Vitamin B12

Vitamin B12 has two reputations, and both are slightly misleading. The first is that it is the vegan's vitamin, a niche concern for people who skip animal foods. The second is that a number on a lab report settles the question. Neither is quite right, and the gap between them is where a lot of real, preventable harm happens.

B12 deficiency is common, and not only among vegans. It rises steeply with age, because absorbing it is an elaborate process that the aging gut performs less and less well, and it is driven up further by some of the most widely used medications in the world. A meaningful share of older adults are either deficient or close to it, usually without knowing.

And the standard test is unusually unreliable. A serum B12 in the "normal" range does not rule out a true deficiency, because the number includes a large inactive fraction and the reference range runs too low. So a marker that looks reassuring can quietly miss a shortfall that, left long enough, damages nerves in ways that do not fully heal. This is a number worth understanding better than most.

B12, or cobalamin, is the only vitamin built around a metal atom, cobalt, and the only one made exclusively by bacteria, which is why it reaches us almost entirely through animal foods. Inside the body it serves as a cofactor for two enzymes: methionine synthase, which remethylates homocysteine and supports DNA methylation, and methylmalonyl-CoA mutase, which keeps a step of mitochondrial energy metabolism running ^[1]. A shortage disrupts both at once.

The consequences fall hardest on rapidly dividing and highly specialized cells. In the bone marrow, impaired DNA synthesis produces the oversized, immature red cells of megaloblastic anemia. In the nervous system, the disruption damages the myelin that insulates nerves, producing numbness, balance problems, and cognitive changes. Crucially, the neurological damage can appear without any anemia at all, and if it persists it can become permanent ^[1]. That combination, serious harm that does not always announce itself through the blood, is what makes B12 worth taking seriously.

The diagnostic catch follows from the biology. A standard serum B12 test measures total cobalamin, most of which is bound to a carrier and not biologically available, so the number can look adequate while the active fraction is low. Functional markers do better: methylmalonic acid and homocysteine both rise when B12 is genuinely insufficient inside cells, and the active fraction, holotranscobalamin, falls earliest of all ^[2]. No single test is definitive, which is exactly why a lone normal serum B12 should not end the conversation when risk or symptoms are present.

Just how common the shortfall is surprises most people. In US and UK surveys, roughly 6% of adults over 60 are outright deficient, and about another 20% sit in a marginal range that standard screening tends to wave through ^[3]. The risk factors read like a list of ordinary life: advancing age and the loss of stomach acid that comes with it, a vegetarian or vegan diet, pernicious anemia, gut surgery, and long-term use of metformin or acid-reducing drugs, all of which interfere with one step or another of that absorption chain ^[3].

There is one more trap worth naming, because it has caused real harm. Folate and B12 work together, and taking folate can correct the anemia of B12 deficiency while doing nothing for the nerve damage underneath. The blood count looks fixed, the shortfall is masked, and the neurological injury quietly progresses. This is why B12 status has to be assessed in its own right, not assumed from a normal blood count or a folate level.

The reassuring half of the story is the treatment. For most people, including many with absorption problems, high-dose oral B12 restores levels effectively, because a small fraction is absorbed by simple diffusion that does not require the intrinsic-factor escort; injections are reserved for severe deficiency or pernicious anemia ^[4]. A deficiency that can do permanent damage is, caught in time, one of the cheapest and simplest to correct.