Introduction: Breast cancer, as a significant threat to women’s health globally, has a complex pathogenesis. Vitamin D, a steroid hormone with diverse physiological functions, has garnered increasing attention in breast cancer research. Numerous studies have been conducted to elucidate the relationship between Vitamin D and breast cancer, yet no definitive conclusions have been reached.

Objective: Hence, in this study, we conducted a two-sample Mendelian randomization (MR) analysis to investigate the association between Vitamin D and breast cancer.

Methods: Genetic variants associated with Vitamin D levels and breast cancer data were obtained from the genome-wide association study database and version R11 of the FinnGen study dataset, respectively. The inverse variance-weighted (IVW) method served as the primary analytical approach, supplemented by various sensitivity analyses. Horizontal pleiotropy was tested using MR-Egger and MR-Pleiotropy RESidual Sum and Outlier methods, while sensitivity analysis was conducted using the leave-one-out method to assess the reliability of the results.

Results: Based on instrumental variable assumptions, 111 single nucleotide polymorphisms were suitable for subsequent analyses after matching the results. The MR analysis demonstrated no evidence of a causal relationship between Vitamin D and breast cancer. The IVW method yielded a non-significant association (p=0.968; odds ratio = 1.002, 95% confidence interval: 0.896 – 1.119), and the other methods pointed out the same direction of effect, and the subsequent pleiotropic analysis and sensitivity analysis confirmed the accuracy of the results.

Conclusion: At the genetic level, no causal relationship between Vitamin D and breast cancer was found; thus, our findings do not support a clinically significant role of Vitamin D supplementation in breast cancer risk reduction.