Vitamin B12 is essential for numerous bodily functions, from red blood cell formation to neurological health. Deficiency in this critical vitamin can lead to symptoms such as fatigue, weakness, numbness, memory problems, and even psychiatric disturbances. While these symptoms are often attributed to diet (though many people take a multivitamin or supplement to get enough B12), aging, or gastrointestinal disorders like pernicious anemia, a growing body of research suggests that vitamin B12 deficiency could sometimes be a subtle, early sign of cancer.
Hematologic cancers such as leukemia and lymphoma, as well as certain solid tumors affecting the stomach, pancreas, and colon, have been linked to low B12 levels. The connection arises partly from how cancers affect nutrient absorption and metabolism, and partly from chronic inflammation or direct tumor involvement in the digestive system. The challenge for clinicians is that B12 deficiency itself is common, especially in older adults, vegetarians, and people with digestive disorders — making cancer an overlooked possibility in diagnostic workups.
Exploring the Data: B12 Deficiency and Cancer Risk
Scientific studies have increasingly highlighted the potential association between low vitamin B12 levels and cancer diagnosis. A 2016 Danish cohort study, published in Cancer Epidemiology, followed over 25,000 individuals and found that those with newly diagnosed vitamin B12 deficiency had a significantly higher risk of being diagnosed with cancer within the first year after the deficiency was detected. The relative risk (RR) of cancer diagnosis in the year following B12 deficiency was 2.4, with the strongest associations seen for gastrointestinal cancers and hematological malignancies. (Gilsing et al., 2016).
Another investigation, published in BMJ Open in 2017, examined data from general practice records and found that patients presenting with unexplained B12 deficiency were more likely to have underlying cancer than age-matched controls without deficiency. Specifically, stomach and pancreatic cancers were notably overrepresented — both cancers that interfere with nutrient absorption through direct tumor effects on the gastrointestinal lining.
B12 deficiency alone is not a definitive sign of cancer. Instead, it’s a potential "red flag," especially when combined with other worrying symptoms such as unexplained weight loss, abdominal pain, or persistent digestive changes.
Why Cancer Can Cause B12 Deficiency
There are several mechanisms by which cancer can lead to low vitamin B12 or other B vitamin levels. In gastrointestinal cancers, tumors in the stomach or small intestine can directly impair the absorption of B12, which requires a healthy stomach lining to produce intrinsic factor — the protein necessary for B12 uptake. Gastric cancer, for example, often damages the stomach mucosa, reducing intrinsic factor production and causing B12 deficiency even in patients with an adequate dietary intake.
Hematologic cancers, such as leukemia and lymphoma, can also disrupt vitamin B12 metabolism indirectly by affecting bone marrow function, which plays a critical role in nutrient processing and red blood cell production. Furthermore, cancer-related chronic inflammation can alter gut function and microbiota, further impairing absorption. Certain cancer treatments, including chemotherapy, can worsen B12 deficiency by damaging the digestive lining or altering appetite and dietary intake.
Should Doctors Screen for Cancer in B12-Deficient Patients?
Current guidelines do not explicitly recommend cancer screening for all B12-deficient patients, but experts agree that a thorough assessment is warranted when the deficiency occurs without an obvious dietary or gastrointestinal cause.
Clinicians are encouraged to pay particular attention to older patients, those with concurrent gastrointestinal symptoms, or those with risk factors such as smoking, obesity, or family history of cancer. Diagnostic workups might include imaging studies, endoscopy, and laboratory tests for tumor markers in select cases.
As research continues to uncover the intricate relationship between micronutrients and malignancy, vitamin B12 deficiency may ultimately become recognized not just as a nutritional disorder, but as a possible early warning signal for certain cancers — a reminder that even seemingly minor biochemical changes in the body can sometimes tell a much larger story.
Sources:
- Gilsing, A. M., et al. (2016). Vitamin B12 deficiency and cancer risk: A population-based cohort study. Cancer Epidemiology, 44, 22-28.
- Clarke, R., et al. (2017). Vitamin B12 status and risk of cancer in UK primary care patients. BMJ Open, 7(4), e011532.
- National Institutes of Health (NIH). (2022). Vitamin B12 - Fact Sheet for Health Professionals. https://ods.od.nih.gov/factsheets/VitaminB12-HealthProfessional/
- https://pmc.ncbi.nlm.nih.gov/articles/PMC9658086/
- https://www.mskcc.org/cancer-care/integrative-medicine/herbs/vitamin-b12
- https://cancer.osu.edu/news/long-term-high-dose-vitamin-b6-b12-use-associated-with-increased-lung-cancer-risk-among-men
