We’ve all heard the advice: eat your fruit and vegetables, get your vitamins, and stay healthy. For the most part, that guidance holds up. But some nutrients have a more complicated story, and vitamin B12 is a fascinating example.

Also known as cobalamin, B12 is essential for life. It helps the body produce red blood cells, keeps the nervous system functioning, and plays a central role in how cells copy and repair DNA.

B12 is found naturally in animal products such as meat, fish, eggs, milk and cheese. Some cereals and breads are also fortified with it, helping people who do not eat meat get enough. Most people following a varied diet get the recommended amount, but vegans, people with certain gut conditions and older adults who absorb nutrients less efficiently may need supplements.

Without enough B12, things can go wrong, sometimes seriously, especially if deficiency is not recognised and treated. Yet in recent years, researchers have been asking whether high levels of B12 intake or high levels of B12 in the blood could be linked to cancer.

The body is constantly making new cells. Every time a cell divides, it needs to copy its DNA accurately. Vitamin B12 is critical to that process. When levels are too low, DNA can be copied incorrectly, leading to mutations that, over many years, may increase the risk of certain cancers, particularly colon........

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