Vitamin B12 and Cancer: What High and Low Levels May Mean

Vitamins are generally marketed as a simple, beneficial addition to your diet.

Get enough and you may look and feel fantastic; get too little and your body may feel the effects.

Yet the truth is more complicated, and few vitamins illustrate this more clearly than B12.

For decades, vitamin B12 has been regarded as a nutritional hero. If you feel mentally foggy, exhausted, or follow a plant-based diet, you may be advised to take B12 for an energy lift.

However, the picture may not be quite so simple.

A growing body of scientific literature has identified both low and high blood levels of B12 in people with cancer, including cancer of the lung, breast, oesophagus, and colon.

That is, both too little and too much.

Why vitamin B12 matters

B12, also known as cobalamin, is a vitamin no animal can survive without. It is vital for making DNA, maintaining the protective myelin coating around nerves, maturing red blood cells, and sustaining fat and amino-acid metabolism.

It occurs naturally only in foods derived from animals. This is why people following a strict plant-based diet, as well as those whose bodies do not absorb nutrients efficiently, can become deficient.

B12's involvement in DNA construction and repair was what first prompted researchers to examine its potential relationship with cancer. Initially, the reasoning was that inadequate B12 might prevent the body from repairing DNA, potentially resulting in mutations and perhaps tumour growth.

But several studies revealed the reverse pattern: abnormally high, rather than low, B12 concentrations were first observed in the blood of people with lung cancer, then breast cancer, and then colon cancer. These associations appeared repeatedly, including in large data sets.

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So, what could explain this?

Blood B12 levels and cancer

First, it is important to distinguish between the B12 concentration in someone's blood and the amount they consume.

Blood levels are shaped not just by diet, but by the way the body stores, uses and transports the vitamin. Cancer can make this process more complex.

The liver holds a substantial B12 reserve. Tumours, particularly those within the liver and cancers that spread aggressively, may interfere with the storage, transport and release of this reserve.

Certain tumours may also increase their production of the proteins that transport B12 through the blood.

Consequently, a high B12 result in a blood test may not be causing disease at all; it might simply reflect it.

This interpretation was supported by a study published this year in Cancer Research Communications.

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Scientists at the University of Texas Medical Branch analysed records for 37,106 people with colon cancer from a worldwide health database covering 108 healthcare systems. Each person had a B12 blood test recorded around the time they were diagnosed.

The trend was striking. Those with raised B12 levels - above 1,000 picograms per litre - had a median survival of approximately five years. For patients whose levels were normal, median survival was almost 11 years.

Those with low B12 survived slightly longer still.

After researchers accounted for age, treatment and other health conditions, patients with high B12 were about twice as likely to die as those with low levels, and around 65 percent more likely to die than patients with normal levels.

They were also more likely to have stage 4 disease already at diagnosis, as well as to develop new metastases during the following year - especially in the liver.

The findings become more intriguing here.

Researchers also directly examined hundreds of colon tumours and identified substantially greater activity in a gene known as MTR. This gene produces an enzyme that needs B12 to create a DNA building block. Tumours with the most active form of the gene were associated with the poorest chances of survival.

Put another way, aggressive tumours may be especially effective at using - and stockpiling - B12 to power their own rapid replication, rather than B12 driving the process.

A 2024 study involving people receiving cancer immunotherapy reported the same association between elevated B12 and poorer outcomes. Studies in patients with oral cancer have also pointed in this direction.

A 2024 review of the evidence stated it clearly: the meaning of elevated B12 in people already diagnosed with cancer "remains uncertain and could potentially be linked to reverse causality" - in scientific terms, cancer may be producing the high B12 reading rather than B12 causing cancer.

The unanswered question of low B12

That does not, however, resolve the issue at the other end of the scale.

A 2024 study that followed 3,758 people with cancer and 2,995 people without cancer in Vietnam identified a U-shaped relationship. Both unusually low and unusually high dietary B12 intake were associated with greater cancer risk than intake levels in the middle range.

Because it was an observational study relying on food questionnaires recalled by participants, it can indicate only an association; it cannot demonstrate that one factor caused the other.

Related: A Common Gym Supplement May Give Cancer-Fighting Cells an Energy Boost

It is also notable that even the group with the highest intake consumed an average of only about 2.97 micrograms of B12 daily. This is not substantially higher than standard nutritional guidance of 2.4 to 2.8 micrograms per day.

Even so, it remains unclear why low B12 blood levels could be linked with a greater cancer risk.

Given the inconsistency in the available research, the relationship between B12 and cancer is far from resolved.

Still, it increasingly appears that B12 levels may signal another problem occurring in the body, rather than being a force that drives cancer itself.

This article was fact-checked by Clare Watson and edited by Clare Watson. Although we take pride in our process, we are only human. If you spot an error, please let us know.