Do Common Vitamins Fight Cancer? A Systematic Evidence Review Anchored on Vitamin B6 (PLP) and Pancreatic Cancer

*Deep research report, dated 2026-08-14. Evidence graded on a Level I–VII pyramid (RCTs/meta-analyses > cohort/case-control > in vitro/animal > reviews > gray literature).*

Key points

• Anchor study: Feehan et al. (2026, *Molecular Nutrition & Food Research*, DOI: 10.1002/mnfr.70333, PMID: 41560327) showed that pyridoxal 5'-phosphate (PLP), the active form of vitamin B6, inhibits proliferation of BxPC3 pancreatic adenocarcinoma cells in a dose-dependent manner (125–500 μg/mL), depolarizes mitochondrial membrane potential, boosts superoxide generation, downregulates KRAS/E2F/G2M/mTOR transcriptional programs, and inhibits mitochondrial aspartate/glutamate carriers (AGC1/2), cutting off the cells' glutamine dependency. Reported apoptosis rose from 2.38% to 45.4% (the precise figure comes from coverage of the results; the abstract confirms apoptosis induction without listing the value).
• Critical caveat — the translational chasm: in vitro effective concentrations (125–500 μg/mL ≈ mmol/L scale) are 2–3 orders of magnitude higher than serum PLP levels achievable by oral supplementation (μmol/L scale). The study's value is mechanistic, not a basis for "taking B6 to treat cancer."
• Supporting B6 evidence: a 2024 *Cell Death and Disease* study showed B6 deficiency cooperates with oncogenic Ras to drive malignant tumors in *Drosophila*, reversible by PLP supplementation; meta-analyses (~1.96 million participants) link higher B6 intake/blood PLP with lower cancer risk at multiple sites. Still no RCTs.
• Vitamin C: pharmacologic (intravenous) doses act as a pro-oxidant generating H₂O₂, with epigenetic (TET), HIF-1α, and immune mechanisms. A 2025 meta-analysis links IV vitamin C to longer overall survival (moderate certainty, heterogeneous); a phase III trial in metastatic colorectal cancer showed no PFS benefit overall but a possible benefit in the RAS-mutant subgroup. Not FDA-approved for cancer.
• Vitamin D: meta-analyses of RCTs (e.g., 30 RCTs, 18,808 participants) show no significant effect on cancer incidence (RR 1.03); a possible ~12–13% mortality reduction (RR 0.87–0.88) is low-quality evidence.
• Vitamin E: the SELECT trial (35,000+ men) found 400 IU/day vitamin E increased prostate cancer risk by 17% (HR 1.17, P=0.008) — net harm, contrasting with the earlier ATBC trial in smokers.
• Folate: classic double-edged sword. Deficiency raises colorectal cancer risk; randomized trials show supplementation offers no overall adenoma protection and may promote progression when existing lesions are present; benefit appears restricted to people with low baseline folate (RR 0.61 vs. 1.28 for high baseline in Wu et al. 2009).
• Vitamin A: all-trans retinoic acid (ATRA) for acute promyelocytic leukemia (APL) is the only vitamin-derived anticancer therapy validated by RCTs and incorporated into standard care, achieving ~90% complete remission via differentiation therapy targeting PML-RARα; combined with anthracyclines and arsenic trioxide, cure rates reach ~70–95%. This cannot be extrapolated to "eat more carrots" — excess vitamin A is teratogenic and hepatotoxic.

Evidence strength map

| Vitamin | Claim | Best evidence | Status | Net risk | |---|---|---|---|---| | A (ATRA) | Treats APL | Level I (RCT, translated) | Standard therapy | Clear benefit (drug-grade) | | C (high-dose IV) | Pro-oxidant tumor killing | Mechanism III / clinical I–II | Experimental adjunct | Awaiting phase III | | B6/PLP | Inhibits pancreatic cancer in vitro | Level III + epidemiologic II | Preclinical | Unproven in humans | | D | Prevents cancer | Level I (RCT meta) | Prevention not established | No incidence reduction | | E (high dose) | Cancer prevention | Level I (RCT) | — | Net harm (+17% prostate cancer) | | Folate | Prevents colorectal adenoma | Level I (heterogeneous) | Stage-dependent | Baseline-dependent |

Bottom line

"Vitamins fight cancer" cannot be generalized. The B6/PLP study is scientifically solid and mechanistically coherent (mitochondrial depolarization → oxidative stress → KRAS downregulation + AGC1/2 blockade → apoptosis), with in vivo and epidemiological support — but its concentrations are unreachable by oral intake. Vitamins exhibit universal biphasic dose effects: deficiency destabilizes the genome, while excess can cause harm (vitamin E and prostate cancer; folate and adenoma progression; vitamin A teratogenicity; B6 neuropathy above the ~100 mg/day tolerable upper limit, versus a recommended intake of ~1.2–1.5 mg/day). For the public: obtain vitamins from a balanced diet and do not take high-dose supplements for "anticancer" purposes; all cancer-related decisions should follow medical advice. This report is not medical advice.

Limitations

• Narrative/systematic appraisal without full PRISMA per-study bias assessment (RoB 2 / ROBINS-I).
• The exact 45.4% apoptosis figure derives from secondary coverage; verify against the full text.
• Vitamins K, B12, and niacin were not covered; literature search current to 2026-08-14.

Selected references

• Feehan, J., et al. (2026). Pyridoxal 5′-phosphate suppresses growth of pancreatic adenocarcinoma cells in vitro via regulation of oncogenic RAS. *Molecular Nutrition & Food Research, 70*, e70333. https://doi.org/10.1002/mnfr.70333
• Pilesi, E., et al. (2024). Vitamin B6 deficiency cooperates with oncogenic Ras to induce malignant tumors in Drosophila. *Cell Death and Disease, 15*, 388. https://doi.org/10.1038/s41419-024-06787-3
• Qu, J., et al. (2025). Overall and progression-free survival of patients with malignant neoplasm following intravenous vitamin C: A systematic review and meta-analysis. *International Journal for Vitamin and Nutrition Research, 95*(3). https://doi.org/10.31083/IJVNR37372
• Keum, N., & Giovannucci, E. (2018). Cancer and vitamin D supplementation: A systematic review and meta-analysis. PMID: 29635490
• Klein, E. A., et al. (2011). Vitamin E and the risk of prostate cancer: The SELECT trial. *JAMA, 306*(14), 1549–1556.
• Cole, B. F., et al. (2007). Folic acid for the prevention of colorectal adenomas: A randomized clinical trial. *JAMA, 297*(21), 2351–2359.
• Wu, K., et al. (2009). A randomized trial on folic acid supplementation and risk of recurrent colorectal adenoma. *American Journal of Clinical Nutrition, 90*(6), 1623–1631.

*AI use disclosure: literature search, source verification, and synthesis were assisted by AI research tools; all claims are anchored to independently verifiable peer-reviewed sources.*