clinical impact on hirsutism and acne is modest and slow (3–6 months).
Fertility outcomes
Higher pre-conception 25(OH)D is associated with better live-birth rates in observational IVF studies; RCT data is small but favourable. For women trying to conceive with PCOS, correcting deficiency (target ≥ 30 ng/mL) before starting ovulation induction is a reasonable step. See our pregnancy page for continuing supplementation.
Practical protocol
- Test serum 25(OH)D at PCOS diagnosis and annually.
- Aim for 30–50 ng/mL — the range associated with insulin-sensitivity benefit.
- Typical dose: 2,000–4,000 IU/day cholecalciferol (higher if BMI > 30).
- Retest at 3 months.
- Combine with the standard PCOS lifestyle interventions — moderate weight loss (5–10% body weight), Mediterranean-pattern diet, and regular exercise.