Before becoming pregnant, women must ensure they get enough vitamin B12, in addition to folic acid, to cut the risk of their baby having a serious birth defect of the brain and spinal cord, new research shows. It is the latest study to stress the importance of B12, a vitamin that some vegetarians and vegans might struggle to obtain.

What is B12? Why does this vitamin have a number as well as a letter? A vitamin is just an organic substance that the body needs in small amounts for normal metabolic function. The eight B vitamins were originally thought to be one single vitamin. They were then found to be chemically different, though they are often found in the same foods. Hence the differentiation by number. Folic acid is itself a B vitamin - vitamin B9, also known as vitamin M.

What do we need B12 for? It has a lot of important functions. It helps the body make red blood cells and keeps the nervous system healthy, it helps release energy from our food and, as the latest study highlights, it is needed to process folic acid. But we are learning more about it all the time.

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Such as? Women may be able to cut their risk of age-related macular degeneration (AMD), the country's most common cause of sight loss, by taking a combination of vitamins B12, B6 and folic acid: that was the finding of a study published last month in the American Medical Association's Archives of Internal Medicine. In a randomised controlled trial, women taking the supplements had a 34% lower risk of any AMD and a 41% lower risk of visually significant AMD, compared to those not taking them.

And it is very important for women who intend to become pregnant? Exactly. It is already well known that women should take adequate amounts of folic acid pre-pregnancy to avoid neural tube defects, the role of B12 has been less well understood - until now.

The latest study looked at 1200 women in Ireland who gave blood samples during early pregnancy, which were analysed to determine vitamin B12 levels. Those with the lowest vitamin B12 levels were five times more likely to have a baby with a neural tube defect like spina bifida, in which the spinal cord and back bones do not form properly, than those with the highest levels.

The study showed that vitamin B12 deficiency was a risk factor for neural tube defects independent of, and additional to, folic acid. Vegans and women who eat very little meat or dairy should be particularly careful about getting enough vitamin B12.

Why's that? Because its main source is animal products. Vitamin B12 is found in virtually all meat products. Good sources include meat, salmon, cod, milk, cheese and eggs. Adults need approximately 0.0015 mg a day. If you eat meat, fish or dairy products then your diet should provide for your vitamin B12 needs.

But because vitamin B12 isn't found in fruit, vegetables and grains, vegans might struggle to get enough of it and become deficient. Although it has been suggested that some plants can provide B12, such as algae, the Vegetarian Society warns that "these cannot be utilised to satisfy dietary needs", adding that "the current nutritional consensus is that no plant foods can be relied on as a safe source of vitamin B12". For that reason, vegans must find ways of obtaining adequate levels of B12.

The Vegetarian Society recommends that vegans ensure their diet includes foods fortified with vitamin B12.

Can you take supplements? You can. There is no evidence to show what the consequences might be of taking high daily doses of vitamin B12. For that reason, it's wise not to take too much. The Food Standards Agency advises that taking 2 mg or less of vitamin B12 supplements a day is unlikely to cause any harm.