B12: the one that is not optional

If you read one article on this site, read this one.

Vitamin B12 is produced by bacteria, not by plants or animals. Animals accumulate it through their diet and through gut bacteria, which is why it appears in animal products.

Plant foods do not contain reliable amounts. Anyone eating an entirely plant-based diet requires a supplement or reliably fortified foods.

This is not contested. It is the position of every major dietetic association that has published on the subject.

Why it matters

B12 is required for red blood cell formation and for the nervous system.

Deficiency causes megaloblastic anaemia, producing fatigue, weakness and breathlessness. It also causes neurological damage — numbness and tingling, difficulty walking, memory problems, mood changes and, in severe cases, permanent damage.

The critical point: the neurological damage can become irreversible. Anaemia responds to treatment; long-standing nerve damage may not fully resolve.

Deficiency in infants of deficient mothers is particularly serious and can cause permanent developmental harm. This has been documented in case reports and it is the strongest reason for absolute clarity on this point.

The delay that catches people out

The liver stores B12, and stores can last years.

Someone who becomes vegan may feel entirely well for two, three or more years while stores deplete, then develop symptoms.

Which means the absence of symptoms tells you nothing about whether your intake is adequate.

It also means that people who have been vegan for a long time without supplementing may be depleted without knowing it.

The sources that do not work

Several plant foods have been proposed as B12 sources and do not hold up.

Spirulina and most algae contain pseudo-B12 — analogues that are detected by some assays but are not biologically active in humans, and which may interfere with absorption of the real thing.

Fermented foods including tempeh, miso and sauerkraut contain variable and unreliable amounts, mostly analogues.

Mushrooms, with a few exceptions containing small and unreliable amounts.

Unwashed vegetables, which historically supplied small amounts via soil bacteria and which are not a reasonable strategy.

Nori and some seaweeds contain some active B12, and amounts vary considerably and are not a reliable primary source.

The consistent finding across studies of unsupplemented vegans is high rates of deficiency. The plant sources do not work.

What does work

Supplements. Cyanocobalamin is the most studied, most stable and cheapest form. Methylcobalamin is also used; there is no strong evidence of superiority.

Absorption is limited per dose by the active transport mechanism, and a small proportion is absorbed passively regardless. Which means larger, less frequent doses work.

Commonly recommended regimes for adults:

Around 25 to 100 micrograms daily, or

Around 1000 micrograms two or three times per week, or

Around 2000 micrograms once weekly.

These are higher than the dietary reference intake because absorption of a single large dose is inefficient. This is expected and it is not harmful.

Fortified foods. Many plant milks, breakfast cereals, nutritional yeast products and meat alternatives are fortified.

These work if consumed reliably in adequate quantity, and relying on them requires actually checking the labels and actually eating them consistently.

Most dietitians recommend a supplement in addition, because fortification varies and consumption is inconsistent.

Safety

B12 has no established upper limit and excess is excreted. It is among the safest supplements available.

One caution: high-dose B12 has been associated with acne in some people, which resolves on reducing the dose.

Testing

Worth doing, and worth knowing that serum B12 alone is an imperfect test.

It can appear normal in the presence of functional deficiency, particularly where analogues are present from supplements like spirulina.

More informative markers include methylmalonic acid and homocysteine, which rise in deficiency. Ask about these if there is doubt, particularly if you have symptoms with a normal B12 result.

Who else should pay attention

Beyond vegans:

Vegetarians, whose intake from dairy and eggs is frequently marginal.

Older adults, in whom absorption declines because of reduced stomach acid.

Anyone taking metformin long term, which is associated with reduced B12 absorption.

Anyone on long-term acid-suppressing medication.

Anyone with a gastrointestinal condition affecting absorption, or who has had relevant surgery.

People with pernicious anaemia, an autoimmune condition affecting absorption, who require injections rather than oral supplements.

Pregnancy and children

The stakes are highest here.

Pregnant and breastfeeding women on plant-based diets must supplement, and requirements are higher.

Infants and children on plant-based diets require reliable B12 from supplements or fortified foods, appropriate to age.

Do not manage this without dietetic or medical input. Case reports of severe deficiency in breastfed infants of unsupplemented mothers exist and the outcomes are serious.

The one-line version

Take a supplement. Every week, without fail, for as long as you eat this way.

If you have symptoms including numbness, tingling, persistent fatigue, or memory problems, see a doctor promptly. Do not start high-dose supplementation before testing if deficiency is suspected, as it can mask the diagnosis. Pregnant and breastfeeding women and children require professional guidance.