Vitamin D and Cancer
Cancer is one of the areas where observational and randomised evidence for vitamin D diverge sharply. Cohort studies consistently link higher serum 25(OH)D with lower cancer risk. Large RCTs have been more disappointing for cancer incidence, but a modest signal for reduced cancer mortality has held up across meta-analyses.
Cancer incidence
Three major trials have answered this directly:
- VITAL (n=25,871, 5 years, 2,000 IU/day) — no reduction in total cancer incidence.
- D-Health (n=21,315 Australians, 5 years, 60,000 IU/month ≈ 2,000 IU/day) — no reduction in total cancer incidence.
- ViDA (n=5,110 New Zealanders, 3.3 years, 100,000 IU/month ≈ 3,300 IU/day) — no reduction in total cancer incidence.
The RCT verdict on preventing cancer onset in unselected populations is essentially null.
Cancer mortality
Here the signal is more interesting. A 2019 meta-analysis of 10 vitamin D supplementation trials (~80,000 participants) found a 13% reduction in cancer mortality (HR 0.87, 95% CI 0.79–0.96). VITAL itself showed a significant 25% reduction in cancer death after excluding the first two years to account for occult tumours (which take time to become clinically obvious). D-Health also showed modestly lower cancer mortality with continued supplementation.
The proposed mechanism: vitamin D doesn't stop cancer starting, but may slow progression by promoting cell differentiation, inducing apoptosis, and modulating the tumour immune microenvironment.
By cancer type
- Colorectal cancer: the strongest evidence base. Observational studies consistently link higher 25(OH)D to lower incidence and better survival. RCTs (SUNSHINE) suggest supplementation improves progression-free survival in metastatic disease.
- Breast cancer: higher 25(OH)D at diagnosis is associated with better outcomes in observational data. RCTs of supplementation for prevention have not shown incidence benefit.
- Prostate cancer: observational associations are inconsistent. RCT evidence is null.
- Lung cancer: observational protection; RCT null.
- Skin cancer: paradoxically, high UV exposure (which raises 25(OH)D) is the main risk factor. Vitamin D from supplements does not increase skin cancer risk.
Body weight modifier
Post-hoc VITAL analysis (Chandler 2020) found the cancer-mortality benefit was concentrated in normal-BMI participants. Obese individuals achieve lower serum 25(OH)D on the same dose (see absorption / weight), which may explain the difference.
Practical recommendation
- Vitamin D supplementation is not a cancer-prevention strategy for the general population.
- If you have a cancer diagnosis and are deficient, correction to ≥ 30 ng/mL is reasonable adjunct — with your oncologist's knowledge.
- Colorectal cancer patients specifically may benefit from supplementation targeting 30–40 ng/mL (SUNSHINE-suggested).
- Sun protection remains the primary intervention for skin cancer prevention.