Vitamin B12 (Cobalamin)

B12 is the largest and most structurally complex vitamin, built around a cobalt atom — hence cobalamin. It is produced by bacteria, not by plants or animals, and accumulates in animal tissue up the food chain.

This is why it is genuinely absent from plant foods. Sources are meat, fish, eggs, dairy and fortified products. Seaweed, fermented foods and mushrooms are sometimes claimed as sources but largely contain inactive analogues that do not do the job, and in some cases compete with the real thing.

Supplemental forms differ. Cyanocobalamin is cheap, extremely stable and well studied. Methylcobalamin and adenosylcobalamin are the active forms and are marketed as superior, though evidence for a practical advantage in most people is thinner than the price difference suggests. Hydroxocobalamin is the form generally used for injections.

B12 is required by only two enzymes in humans, but both matter enormously. One converts homocysteine back to methionine, working alongside folate to sustain the methylation cycle. The other handles a step in the breakdown of certain fats and amino acids.

Its absorption is the most elaborate of any vitamin. B12 must be released from food protein by stomach acid, bound to a carrier protein produced by the stomach lining, and then absorbed through a specific receptor in the small intestine. Each of those steps can fail independently of dietary intake.

This is why B12 status and B12 intake can diverge so sharply, and why older adults are more likely to fall short despite eating plenty of it — stomach acid production declines with age, and that first step becomes less reliable.

B12's essentiality is beyond dispute, and correcting a genuine shortfall produces clear, well-documented improvement.

Supplementation is unambiguously necessary for anyone eating a fully plant-based diet. This is not a matter of optimisation — there is no reliable plant source, and going without eventually causes problems that are not always fully reversible.

For energy in people with adequate status, evidence is absent despite B12's reputation as an energy vitamin. That reputation comes from how dramatically people improve when a real shortfall is corrected, which is a different thing entirely. If you eat animal products regularly and feel tired, B12 is unlikely to be the answer.

B12 has an exceptional safety record. No tolerable upper intake level has been established, because absorption is self-limiting and excess is excreted. Even very large oral doses have not been associated with toxicity.

Adverse effects are rare. Occasional reports include mild digestive upset and, with high-dose injections, skin reactions.

Several commonly prescribed medicines reduce B12 absorption over time. If you take regular medication long term, it is worth asking your doctor whether your B12 status should be checked rather than assuming your diet covers it.