What vitamin B12 does
B12 is needed to make red blood cells and DNA, and to keep the nervous system healthy. Deficiency can cause anaemia, tiredness and, over time, irreversible nerve damage.
How much
Adults need about 2.4–4 µg a day.
Where it comes from
B12 is made by bacteria, not plants or animals. Animal foods contain it because animals absorb it or are given supplements. Plants are not a reliable source — seaweed, spirulina and fermented foods mostly contain inactive B12 analogues.
What to do
If you eat mostly or only plants, take a B12 supplement or eat fortified foods (some plant milks, cereals and nutritional yeast) every day. Common options are a small daily dose or a larger weekly one — ask your doctor or pharmacist which suits you, and have your levels checked if you’re unsure.
Absorption also falls with age and with some medications, so B12 matters for older adults regardless of diet.