Vitamin B12 (B12)

Vitamin B12, or cobalamin, is a vitamin your body can't make and can barely absorb without regular intake from food. It's essential for three things: building healthy red blood cells, maintaining the protective sheath around your nerves, and making DNA. It comes almost entirely from animal foods — meat, fish, eggs, and dairy — which is why diet is central to it.

Here's the catch that makes B12 tricky to read: the standard test measures total B12, but only a fraction of that is the active form your cells can actually use. So a result can look normal while your cells run short — especially in the borderline "grey zone" of 200–300 pg/mL. When the number is borderline but symptoms fit, doctors confirm with more specific tests — methylmalonic acid (MMA) or homocysteine — which rise when B12 is genuinely low.

B12 sits at the crossroads of your blood and your nervous system. Too little, and red blood cells grow large and ineffective — a type of anaemia that causes fatigue and pallor — and, more seriously, nerves begin to suffer: tingling, numbness, balance problems, memory and mood changes. The crucial point most people don't know: the nerve damage can happen without any anaemia, and if left untreated it can become permanent. That's why catching a low B12 early matters far more than the mild fatigue might suggest.

Low B12 (deficiency) shows up in two systems. In the blood, it makes red cells large and fragile — a megaloblastic anaemia causing fatigue, pallor, and breathlessness. In the nerves, it causes tingling and numbness in the hands and feet, unsteadiness, and sometimes memory, concentration, or mood problems; a sore, red tongue is a classic sign. Because the body stores B12 for years, deficiency creeps in slowly — but the nerve damage can arrive before any anaemia and can become irreversible, so it shouldn't be left to run.

• A plant-based diet — B12 comes almost only from animal foods, so vegetarians and especially vegans are at real risk without fortified foods or a supplement.
• Metformin — the most common diabetes medicine lowers B12 over time, which matters greatly given how widely it's used; long-term users should have their level checked.
• Acid-reducing drugs (PPIs) — long-term omeprazole and similar drugs reduce the stomach acid needed to release B12 from food.
• Pernicious anaemia — an autoimmune condition that destroys the stomach's ability to absorb B12; the classic cause, needing injections or high-dose treatment.
• Older age and H. pylori — both reduce the stomach's ability to free B12 from food; deficiency is common in the elderly.
• Gut conditions and surgery — coeliac disease, Crohn's, and stomach or bowel surgery all impair absorption.

If B12 is low, the fix depends on why. For a dietary shortfall, more animal foods — liver, eggs, fish, meat, dairy — or fortified foods and a supplement usually correct it. For an absorption problem (pernicious anaemia, older age, after surgery), the vitamin has to bypass the gut: either high-dose oral B12 (which works for many, since a little is absorbed without help) or injections for severe or nerve-related cases. Just as important is addressing the cause — reviewing long-term metformin or acid-reflux drugs with a doctor. Retest to confirm it's rising.

B12 is one of the safest vitamins — water-soluble, with no upper limit, because the body excretes what it doesn't need. Both common forms — cyanocobalamin and methylcobalamin — work; cyanocobalamin is cheaper and stable. For most people, high-dose oral B12 works as well as injections; injections are reserved for severe deficiency, nerve involvement, or absorption problems. Vegetarians and vegans genuinely need a supplement or reliably fortified foods. One honest note: for people who aren't deficient, B12 shots don't boost energy, mood, or weight — that popular promise isn't supported.

B12 is best understood alongside its partners. Folate is its twin — both build red blood cells, and both cause the same large-cell anaemia — but there's a trap: taking folate can hide (mask) a B12 deficiency's anaemia while the nerve damage quietly progresses, which is why B12 is checked before treating with folate. Homocysteine and methylmalonic acid (MMA) both rise when B12 is genuinely low, so they confirm a borderline result — MMA is the most specific. On a blood count, a high MCV (large red cells) is the classic clue. And ferritin / iron matters because a coexisting iron deficiency can shrink the cells back to normal size, hiding the B12 problem — so a normal blood count doesn't rule it out.