The Emerging Role and Clinical Impact of Vitamin D Supplementation in Breast Cancer Patients Undergoing Chemotherapy: A Systematic Review

Introduction: Breast cancer is the most common cancer in women and is often diagnosed at an advanced stage. Chemotherapy plays a crucial role in disease control but is limited by toxicity and response variability. Vitamin D, with its biological functions and the high prevalence of deficiency in patients, has potential as an adjuvant therapy to improve treatment response and clinical outcomes. Objective: To integrate existing evidence on the role of vitamin D supplementation in breast cancer patients undergoing chemotherapy, focusing on its impact on treatment response and clinical outcomes. Methods: This systematic review followed PRISMA guidelines and included RCTs, cohort, and case-control studies evaluating vitamin D supplementation in adult women with breast cancer undergoing chemotherapy. A comprehensive search across five databases up to June 2025 was conducted, applying the pre-established PICOS criteria, with filtering, data extraction, and quality assessment performed. This systematic review has been prospectively registered in the PROSPERO database (CRD420251142938). Results: This systematic review identified six eligible studies involving breast cancer patients undergoing chemotherapy, with interventions ranging from low- to high-dose vitamin D, either alone or in combination with synbiotics. Vitamin D supplementation in breast cancer patients undergoing chemotherapy has been associated with improved clinical outcomes, including enhanced nutritional status, reduced inflammatory and cardiotoxic biomarkers, and increased pathological complete response (pCR) and disease-free survival (DFS). Conclusion: Vitamin D supplementation during chemotherapy shows potential benefits in breast cancer patients, but large-scale standardized trials with long-term follow-up are needed to confirm its impact on survival outcomes.