Vitamin D plays a key role in immune modulation, cell proliferation, and hormone regulation. Dysregulated testosterone may contribute to breast cancer progression. We investigated whether long-term vitamin D supplementation affects serum testosterone levels in breast cancer survivors. Complete data at baseline, 12, and 24 months were derived from 253 women with early-stage breast cancer participating in the DEDiCa trial and randomized to receive either a high-dose vitamin D to maintain serum 25(OH)D at 60 ng/mL (group A) or a standard dose to maintain serum levels at 30 ng/mL (group B). Serum 25(OH)D levels significantly increased in both groups (p < 0.001). No significant changes in testosterone concentrations were observed between treatment groups over the 24 month treatment period (A: 0.125 to 0.140 ng/mL; B: 0.162 to 0.193 ng/mL; p = 0.682). Baseline serum testosterone levels emerged as the most significant predictor of testosterone trajectories, possibly modulated by hormone-suppressive therapy. These results are reassuring that vitamin D supplementation did not adversely affect testosterone levels in this population of breast cancer survivors and may partially concur with a healthy lifestyle to equilibrate testosterone levels.
Keywords: 25-hydroxyvitamin D [25(OH)D]; androgens; aromatase inhibitors; body mass index (BMI); breast cancer survivors; cholecalciferol (vitamin D); hormone-suppressive therapy; lifestyle intervention; sex hormone–binding globulin (SHBG); testosterone.