Environmental and Clinical Determinants of Vitamin D Status in Breast Cancer Patients: A Multivariable Analysis

Background/Objectives: Vitamin D deficiency is commonly observed in patients with breast cancer and may affect disease course and treatment outcomes. The determinants of vitamin D status in this population remain incompletely understood, particularly with respect to lifestyle, psychosocial, and tumour-related factors. The aim of this study was to identify environmental and clinical factors associated with serum 25-hydroxyvitamin D [25(OH)D] concentrations in women with breast cancer, with particular emphasis on dietary patterns, psychological distress, sleep quality, and tumour molecular subtype. Methods: A cross-sectional observational study was conducted among 101 women with histopathologically confirmed invasive breast cancer, recruited at the St. John of Dukla Oncology Centre of the Lublin Region (COZL) in Lublin, Poland, between 2018 and 2019. Serum 25(OH)D concentrations were measured by electrochemiluminescence immunoassay (ECLIA; Roche Diagnostics) within 2–8 weeks after surgical treatment. Clinical and lifestyle data were collected using standardised questionnaires and medical records. Dietary patterns were assessed with the KomPAN questionnaire (pro-healthy diet index pHDI-10 and non-healthy diet index nHDI-14). Psychological distress was measured with the Distress Thermometer. Multiple linear regression with backward stepwise elimination was applied to identify factors independently associated with of 25(OH)D concentration. Results: Insufficient vitamin D status (25(OH)D < 30 ng/mL) was found in 56.4% of participants. The multivariable regression model was statistically significant (F(6,92) = 15.298; p < 0.001), explaining 49.9% of the variance in 25(OH)D concentrations. Factors independently associated with lower 25(OH)D included higher BMI (b = −0.715; p = 0.005), sleep disturbances (b = −6.257; p = 0.020), higher psychological distress score (b = −2.263; p < 0.001), and luminal B versus luminal A subtype (b = −5.909; p = 0.025). A higher pro healthy diet index (pHDI-10) was independently associated with higher 25(OH)D (b = 0.245; p = 0.040). Conclusions: Vitamin D status in patients with breast cancer is shaped by complex interactions among modifiable lifestyle factors and tumour characteristics. Targeted interventions addressing diet quality, psychological well-being, sleep health, and weight management may improve vitamin D status in this population. The association between molecular subtype and 25(OH)D concentrations warrants further prospective investigation. These findings should be interpreted with caution, as vitamin D supplementation and direct measures of sun exposure—both established determinants of 25(OH)D—could not be included as covariates and may account, at least in part, for the associations reported.