Common Vitamin Has a Complicated Link to Cancer, Experts Reveal
Health20 August 2026ByFiona MacDonald
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Vitamins are usually sold to us as a pretty straightforward, good thing to add to your diet
Get enough, and you’ll look and feel great; get too little, and your body might notice
But the reality is messier – and few vitamins prove that better than B12
For decades, vitamin B12 has been one of the heroes of nutrition. If you’re foggy-headed or tired – or on a plant-based diet – you might be recommended B12 to help pep you up
But it might not be so straightforward
A growing number of scientific papers have detected both too little and too much of it in the blood of people with cancer – of the lungs, the breast, the esophagus, and the colon
That’s right, too little AND too much
B12, or cobalamin, is a vitamin that no animal can survive without. It’s essential for building DNA; it also maintains the myelin sheath that protects our nerves, matures red blood cells, and keeps fat and amino acid metabolism ticking over
It only comes naturally from animal-derived foods, which is why people on a strict plant-based diet – or anyone else who absorbs nutrients poorly – can run short
It’s B12’s role in building and repairing DNA that first led researchers to investigate its link to cancer. The original assumption was that without sufficient amounts of B12, the body wouldn’t be able to repair DNA, and this could lead to mutations and possibly tumor growth
Except that some studies showed the opposite problem – unusually high (not low) B12 was found in the blood of first people with lung cancer. Then breast cancer. Then colon cancer. These links were found repeatedly, and in large datasets
So what exactly is going on?
Firstly, we need to clarify the fact that the amount of B12 someone has in their blood and the amount that they eat are two different things
Blood levels depend on diet, but also how the body stores, uses, and shuffles the vitamin around. And that’s where cancer complicates things
The liver keeps a large reserve of B12. Tumors – especially in the liver itself, and cancers that spread aggressively – can disrupt how that reserve gets stored, transported, and released
Some tumors also ramp up production of the very proteins that carry B12 through the bloodstream
Which means a high B12 reading on a blood test might not be the cause of a disease; it could just be a symptom
This is something that was supported by a study published this year in Cancer Research Communications
Researchers at the University of Texas Medical Branch pulled records for 37,106 colon cancer patients from a global health database spanning 108 healthcare systems, each of whom had a B12 blood test on file from around the time of their diagnosis
The pattern was pretty stark. Patients with elevated B12 – above 1,000 picograms per liter – had a median survival of around five years. Patients with normal levels survived a median of nearly 11 years
Patients with low B12 did slightly better again
Even after accounting for age, treatment, and other health conditions, patients with high B12 were roughly twice as likely to die as those with low levels, and about 65 percent more likely to die than those with normal levels
They were also more likely to already have stage 4 disease at diagnosis, and more likely to develop new metastases within a year – particularly in the liver
Here’s where it gets more interesting
The team also examined hundreds of colon tumors directly and found significantly higher activity in a gene called MTR – which builds an enzyme that requires B12 to make a building block of DNA. Tumors with the most active version of that gene tracked with the worst survival odds
In other words: aggressive tumors may simply be more efficient at using – and hoarding – B12 to fuel their own rapid copying, not the other way around
A 2024 study of people on cancer immunotherapy found the same pattern of elevated B12 tracking with worse outcomes. Research in oral cancer patients has pointed the same way too
A 2024 review of the evidence put it plainly: the meaning of elevated B12 in people already diagnosed with cancer “remains uncertain and could potentially be linked to reverse causality” – science-speak for: the cancer may be causing the high B12 reading, not the B12 causing the cancer
That still leaves the opposite end of the scale unresolved
A 2024 study tracking 3,758 cancer patients and 2,995 people without cancer in Vietnam found a U-shaped curve: both unusually low and unusually high dietary B12 intake were linked to higher cancer risk, compared to people sitting in the middle
It’s an observational study, built on food questionnaires people had to recall themselves, so it can only point to an association – not prove one thing caused another
It’s also worth noting that even the highest intake group averaged only around 2.97 micrograms of B12 per day – not dramatically above standard nutritional recommendations of 2.4 to 2.8 micrograms per day
Still, as of yet, it’s not clear why low blood levels of B12 might be associated with a higher risk of cancer
With this much inconsistency across the research, the B12-cancer relationship is far from settled
But it’s looking more likely that B12 levels may be a symptom of something else going wrong in the body, rather than a driver of cancer itself
This article was fact-checked by Clare Watson and edited by Clare Watson. While we pride ourselves on our process, we are only human. If you spot a mistake, please let us know
