We’ve been advised to consume fruits and vegetables, ensure adequate vitamin intake, and maintain good health. Generally, this advice is solid. However, some nutrients present a more complex narrative, with vitamin B12 offering a particularly intriguing case.
Vitamin B12, also known as cobalamin, is vital for survival. It plays a role in producing red blood cells, sustaining the nervous system, and ensuring cells accurately replicate and repair DNA.
This vitamin is naturally present in animal-based foods such as meat, fish, eggs, milk, and cheese. Certain cereals and breads are fortified with B12 to aid those who avoid meat in meeting their nutritional needs.
While most individuals with a diverse diet receive the recommended B12 levels, vegans, individuals with specific gut conditions, and older adults who have reduced nutrient absorption may require supplements.
A deficiency in B12 can lead to significant health issues, especially if it goes unnoticed and untreated, as problems can arise.
Recently, researchers have explored whether excessive B12 intake or elevated blood levels of B12 could be linked to cancer.
Finding Balance
The body continuously generates new cells, requiring accurate DNA replication with each cell division. Vitamin B12 is crucial in this process.
Insufficient B12 levels can result in incorrect DNA replication, potentially leading to mutations that may increase the risk of certain cancers, such as colon cancer, over time. This highlights the importance of addressing B12 deficiency.
frameborder=”0″ allow=”accelerometer; autoplay; clipboard-write; encrypted-media; gyroscope; picture-in-picture; web-share” referrerpolicy=”strict-origin-when-cross-origin” allowfullscreen>A 2025 case-control study conducted in Vietnam identified a U-shaped relationship between B12 intake and cancer risk, with both low and high intakes linked to increased risk.
This type of study can show correlation but not causation, so the key takeaway is not that B12 is harmful, but that maintaining balance is important.
While it might seem logical that increasing B12 intake could enhance cancer protection by supporting healthy cell growth, research does not fully back this notion.
Vitamin B12 promotes overall cell growth, not just the growth of healthy cells. A concern is that if pre-cancerous cells exist, high levels of B12 could, in theory, promote their growth. However, proving this in humans is challenging.
Long-term studies on high-dose B vitamin supplements have not demonstrated clear protective effects against cancer incidence or mortality.
While one analysis suggested a reduced risk of melanoma, this was specific to that type of cancer and not indicative of a general cancer-preventative effect from high-dose B vitamins.
Some observational studies have indicated a slight increase in lung cancer risk with long-term, high-dose B6 and B12 supplementation, particularly among men and smokers, though these studies do not prove causation.
Doctors have observed that many cancer patients have unusually high levels of B12 in their blood, prompting questions about whether elevated B12 contributes to cancer or if cancer itself increases B12 levels.
A 2022 study concluded that high B12 in cancer patients is often an “epiphenomenon,” meaning the presence of the vitamin occurs alongside the disease but does not necessarily cause it. Further research in 2024 reached a similar finding.
This phenomenon may involve two primary mechanisms. First, tumors can affect the liver, which stores significant amounts of B12. When the liver is compromised, it may release more B12 into the bloodstream.
Second, some tumors might increase proteins that bind to B12 in the blood, leading to higher blood test readings without necessarily reflecting increased B12 use by the body’s cells.
A Useful Marker
Researchers are also considering that elevated B12 might not cause cancer but could serve as an indicator of cancer presence or progression.
A 2026 study revealed that colon cancer patients with very high B12 levels had a median survival of about five years, compared to nearly eleven years for those with normal levels.
Similar trends have been found in oral cancer and in patients receiving immunotherapy, where elevated B12 was linked to poorer outcomes.
This suggests that unexplained, persistent high B12 levels, particularly when not caused by supplements, should not be dismissed. They could indicate liver disease, blood disorders, or undiagnosed cancer.
For most individuals, this is not a concern. B12 from a balanced diet including meat, fish, eggs, dairy, or fortified foods is typically not problematic, as it is challenging to consume excessive B12 from food alone. Deficiency remains a more prevalent issue than excess.
The primary concern lies in long-term high-dose supplementation without medical guidance or unexplained high blood B12 levels when not taking supplements.
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The overarching message is straightforward: more is not always better. Cancer prevention cannot rely solely on increasing any single vitamin intake. Long-term habits such as maintaining a balanced diet, regular exercise, avoiding smoking, protecting the skin, and attending routine health screenings are more important.
Regarding vitamin B12, it is crucial to obtain adequate amounts through diet or supplementation if necessary, especially for vegans, older adults, or those with absorption issues. However, avoid excessive doses unless advised by a physician.
With B12, as with many nutrients, the aim is to achieve the right balance rather than maximizing intake.
Ahmed Elbediwy, Senior Lecturer in Cancer Biology & Clinical Biochemistry, Kingston University and Nadine Wehida, Senior Lecturer in Genetics and Molecular Biology, Kingston University
This article is republished from The Conversation under a Creative Commons license. Read the original article.
