Vitamin B12 is a water-soluble nutrient that your body needs for three things it absolutely cannot do without: producing healthy red blood cells, maintaining a functioning nervous system, and synthesising DNA. Unlike most vitamins, B12 is found almost exclusively in animal-based foods, which makes deficiency a genuine concern for vegans, vegetarians, and older Australians whose ability to absorb it naturally declines with age.
What vitamin B12 actually does in your body
Your body uses B12 to produce myelin, the protective sheath that wraps around nerve fibres. Without enough myelin, nerve signals slow down or misfire entirely. That's why a prolonged B12 deficiency doesn't just make you tired. It can cause tingling in the hands and feet, balance problems, and in severe cases, lasting neurological damage.
B12 also works alongside folate to produce red blood cells in bone marrow. When B12 is short, the body makes red blood cells that are too large and poorly formed to carry oxygen efficiently. The result is a condition called megaloblastic anaemia, which causes fatigue, breathlessness, and brain fog that many people mistake for stress or poor sleep.
A third role is less discussed but equally important: B12 helps break down homocysteine, an amino acid that builds up in the blood when B vitamins are low. Elevated homocysteine is linked to cardiovascular disease and cognitive decline. Keeping B12 levels adequate is one of the simplest ways to manage that risk.
Where B12 comes from
B12 is produced by bacteria and archaea, not by plants or animals. Animals absorb it from their gut bacteria or from soil microbes on the food they eat, which is why meat, fish, eggs, and dairy are the main dietary sources. A 100-gram serving of beef liver contains roughly 70 micrograms of B12, far exceeding the adult recommended daily intake of 2.4 micrograms. Salmon, tuna, and dairy products are solid everyday sources.
Plant foods contain no meaningful B12. Some fermented products and certain algae are sometimes cited as sources, but the B12 analogues in those foods are not reliably absorbed by the human body. Vegans need to supplement or eat fortified foods consistently. This isn't optional. It's a biological fact about the diet.
Fortified foods like plant-based milks, breakfast cereals, and nutritional yeast can make a real difference, but you need to check the label. Not every brand adds B12, and the amounts vary considerably.
Who is most at risk of deficiency
Deficiency develops slowly, sometimes over years, because the liver stores enough B12 to last 3 to 5 years. That's why symptoms often creep in quietly and get attributed to other causes before anyone thinks to test for B12.
The groups most likely to be deficient include:
- Vegans and strict vegetarians who don't supplement.
- People over 50, whose stomach lining produces less intrinsic factor, the protein needed to absorb B12 from food.
- Anyone taking metformin for type 2 diabetes, since the drug interferes with B12 absorption over time.
- People with Crohn's disease, coeliac disease, or who have had gastric surgery, all of which affect absorption in the gut.
The absorption issue is worth understanding. B12 from food is bound to proteins and needs stomach acid and a protein called intrinsic factor to be released and absorbed. B12 in supplements bypasses some of that process, which is why high-dose oral supplements can work even in people with absorption problems.
Symptoms to watch for
Early B12 deficiency is easy to miss. Fatigue is the most common complaint, followed by weakness and a sore, inflamed tongue. Some people notice mood changes, irritability, or difficulty concentrating long before any physical symptoms appear.
As deficiency deepens, neurological symptoms become more prominent. Numbness or tingling in the extremities, difficulty walking steadily, and memory problems are all signals that nerve damage may be underway. At that stage, the damage isn't always fully reversible even after B12 is restored, which is why catching it early matters. These neurological symptoms echo what happens in iron deficiency, though the mechanisms are different and the two conditions can coexist.
A simple blood test measures serum B12. Australian GPs include it on standard blood panels when fatigue or neurological symptoms are present. If your level sits below 200 pmol/L, most doctors consider supplementation necessary. The optimal range is generally accepted to be above 300 pmol/L.
How to get enough B12
For most Australians who eat meat and dairy regularly, dietary B12 isn't a concern. The real work is knowing your risk factors. If you're over 50, plant-based, or on long-term metformin, it's worth asking your GP to test your levels rather than waiting for symptoms to show up.
Supplements come in several forms. Cyanocobalamin is the most common and cheapest, and it's well-absorbed by most people. Methylcobalamin is a naturally occurring form sometimes marketed as superior, though the evidence that it's meaningfully better for most people is thin. What matters more is consistency. Taking a daily 25–100 microgram supplement or a weekly higher-dose supplement covers most needs reliably.
Injections are the gold standard for people with absorption disorders, since the B12 goes directly into the bloodstream without needing intrinsic factor at all. High-dose sublingual tablets, which dissolve under the tongue, achieve similar results through passive absorption across the mucous membrane.
B12 works closely with other B vitamins, particularly folate and B6. If you're looking at broader nutritional gaps, the way magnesium shortfalls tend to pair with other micronutrient gaps, B12 is rarely the only thing worth checking.
A note on supplements
B12 has no known toxicity at high doses. The body excretes what it doesn't need via urine, which is why the upper limit for intake hasn't been formally set in Australia. That said, taking supplements you don't need isn't a free ride either. Masking an underlying absorption disorder without identifying it can delay treatment for whatever is causing the problem.
The National Health and Medical Research Council sets the Australian recommended dietary intake for B12 at 2.4 micrograms per day for adults, rising slightly during pregnancy and breastfeeding. For most people, the goal isn't megadosing. It's consistent, reliable intake over time, whether from food, supplements, or both.
If you haven't had your B12 checked in the past couple of years and you fall into any of the risk groups above, that's the place to start.