Of the eight B vitamins, seven are broadly handled by a reasonable diet. B12 is the exception, and it is the exception for a reason that catches people out: the problem is usually not how much you eat, it is whether you can absorb it.
Four groups are at meaningfully elevated risk — older adults, people eating little or no animal food, people taking metformin, and people on long-term acid-reducing medication. If you are in more than one, the risk stacks. This guide explains the mechanism in each case, what deficiency looks like as it develops, why folate complicates the picture, and what to actually ask for. For the wider set, see our reference guide to what each B vitamin does.
Why B12 Absorption Is So Fragile
Absorbing B12 from food takes a multi-step process that has several places to fail:
- Stomach acid and pepsin release B12 from the food protein it is bound to.
- The stomach lining produces intrinsic factor, a protein that escorts B12 onward.
- The B12-intrinsic factor pair travels to the terminal ileum, the last section of small intestine.
- Specific receptors there absorb the complex into the bloodstream.
Break any link and absorption drops, no matter how much B12 is on your plate. This is why B12 is different from, say, thiamine, where eating more genuinely fixes it.
Crucially, the supplement form of B12 is not protein-bound. It does not need the acid-release step, and at higher doses a small fraction is absorbed by passive diffusion independent of intrinsic factor entirely. That is the practical reason supplements can work for people whose absorption of food-bound B12 has failed.
Group One: Adults Over About 50
Stomach acid production commonly declines with age, a condition called atrophic gastritis that affects a substantial share of older adults, often without symptoms. Less acid means less B12 released from food protein — so an older person can eat plenty of meat, fish and dairy and still absorb poorly.
This is well enough established that dietary guidance in several countries specifically recommends adults over 50 get most of their B12 from fortified foods or supplements rather than from food alone, precisely because those forms bypass the acid-release step.
The problem is that early B12 deficiency in an older adult looks like ageing. Tiredness, low mood, forgetfulness, unsteadiness on the feet — all easily attributed to something else. It is one of the reversible contributors to cognitive and balance complaints that is genuinely worth ruling out.
Group Two: Vegetarians And Vegans
B12 is made by bacteria, not by plants or animals. Animals accumulate it; plants do not. No unfortified plant food is a reliable source. Not spirulina, not nutritional yeast unless specifically fortified, not fermented soy, not mushrooms, not seaweed — several of these contain B12 analogues that show up on an assay but are not usable by human enzymes, and may even interfere.
- Vegans need a reliable fortified food or supplement source. This is not optional or a matter of dietary philosophy; it is a simple consequence of where the vitamin comes from.
- Vegetarians who eat eggs and dairy get some, but intakes are often marginal and low status is common enough to be worth checking.
Liver stores can mask a shortfall for years, which is the trap — someone can feel fine for a long stretch and then present with neurological symptoms once stores run out. Folate, meanwhile, is usually abundant on a plant-based diet, and that abundance can conceal the anaemia that would otherwise flag the B12 problem.
Group Three: People Taking Metformin
Metformin is associated with reduced B12 absorption, and the effect appears to grow with dose and duration of use. The leading explanation is that metformin interferes with a calcium-dependent step in B12 uptake at the terminal ileum.
The clinically awkward part: peripheral neuropathy — numbness, tingling and burning in the feet — is also a common complication of diabetes itself. B12 deficiency produces the same symptom. It is entirely possible for nerve symptoms caused by low B12 to be attributed to diabetic neuropathy and left unaddressed. Periodic B12 monitoring is recommended in many diabetes care guidelines for people on long-term metformin. Do not stop or change metformin on your own — the action here is asking whether your B12 has been checked.
Group Four: People On Long-Term Acid Reducers
Proton pump inhibitors and H2 blockers work by suppressing stomach acid, which is exactly the step needed to release B12 from food protein. Long-term use is associated with lower B12 status, and the association strengthens with duration.
Short courses are not the concern. Years of continuous use without review is. Many people stay on an acid reducer indefinitely, long after the original reason has resolved, without anyone revisiting it. Two reasonable questions for your prescriber: is this still needed at this dose, and has my B12 been checked?
Where Folate Fits — And Why It Complicates Things
Folate and B12 work together in the reaction that converts homocysteine to methionine. When B12 is missing, folate gets trapped in an unusable form, which is why both deficiencies produce the same large-cell anaemia.
This creates a specific hazard. Plentiful folate — from a plant-heavy diet, fortified grains, or a supplement — can correct or prevent the anaemia of B12 deficiency while the neurological damage continues silently underneath. The blood count looks reassuring; the nerves are not. It is the reason clinicians check B12 rather than inferring it from a normal full blood count, and the reason a high-dose folic acid supplement should not be taken indefinitely without knowing your B12 status.
Folate has its own priority group: anyone who could become pregnant. Neural tube closure happens in the first few weeks, often before pregnancy is recognised, which is why folic acid before conception and through early pregnancy is standard advice. See prenatal vitamins and discuss dosing with your provider, since some situations warrant a higher dose.
Signs Worth Taking Seriously
- Early: persistent fatigue not explained by sleep, breathlessness on mild exertion, pale skin, sore or smooth tongue, mouth ulcers, low mood or irritability
- Neurological: pins and needles or numbness in hands and feet, usually symmetrical; unsteady walking, especially in the dark; muscle weakness
- Cognitive: difficulty concentrating, memory changes, confusion
The neurological signs are the ones that matter most, because prolonged deficiency can cause permanent nerve damage. Anaemia can be reversed; nerve damage caught late may not be.
What To Actually Do
- Ask for a test rather than guessing. Serum B12 is the standard first step. If it comes back borderline, methylmalonic acid and homocysteine are more sensitive follow-ups. Say up front if you already take a B12 supplement, since it affects interpretation.
- Do not start a supplement immediately before testing if deficiency is being investigated — it can obscure the result. Ask your clinician about timing.
- Mention biotin too. High-dose biotin distorts many immunoassays, so it is worth disclosing before any blood draw. Details in our guide to biotin dosage, safety and lab-test interference.
- Match the form to the reason. Where absorption is impaired rather than intake, oral supplements at appropriate doses often still work through passive diffusion — but confirmed pernicious anaemia or significant malabsorption may need injections. That is a clinical decision, not a shelf decision.
- Consider whether a full B-complex is what you need. If the issue is specifically B12, a targeted product may make more sense than an eight-vitamin blend. Our comparison of liquid versus pill B-complex covers format, and you can browse vitamins and supplements or vitamins and minerals for options.
- Recheck rather than assume. If you are in a risk group, this is worth revisiting periodically, not solving once.
Talk to your doctor before starting or stopping anything. That applies especially if you take metformin or an acid reducer — do not alter prescribed medication on the basis of an article. This is general educational information, not medical advice, and B vitamins are not a treatment for any disease.
