Vitamin D and Breast Cancer
Breast cancer is the most common cancer in women worldwide. Vitamin D has been examined as a factor in incidence, prognosis, and treatment tolerance — with results that are mixed for prevention and more consistent for survival after diagnosis.
Prevention trials
- VITAL (2019) — 25,000 adults; 2,000 IU/day cholecalciferol for 5.3 years: no reduction in invasive breast cancer overall.
- WHI-CaD (2013) — 36,000 women; 400 IU vitamin D + 1,000 mg calcium for 7 years: no reduction in breast cancer.
- Marshall-Lappe 2017 — 2,300 postmenopausal women; 2,000 IU + 1,500 mg calcium: trended toward lower cancer incidence but not significant for breast cancer specifically.
- Overall — no clear preventive effect of standard-dose vitamin D on breast cancer incidence in adults with normal baseline 25(OH)D.
Survival after diagnosis
Meta-analyses (Hu 2018; Yao 2014) find breast cancer patients with sufficient 25(OH)D at diagnosis have 30–40% lower all-cause and breast cancer-specific mortality than deficient patients. Whether correcting deficiency changes that trajectory is less clear from randomised evidence, but correction is safe and cheap.
Aromatase inhibitor arthralgia
Aromatase inhibitors (anastrozole, letrozole, exemestane) reduce oestrogen after breast cancer but cause joint pain in ~50% of women — a common reason for discontinuation. The 2010 Rastelli RCT showed vitamin D loading (50,000 IU weekly) reduced the incidence of new musculoskeletal symptoms in letrozole-treated women. Standard practice now includes checking and correcting 25(OH)D in women starting aromatase inhibitors.
Bone health on breast cancer therapy
Aromatase inhibitors accelerate bone loss substantially. All women starting an aromatase inhibitor should have DEXA and vitamin D testing; osteopenia/ osteoporosis triggers bisphosphonate or denosumab. Vitamin D + calcium is foundational.
Practical guide
- Test 25(OH)D at breast cancer diagnosis; correct to ≥ 30 ng/mL.
- Continue supplementation through treatment — supports bone health during chemotherapy and endocrine therapy.
- Higher doses (2,000–4,000 IU/day) commonly used in survivorship; individualise to serum level.
- Vitamin D is adjunctive — do not use it as a substitute for evidence-based breast cancer therapy.