Vitamin B12: who actually needs to worry, and who is being oversold

Adults need about 2.4 micrograms of vitamin B12 a day, and someone who eats meat, fish, eggs or dairy regularly almost always gets it without trying [s1]. Genuine deficiency is real but concentrated in two groups — older people, whose gut stops absorbing the vitamin, and people who avoid animal foods, whose intake falls short — which is also why blanket B12 supplements marketed for "energy" to everyone else are mostly selling reassurance [s1][s3][s4].

What B12 does and how much you need

Vitamin B12, or cobalamin, is a B vitamin with a central role in cellular metabolism, particularly DNA synthesis, methylation and mitochondrial metabolism [s2]. Its Recommended Dietary Allowance for adults is 2.4 micrograms a day, rising to 2.6 micrograms in pregnancy [s1]. Those are small quantities, and because the body stores B12, a well-nourished adult carries a reserve that takes a long time to run down. The vitamin occurs naturally almost only in animal-source foods, which is the root of most dietary shortfalls [s3].

Deficiency is diagnosed less by symptoms than by biochemistry. The standard markers are a low blood B12 level together with raised homocysteine and methylmalonic acid, two metabolites that accumulate when the vitamin is scarce — though experts note the exact cut-offs that separate clinical from subclinical deficiency remain debated [s2]. That definitional fuzziness is why prevalence estimates vary so widely.

How common is it, really

Frank, symptomatic deficiency — with the classic anaemia and nerve damage — is relatively uncommon [s2]. Milder, subclinical deficiency is not: depending on the threshold used, it affects between 2.5% and 26% of the general population, and its clinical importance at the mild end is genuinely unclear [s2]. In other words, a large share of "low" readings may never cause harm.

Age shifts the picture. In large surveys in the United States and the United Kingdom, roughly 6% of people aged 60 or over were B12 deficient (plasma B12 below 148 pmol/L), and closer to 20% had marginal status (148 to 221 pmol/L) in later life, with prevalence rising as people age [s3]. The mechanism changes with age too. In younger adults, low B12 usually reflects inadequate intake of animal foods; in older people the dominant cause becomes food-bound cobalamin malabsorption — the stomach can no longer liberate B12 from food, often because of atrophic gastritis [s3]. Crucially, most older people can still absorb the vitamin from fortified food or supplements, where it is not bound to protein [s3]. The most severe absorption failure, pernicious anaemia, is different again: it stems from a lack of intrinsic factor, and if left untreated it can cause potentially irreversible neurological damage [s1].

Vegetarians and vegans

The other high-risk group is defined by diet. A review of 18 studies that measured methylmalonic acid or active B12 found deficiency was common among vegetarians across the board: reported rates were 62% among pregnant women, between 25% and 86% among children, 21% to 41% among adolescents, and 11% to 90% among the elderly [s4]. Rates were higher among vegans than among vegetarians, and higher in people who had followed the diet since birth than in those who adopted it later [s4]. The review's blunt conclusion was that vegetarians tend toward B12 depletion regardless of where they live, their age or the exact type of diet, and should take deliberate steps — fortified foods or supplements — to secure an adequate intake [s4]. This is the clearest case in the whole B12 story where routine supplementation is genuinely warranted [s4].

Treatment, and the marketing gap

Where deficiency is real, correcting it is straightforward: management rests on B12 supplementation, either as high-dose oral tablets or as injections, with the route chosen according to the cause and severity [s2]. That effectiveness, though, is precisely what gets stretched in marketing. The pitch that everyone should take B12 for energy or focus does not follow from the evidence: the people who benefit are those who are deficient, and for a meat-eating adult with normal absorption, extra B12 corrects a problem they do not have.

How to read this

The honest summary is that B12 is a nutrient where the risk is specific rather than universal. It matters enormously for the groups who cannot absorb it or do not eat it, and for them supplementation or fortification is well justified [s1][s3][s4]. For the general, animal-food-eating population it is largely a non-issue, and the widespread "low" subclinical readings are of uncertain significance [s2]. The clearest overlap with everyday diet is plant-based eating, where B12 is the textbook gap — see plant-based diets and nutrient gaps — and pregnancy, covered in preconception B12 and folate. The age angle connects to the wider problem of medicines and nutrient status in later life, discussed in deprescribing in older adults.

This article is informational and not dietary or medical advice; suspected B12 deficiency should be assessed and treated by a qualified clinician.