Vitamin D and Its Metabolites Status before and during Chemotherapy in Caucasian Breast Cancer Patients

Background: The predictive role of vitamin D (VD) in breast cancer (BC) patients’ survival is still being investigated. This paper aims to evaluate the changes in VD metabolites during chemotherapy (CTH) and the predictive role of VD status in Caucasian BC patients treated with CTH. Methods: Vitamin D and its metabolites were assessed with reference LC–MS/MS methodology in 98 consecutive BC patients starting CHT, after 3 and 6 months, and compared to the control group. Results: The frequency of VD deficiency in BC patients was greater than in the control group (56.1% vs. 37.2%). After 6 months of CTH, the number of VD-deficient BC patients slightly increased to 60%. The concentrations of VD active forms [25(OH)D(2), 25(OH)D(3)], and catabolites [24,25(OH)(2)D(3) and 3-epi-25(OH)D(3)] decreased after 3 and 6 months of CTH compared to the baseline values. Strong positive correlations between concentrations of 3-epi-25(OH)D(3) and 25(OH)D in both groups were found. Similar correlations were also observed between 24,25(OH)(2)D(3) and 25(OH)D levels. Kaplan–Meier survival analysis showed significantly longer survival in BC patients without deficiency (>20 ng/mL) at baseline (HR = 2.44 (95% CI 1.07–5.59), p = 0.026). Conclusions: (1) Our data provide further evidence that BC patients before CTH are more VD-deficient than the general population and this deficiency increases further during CTH treatment, as observed using the reference LC-MS methodology. (2) Presented results show that VD catabolism is not affected in BC patients. (3) The poorer survival in VD-deficient BP patients supports the importance of VD supplementation in BC patients with 25(OH)D levels below 20 ng/mL.