Vitamin B12 Deficiency: Signs, Causes and Best Food Sources
- , by SANUSq Research team
- 8 min reading time
Vitamin B12 is one of those nutrients you never think about — until you don't have enough. And a surprising number of people don't, often for years, without realising it. This guide covers what B12 actually does, the signs of B12 deficiency worth taking seriously, who is most at risk, and the best vitamin B12 sources — along with the reason intake alone doesn't always settle the question.
Why vitamin B12 matters
Vitamin B12 (cobalamin) is essential for some of the body's most fundamental jobs. It's needed to make healthy red blood cells, to build and protect the myelin sheath that insulates your nerves, to synthesise DNA, and to help convert food into usable energy. Because it touches the blood, the nervous system and energy metabolism all at once, a shortfall can show up in a bewildering range of ways — which is exactly what makes B12 deficiency so easy to miss.
Behind the scenes, B12 works hand in hand with folate in a process called methylation — the chemistry that builds DNA, regulates genes and keeps a compound called homocysteine in check. When B12 is low, homocysteine can rise, which is one reason deficiency is linked with cardiovascular and cognitive concerns as well as anaemia. It's a small molecule with an outsized reach.
A comprehensive review notes that while clinical B12 deficiency with classic anaemia and neurological signs is relatively uncommon, subclinical deficiency affects anywhere from about 2.5% to 26% of the general population depending on how it's defined, and is most common in older adults and in people whose diets are low in animal-derived foods (Green et al., 2017).
Vitamin B12 deficiency symptoms
Deficiency often creeps in slowly, which is why vitamin B12 deficiency symptoms are so often put down to a busy life or getting older. Common signs include persistent fatigue and weakness, pins-and-needles or numbness in the hands and feet (from nerve involvement), a sore or swollen tongue, pale or slightly yellowish skin, breathlessness, and cognitive symptoms such as brain fog, memory lapses or low mood. In more advanced cases it causes a specific anaemia (megaloblastic anaemia) and, if left untreated, nerve damage that can become permanent. The nerve symptoms deserve particular attention, because they're the ones with lasting consequences: B12 keeps the myelin insulation around nerves intact, and as it runs low that insulation frays, producing tingling, numbness and balance problems — and, if the shortfall continues, damage supplements can no longer fully reverse. Because these symptoms overlap with so many other conditions, a blood test is the only way to know for sure — and it's worth asking for one if several of these ring true.
Who is most at risk
Some groups are far more likely to run short:
- Vegans and vegetarians. B12 is found almost exclusively in animal foods, so plant-based eaters need fortified foods or a supplement — this isn't optional, it's essential.
- Older adults. The stomach produces less acid and intrinsic factor with age, both needed to absorb B12 from food, so absorption falls even when intake is fine.
- People on certain medications. Long-term use of metformin (for type 2 diabetes) and acid-suppressing drugs (PPIs) both reduce B12 absorption.
- Those with gut or absorption conditions, such as pernicious anaemia (an autoimmune loss of intrinsic factor), coeliac or Crohn's disease, or after gastric surgery.
In the long-running Diabetes Prevention Program, people on long-term metformin were significantly more likely to develop B12 deficiency and anaemia, with risk rising the longer the drug was taken — leading the authors to recommend routine B12 testing for metformin users (Aroda et al., 2016).
Vitamin B12 sources — and why they aren't always enough
Nearly all the B12 rich foods are animal foods, and the list is short and predictable: shellfish (clams are exceptional), liver and other organ meats, fish such as salmon and tuna, red meat, eggs and dairy. For most omnivores, a varied diet that includes some of them covers the requirement comfortably. The catch is absorption — getting enough B12 in your diet is only half the story if your body can't take it up. That's why older adults and people on B12-lowering medications can become deficient despite eating plenty, and why fortified foods (some plant milks, cereals and nutritional yeast) and supplements are the reliable route for anyone who is plant-based or has an absorption problem.
It's also worth knowing that the body stores several years' worth of B12 in the liver, which is why a deficiency can take a long time to appear — and, frustratingly, why it can be well established before the symptoms make themselves obvious.
Getting tested — the vitamin B12 deficiency test
Because the symptoms are so non-specific, diagnosis rests on blood tests — a serum B12 level, often alongside markers such as homocysteine or methylmalonic acid that pick up a shortfall earlier and more reliably than B12 alone. If you're in a higher-risk group, or several of the signs above sound familiar, it's a simple thing to ask your doctor to check rather than self-diagnosing.
Testing also helps tell a simple dietary shortfall apart from an absorption problem such as pernicious anaemia — which matters, because the two are treated differently: oral or dietary B12 for the former, often injections for the latter.
Correcting a shortfall
The good news is that B12 deficiency is very treatable once it's found — through diet, oral or sublingual supplements, or, where absorption is the problem, B12 injections prescribed by a doctor. The important thing is not to leave it: the fatigue and brain fog usually reverse, but nerve damage from prolonged deficiency can become permanent, so early testing and treatment genuinely matter. If you suspect low B12, the right first step is a blood test rather than guesswork.
Frequently asked questions
What does vitamin B12 do?
It's essential for making red blood cells, keeping nerves healthy (via the myelin sheath), synthesising DNA, and turning food into energy. That's why deficiency can cause anaemia, nerve symptoms, fatigue and cognitive changes all at once.
What are the signs of B12 deficiency?
Fatigue, tingling or numbness in the hands and feet, a sore tongue, pale skin, breathlessness, and brain fog or low mood are common. They come on gradually and overlap with many conditions, so a blood test is the way to confirm it.
Who needs to watch their B12?
Vegans and vegetarians (B12 is almost only in animal foods), older adults (absorption declines with age), people on long-term metformin or acid-reducing medication, and anyone with a gut or absorption condition such as pernicious anaemia or coeliac disease.
Can I get enough B12 from food?
Most omnivores can. The foods high in vitamin B12 are everyday ones — meat, fish, eggs and dairy — and a normal mixed diet supplies plenty. But if you're plant-based, older, or have an absorption issue, food alone often isn't enough — fortified foods or a supplement become necessary, because the problem is frequently absorption rather than intake.
Can you take too much B12?
B12 is water-soluble and considered very safe — excess is generally excreted rather than stored, so there's no established toxic level from food or normal supplements. The bigger risk is the opposite: under-treating a real deficiency. That said, an unexpectedly high B12 on a test without supplementing is worth discussing with a doctor.
How is B12 deficiency treated?
With dietary changes, oral or sublingual supplements, or B12 injections when absorption is impaired. It's very treatable, but worth acting on early — tiredness and brain fog usually recover, while long-standing nerve damage may not.
References
- Green R, Allen LH, Bjørke-Monsen AL, et al. Vitamin B12 deficiency. Nature Reviews Disease Primers. 2017;3:17040. PMID 28660890
- Aroda VR, Edelstein SL, Goldberg RB, et al. Long-term metformin use and vitamin B12 deficiency in the Diabetes Prevention Program Outcomes Study. Journal of Clinical Endocrinology & Metabolism. 2016;101(4):1754–1761. PMID 26900641
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