Vitamin B12

A vitamin your body cannot make and cannot get from plants. It is needed to build red blood cells and to maintain the sheath around your nerves, and the liver holds a store large enough to last several years — which is why a shortage develops slowly and is often well advanced before it is noticed.

Required to make red blood cells, to maintain the myelin sheath around nerves, and to recycle folate.

• Meat, fish and shellfish
• Eggs
• Dairy
• Fortified plant milks and nutritional yeast

Needs stomach acid and intrinsic factor. Older adults often cannot free it from food protein even though the vitamin is present, so fortified food or supplements work better than more meat.

2.4 µg for most adults. The upper limit, past which harm becomes likely, is None established.

Reference intakes are population figures. Pregnancy, illness, age and medication all move an individual’s needs, which is one of several reasons this page is information rather than advice.

A sustained shortage has a name here: Pernicious anaemia. That is worth pausing on — nutrients whose deficiency earned a name are the ones where the consequences were severe enough, historically, to be described as a disease.

Most at risk: Vegans and strict vegetarians — it occurs naturally only in animal foods. Also adults over 50, anyone on long-term metformin or acid-blocking medication, and after stomach or bowel surgery.

Rarely low intake alone, except on a vegan diet without fortified foods. More often absorption: pernicious anaemia (an autoimmune loss of intrinsic factor), stomach or bowel surgery, coeliac or Crohn’s disease, and long-term metformin or acid-suppressing medication. Absorption also falls with age as stomach acid declines.

Fatigue and anaemia, then pins and needles, unsteadiness, memory problems and mood change. Nerve damage can become permanent if it goes untreated for long, which is why this one should not be left to guesswork.

Every sign here has other, often likelier, explanations. A list of symptoms is a reason to look into something, never a diagnosis.

A serum B12 blood test is the usual first step. It is an imperfect measure — results in the low-normal range can accompany real deficiency, and levels can read normal in liver disease. Where the picture is unclear, methylmalonic acid or homocysteine give a better sense of whether cells are actually short.

Meat, fish, eggs and dairy all supply it. On a plant-based diet, fortified foods are the reliable route: nutritional yeast, fortified plant milks and some breakfast cereals. Check the label rather than assuming — fortification is not universal.

Available as cyanocobalamin and methylcobalamin; both work for most people. Where the problem is absorption rather than intake, oral doses may still be effective at higher amounts, but injections are sometimes needed. Excess is excreted rather than stored, so toxicity is not the concern here — a missed diagnosis is.

No known toxicity. Excess is excreted.

Interactions: Metformin and proton pump inhibitors both reduce absorption over time.