Background:Biological evidence has revealed antitumor effect of vitamin D, but whether it could predict the response to neoadjuvant chemotherapy (NAC) in breast cancer (BC) patients remains inconclusive. The aim was to investigate the association between pretreatment vitamin D level and response to NAC and subsequent survival outcomes in BC patients.Materials and methods:The authors systematically searched the Medline, Embase, Cochrane Library, and Web of Science databases and clinical trial registries to identify relevant articles from inception to 8 October 2024. Eligible studies investigating the associations between pretreatment plasma vitamin D and response to NAC in BC patients were selected according to the predefined criteria, with the study characteristics extracted by two reviewers. The primary outcome was pathological complete response (pCR), while overall pathological response and event-free survival (EFS) were adopted as secondary outcomes. Summary effect estimates of odds ratios (ORs) or hazard ratios (HRs) with 95% CIs were pooled using a random-effects model. Subgroup and sensitivity analyses were performed based on study characteristics and methodological quality.Results:Six retrospective cohort studies involving 1291 BC patients were included. The authors observed a significant association between pretreatment vitamin D deficiency and 50% increased odds of non-pCR after NAC (OR=1.50, 95% CI: 1.11-2.03, P=0.008) with no heterogeneity (I 2=0%). The authors also identified a significant association of vitamin D with the overall pathological response (OR=1.33, 95% CI: 1.01-1.75, P=0.046). A similar association with EFS (HR=1.27, 95% CI: 0.92-1.75, P=0.139) was also noted although the effect estimate was not statistically significant. Sensitivity analyses based on methodological quality showed consistent findings.Conclusion:Pretreatment vitamin D deficiency is associated with an inferior response to NAC in BC patients. Our meta-analysis advocates further prospective studies with large sample sizes before vitamin D supplementation could be administered to improve NAC response and subsequent prognosis of BC patients.