**Background:** Vitamin D is known to play a role in cell differentiation, anti-inflammatory and antiproliferative effects, and vitamin D receptors are expressed in breast cancer cells. Despite preclinical evidence suggesting a link between vitamin D and breast cancer biology, clinical trial results remain inconsistent. Many oncological patients take supplements without physician knowledge, and vitamin D deficiency is associated with unfavorable skeletal outcomes, increased mortality, cardiovascular disease, and metabolic syndrome. This study aimed to determine serum 25-hydroxyvitamin D (25(OH)D) levels in newly diagnosed breast cancer patients and correlate these with prognostic factors and potential modulators such as supplementation and sun exposure.
**Methods:** The BEGYN study was a prospective observational study conducted at Saarland University Medical Center between September 2019 and January 2021. A total of 110 non-metastatic breast cancer patients were enrolled. Inclusion criteria were age >18 years, sufficient German language proficiency, physical condition allowing spirometry on a treadmill, and ability to use a smartphone and fitness tracker. Exclusion criteria included pregnancy, metastases, simultaneous carcinomas, and life expectancy <1 year. At the initiation visit (before any pharmacologic or operative therapy), serum 25(OH)D was measured using the LIAISON® 25 OH Vitamin D TOTAL Assay. Supplementation of vitamins and trace elements was documented, and patients completed a questionnaire on sun exposure and nutrition. Clinical characteristics (age, BMI, Karnofsky performance status) and histopathological parameters (TNM classification per UICC 8th Edition, grading per Elston and Ellis 1991, ER/PR/Her2/Ki67 immunohistochemistry) were recorded. Statistical analyses used SPSS 28.0. Univariate linear regression assessed modulators of 25(OH)D levels. Patients were divided into three groups for prognostic factor analysis: mild (≤/>30 ng/mL), moderate (≤/>20 ng/mL), and severe (≤/>10 ng/mL) vitamin D deficiency. Associations were tested using Mann–Whitney U test, Chi-Square test, and Fisher's exact test as appropriate.
**Key Results:** Median patient age was 54 years (range 26–81). Five patients (4.5%) had bilateral breast cancer; 94 patients (85.5%) had their first malignant disease; 16 patients (14.5%) had recurrent cancer or a second carcinoma. Most patients (92.7%) had a Karnofsky performance status of 90–100%. Median BMI was 26 (range 19–39). Median serum 25(OH)D was 24 ng/mL (range 5–65 ng/mL). Overall, 64.8% of patients had vitamin D below recommended values (30 ng/mL), 35.2% were within range, and 0% were above. Eighteen patients (16.4%) took vitamin D supplements; their median 25(OH)D was significantly higher (43 ng/mL vs. 22 ng/mL; p < 0.001). Seasonal variation was observed: median 25(OH)D was 20 ng/mL in spring, 31 ng/mL in summer, 25 ng/mL in autumn, and 17 ng/mL in winter. Summer measurement was associated with a 7-point higher vitamin D level (p = 0.03). Among 91 patients completing the sun exposure questionnaire, 98.8% knew UV radiation produces vitamin D, and 45.1% spent more time in the sun for this reason; however, 98.8% were also aware of skin cancer risk, and 57.1% spent less time in the sun. In the prognostic factor analysis, patients with moderate vitamin D deficiency (≤20 ng/mL) were less likely to have triple-negative breast cancer (2.3% vs. 14.4% of patients with 25(OH)D >20 ng/mL; p = 0.047) and more likely to have Luminal B carcinomas (32.6% vs. 13.8%; p value not explicitly stated for this comparison). No significant associations were found between vitamin D deficiency and Ki67, grading, Her2-positive subtype, Luminal A subtype, or tumor stage (cT and cN+).
**Clinical Implications:** This study demonstrates that vitamin D deficiency is highly prevalent (64.8%) among newly diagnosed breast cancer patients, exceeding rates reported in the general German population (57.8% in 1991). Supplementation effectively raises levels but did not bring all patients to sufficiency, and no patients exceeded reference values, confirming the rarity of vitamin D intoxication. The lack of consistent association between vitamin D deficiency and most established prognostic factors (tumor stage, grading, Ki67) suggests vitamin D is not a major independent prognostic factor in breast cancer. However, the association with Luminal B subtype (more aggressive than Luminal A) warrants further investigation. Given that breast cancer patients face additional bone health risks from chemotherapy, endocrine therapy (especially aromatase inhibitors), radiotherapy, and immobilization, routine vitamin D screening and supplementation are clinically important to prevent skeletal complications and potential treatment side effects such as paclitaxel-induced polyneuropathy. Study limitations include the small sample size (n=110) and retrospective self-reporting of supplementation and sun exposure.