A Common Vitamin Has a Complicated Link to Cancer, Experts Reveal - ScienceAlert

Add ScienceAlert on Google Supplements can help with dietary deficiencies. (Anastasiia Voloshko/Moment/Getty Images) Vitamins are usually framed in simple terms: Get the right amounts, and your body will shine. In reality, the human body is more nuanced. Too much or too little of certain vitamins or minerals in your diet can be linked with significant problems – but interpreting what that link means isn't always straightforward. For many years, it has been known as one of the good guys of nutrition, helping to keep your body functioning properly. And if you don't have enough, you'll definitely feel it, with persistent fatigue and sluggish cognitive function among the most common symptoms of a B12 deficiency. But recent studies suggest that the bigger picture of B12 is a messy one, revealing puzzling relationships between abnormal levels of B12 and certain cancers – including cancers of the lungs, breast, esophagus, and colon. B12, or cobalamin, is a vitamin that no animal can live without. It helps make DNA and the myelin that protects our nerves, and is vital to red blood cell maturation, as well as processes involving both fatty and amino acids. Since plants don't need or make B12, the vitamin is only naturally found in animal-derived food products, and people who eat a plant-based diet or absorb nutrients poorly often struggle with a deficiency. Its role in synthesizing and stabilizing DNA is at least partly what led scientists to investigate a link with cancer. Cancer starts with DNA damage, and severe B12 deficiency is thought to indirectly interfere with the processes cells use to copy and repair DNA. Investigations into this link have led some researchers to propose that maintaining healthy B12 levels could help protect cells from the kinds of genetic instability associated with cancer. But other studies muddied the waters – showing a link between unusually high levels of B12 in the blood, and lung cancer. So both abnormally low and abnormally high levels of B12 have been shown in cancer cases in multiple studies around the world. But does that mean imbalanced B12 intake causes cancer? Well, no. In fact, it may be the other way around. One major complication is that researchers are often measuring two different things: how much B12 people consume, and how much of the vitamin is circulating in their blood. Those aren't necessarily the same thing, because blood B12 levels are influenced not just by diet, but also by how the body stores, uses, and transports the vitamin. The liver stores large amounts of B12, and several cancers – especially liver cancers and cancers that spread aggressively through the body – can disrupt the way the vitamin is stored, transported, and released. Some tumors also appear to increase production of the proteins that carry B12 through the bloodstream. That means elevated B12 in blood tests may sometimes be a symptom or side effect of disease rather than the thing causing it. Researchers are still trying to untangle exactly what these elevated B12 levels mean. A 2026 study involving more than 37,000 colon cancer patients found that people with very high B12 levels had significantly worse survival outcomes, but stopped short of establishing a causal link, instead suggesting that B12 may function as a biomarker of aggressive disease. A 2022 review of human studies concluded there still isn't strong evidence that high B12 intake, high B12 levels, or supplementation directly cause cancer, despite repeated associations between abnormal B12 levels and disease. Another 2024 review examined B12 blood levels in cancer patients and concluded that "he diagnostic significance of elevated B12 levels among patients already diagnosed with cancer remains uncertain and could potentially be linked to reverse causality". A 2024 study of 3,758 cancer patients in Vietnam complicated the picture further. It compared dietary B12 intake among cancer cases and 2,995 people without cancer and found a U-shaped association – people with both unusually low and unusually high intake showed elevated overall cancer risk compared to those in the middle range. The Vietnamese intake study means that this may not be as clear-cut, but it also has important limitations. Like many studies investigating the link between B12 and cancer, it was observational, meaning it could only identify associations rather than direct cause-and-effect. The researchers also relied on food questionnaires, which can be imperfect measures of long-term diet. Related: Two Supplements For Menopause May Actually Help, Expert Reveals 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. With such inconsistencies in the wider body of research, the link between cancer and B12 remains unclear, with researchers still trying to establish whether the vitamin contributes to disease, is a result of it, or both. One thing, however, seems clear. Like many things in life, too much or too little is where you might encounter problems. Best take a leaf from Goldilocks and land in the realm of just right.