Low serum 25-hydroxyvitamin D is associated with fatigue in older women with metastatic breast cancer.

[BACKGROUND AND AIMS] Vitamin D deficiency is highly prevalent in older adults and has been linked to cancer-related fatigue through mechanisms involving muscle dysfunction and systemic inflammation. This study aimed to evaluate the association between serum 25-hydroxyvitamin D [25(OH)D] levels and fatigue in women aged ≥65 years with metastatic breast cancer (BC) receiving palliative and oncological care.

[METHODS] A single-center, cross-sectional study was performed at a tertiary-level cancer referral and teaching hospital in Mexico. A total of 177 women with metastatic BC undergoing active treatment were included. Fatigue was assessed using the Spanish-validated Brief Fatigue Inventory (BFI), frailty with the FRAIL scale, and sarcopenia risk with SARC-F. Serum 25(OH)D was measured and categorized as deficient (<20 ng/mL), insufficient (20-29.9 ng/mL), or sufficient (≥30 ng/mL). Associations were analyzed using Spearman correlation, Kruskal-Wallis, and multivariate linear regression.

[RESULTS] Median age was 72 years (IQR 68-78). Vitamin D deficiency was present in 51.4 % of patients, insufficiency in 36.7 %, and only 11.9 % had sufficient levels. Patients with 25(OH)D deficiency exhibited significantly higher BFI scores compared to the sufficient group (median 2.40 vs. 1.20; p = 0.009). A significant negative correlation was observed between 25(OH)D levels and BFI scores (ρ = -0.19; p = 0.011). In the multivariate linear regression analysis, serum 25(OH)D levels were significantly associated with fatigue scores (beta -0.04; 95%CI -0.08, -0.01; p = 0.012).

[CONCLUSION] Vitamin D deficiency is highly prevalent and independently associated with increased fatigue severity in older women with metastatic BC. These findings identify vitamin D status as a potentially modifiable contributor to one of the most distressing symptoms in advanced disease. Routine screening of 25(OH)D levels and targeted supplementation should be considered integral components of supportive and palliative care in this population. Prospective interventional trials are warranted to confirm clinical benefit.