What is the effect of vitamin D supplementation on insulin resistance in women with polycystic ovary syndrome (PCOS). Systematic Review of Randomized Controlled Trials

Polycystic ovary syndrome (PCOS) is a common endocrine disorder marked by reproductive and metabolic abnormalities, including insulin resistance (IR). Vitamin D has been implicated in glucose metabolism through effects on insulin regulation. Women with PCOS often exhibit vitamin D deficiency, raising interest in its potential therapeutic role. However, the impact of vitamin D supplementation on IR in this population remains unclear. This systematic review aims to evaluate the effect of vitamin D supplementation alone on insulin resistance in women with PCOS. A systematic review was conducted searching PubMed, Cochrane Library and Scopus from January 2025 to February 2025. Randomized control trials assessing the effect of vitamin D supplementation on insulin resistance alone, without the additional use of other pharmaceutical treatments or additional supplements known to affect insulin resistance or glycose metabolism were included in the systematic review. A total of 11 randomized controlled trials (RCTs) involving 598 women with polycystic ovary syndrome (PCOS) were included. Vitamin D supplementation significantly reduced HOMA-IR (MD = –0.28; 95% CI: –0.48 to –0.07; p = 0.007), though heterogeneity was high (I² = 87.8%) and when random-effect model was used statistical significance was lost. After excluding three outlier studies, the effect was no longer significant (MD = –0.16; 95% CI: –0.38 to 0.06; p = 0.163; I² = 55.3%). A subgroup analysis of 5 studies using identical dosage (50,000 IU/week for 8 weeks) also showed no effect (MD = –0.07; 95% CI: –0.34 to 0.20; p = 0.596; I² = 88.6%). Funnel plots indicated possible publication bias. Vitamin D supplementation appears to have a modest but statistically significant effect on reducing insulin resistance, as measured by HOMA-IR, in women with PCOS. However, the overall certainty of evidence is limited by high heterogeneity, risk of bias, and potential publication bias. While findings indicate a potential therapeutic role, vitamin D cannot yet be routinely recommended for this purpose. Further well-designed, long-term randomized controlled trials targeting women with insulin resistance or vitamin D deficiency are needed to clarify its clinical utility.