A colleague of mine (I'll call him Tom) went plant-based about three years ago. Completely vegan. Ethically motivated, well-researched, genuinely committed. He'd done his homework on protein. He knew about iron. He ate well. Varied, whole-food, thoughtful about his nutrition.

He did not supplement B12. Because, in his words: "I eat nutritional yeast and fortified soy milk. I figured that was enough."

For two years, it probably was. Then it wasn't.

The first thing he noticed was fatigue. He attributed it to a busy period at work. Then came the tingling in his feet, intermittent and mild, easy to ignore. Then the brain fog. Then the memory lapses. Not dramatic, just a slow erosion of sharpness. Forgetting words mid-sentence. Losing his train of thought in meetings.

He mentioned it to me over coffee. I said something like: "Have you had your B12 checked?" He hadn't. He didn't think a nutritional deficiency was possible given how carefully he ate.

His B12 came back at 112 pmol/L. The WHO defines deficiency as below 150. His GP was concerned enough to start intramuscular injections rather than oral supplements, partly because of the level and partly because of the neurological symptoms.

Within six weeks, the fatigue improved significantly. Within three months, the brain fog lifted. The tingling took longer, about four months before it fully resolved. His GP told him that if he'd waited another year, some of the neurological changes might have been permanent.

That last sentence is the reason I'm writing this article. B12 deficiency isn't just another nutritional gap. It's one of the few deficiencies where the consequences can be irreversible if it's caught too late. And it's significantly more common than most people realise, particularly in vegans, older adults, and people on certain medications.

A note before we get into it

What B12 actually does (and why it's not optional)

Vitamin B12 (cobalamin) is involved in three processes you can't do without.

DNA synthesis. Every cell in your body needs B12 to replicate properly. Without it, cell division goes wrong, particularly in rapidly dividing tissues like blood and gut lining.

Red blood cell production. B12 deficiency causes megaloblastic anaemia, a type of anaemia where red blood cells become abnormally large and dysfunctional. They can't carry oxygen efficiently, which produces fatigue, weakness, and pallor.

Neurological function. B12 helps maintain the myelin sheath, the insulating layer that surrounds nerves and enables proper signal transmission. When B12 is depleted, myelin breaks down. This is what causes the tingling, numbness, balance problems, and cognitive changes. And unlike most nutritional deficiencies, where the damage reverses once you replenish the nutrient, neurological damage from B12 deficiency can be permanent if it progresses far enough.

B12 is also involved in homocysteine metabolism. When B12 is low, homocysteine rises, and elevated homocysteine is associated with increased cardiovascular risk. This connection links B12 status to the cardiovascular picture I covered in the heart health article.

Who's at risk

Vegans. B12 is found almost exclusively in animal products: meat, fish, eggs, dairy. There are no reliable, unfortified plant sources. Systematic reviews of vegan populations show deficiency prevalence ranging from 11% to over 86%, depending on the population studied and whether supplements are used. An Australian study of vegetarian and vegan Seventh-Day Adventists found 53% had B12 below the reference range. Without supplementation, deficiency in vegans isn't a possibility. It's an inevitability. The only question is when.

Vegetarians. Less severely affected than vegans because eggs and dairy provide some B12, but still at significantly higher risk than omnivores. Deficiency rates in vegetarian populations range from 0 to 33%, depending on dietary strictness and supplementation.

Older adults (over 50). Absorption of B12 decreases with age due to reduced gastric acid production and changes in the stomach lining. In Australian studies, prevalence of B12 deficiency in older adults is estimated at 20 to 25%, and may reach 40% in some populations. Many older adults are deficient despite eating meat, because the problem is absorption, not intake.

People taking metformin. This widely prescribed diabetes medication can reduce B12 absorption. Long-term metformin users should have B12 monitored periodically. This connects to the HbA1c article and the diabetes management conversation.

People taking proton pump inhibitors (PPIs). Medications like omeprazole and esomeprazole reduce gastric acid, which is needed to release B12 from food proteins. Long-term PPI use is associated with B12 depletion.

People with gastrointestinal conditions. Coeliac disease, Crohn's disease, atrophic gastritis, pernicious anaemia (an autoimmune condition affecting intrinsic factor production), and prior gastric surgery all impair B12 absorption.

People with pernicious anaemia. An autoimmune condition where the body attacks the cells that produce intrinsic factor, a protein needed for B12 absorption in the small intestine. This is the most common cause of severe B12 deficiency in older adults and requires lifelong treatment.

The symptoms (and why they're easy to miss)

B12 deficiency develops slowly. Stores in the liver can last 3 to 5 years, so depletion is gradual. The symptoms creep in so quietly that most people don't connect them to a nutritional cause.

Haematological symptoms (from impaired red blood cell production)

Fatigue and weakness. Pallor. Shortness of breath on exertion. Lightheadedness.

Neurological symptoms (from myelin degradation)

Tingling or numbness in hands and feet (peripheral neuropathy). Difficulty walking or balance problems. Muscle weakness. Cognitive changes: poor memory, difficulty concentrating, brain fog. In severe cases, confusion, depression, or psychosis.

Other symptoms

Glossitis (swollen, smooth, painful tongue). Mouth ulcers. Mood changes, including irritability and depression. Digestive disturbances.

The overlap with other conditions is enormous. Fatigue, brain fog, mood changes, weakness. These could be iron deficiency, thyroid dysfunction, depression, stress, or a dozen other things. That's why B12 often gets checked late in the investigation, or not at all, unless the clinician specifically considers it.

Tom's tingling feet were the clue his GP took most seriously. Neurological symptoms in the context of suspected B12 deficiency are red flags that prompt faster action.

The irreversibility problem

I want to emphasise this because it's what separates B12 deficiency from most other nutritional deficiencies.

Iron deficiency? Supplement, and you recover. Vitamin D? Supplement, and you recover. Thyroid dysfunction? Treat, and symptoms resolve.

B12 neurological damage can be permanent.

The myelin sheath, once degraded beyond a certain point, may not fully regenerate. Studies have shown that if neurological symptoms from B12 deficiency have been present for more than six months before treatment begins, the chance of full recovery decreases significantly. Some patients retain residual neurological deficits permanently.

This is why early detection matters. This is why risk groups should be tested regularly. And this is why the MJA (Medical Journal of Australia) has flagged that high folate intake in vegans can mask B12 deficiency, hiding the haematological signs (anaemia) while neurological damage progresses silently in the background.

How the test works

The standard B12 test measures total serum vitamin B12, the combined amount of B12 bound to carrier proteins in your blood.

No fasting required. B12 levels aren't affected by recent meals.

No specific timing required. Unlike cortisol or testosterone, B12 doesn't follow a circadian pattern.

Stop B12 supplements for 7 days before testing if you want an accurate baseline. Taking supplements the day before will artificially inflate your result.

One limitation to know about: Total serum B12 measures both the active and inactive forms of the vitamin. A "normal" total B12 doesn't always rule out functional deficiency at the tissue level. If your total B12 is borderline (150 to 250 pmol/L) and you have symptoms, your clinician may consider additional tests, including methylmalonic acid (MMA) and homocysteine, which are more sensitive markers of functional B12 status.

Understanding your result

Result	What it means
Below 150 pmol/L	Deficient (WHO threshold). Treatment should be initiated. If neurological symptoms are present, intramuscular injections are typically preferred over oral supplements for faster repletion.
150–250 pmol/L	Grey zone. May represent early depletion or suboptimal status, especially if symptoms are present. Some clinicians use higher thresholds (220–300 pmol/L) for clinical decision-making, particularly with neurological symptoms.
Above 250 pmol/L	Generally adequate.
Above 600 pmol/L (without supplementation)	Can occasionally indicate liver disease, myeloproliferative disorders, or other conditions. Worth mentioning to your GP if unexpected.

The folate masking effect

This deserves its own section because it catches people off guard.

B12 and folate work together in DNA synthesis and red blood cell production. When B12 is low, the body can use folate as a partial workaround for red blood cell production. That means high folate intake can prevent the anaemia that would otherwise signal B12 deficiency.

This is particularly relevant for vegans, who typically have high folate intake from plant foods. Their folate levels are often excellent, which means their FBC and haemoglobin can look completely normal even while their B12 is critically low and neurological damage is progressing.

The MJA has specifically flagged this: "Subtle neurological damage may be more likely in vegans because of their increased folate levels preventing early detection of vitamin B12 deficiency."

In practical terms: if you're vegan or eating a high-folate diet, don't rely on your FBC to catch B12 deficiency. Test B12 directly. The haematological warning signs you'd expect may not appear until the deficiency is advanced.

Treatment: what happens when you're low

For dietary insufficiency (vegans, vegetarians, older adults with inadequate intake)

High-dose oral B12 supplements (typically 1,000 micrograms per day of cyanocobalamin or methylcobalamin) are effective for most people with dietary insufficiency. Absorption through passive diffusion bypasses the intrinsic factor pathway at high doses.

For absorption-related deficiency (pernicious anaemia, GI conditions, post-gastric surgery)

Intramuscular B12 injections are typically required, as oral absorption may be unreliable. The standard loading protocol usually involves several injections over the first few weeks, followed by maintenance injections every 1 to 3 months depending on the cause and response.

For neurological symptoms regardless of cause

Intramuscular injection is generally preferred initially because of faster tissue repletion. Early treatment matters. As discussed above, neurological outcomes are significantly better when treatment begins promptly.

Response timeline

Fatigue and haematological symptoms usually improve within 2 to 4 weeks. Neurological symptoms may take 3 to 6 months to improve, with some residual deficits possible in longstanding deficiency. Retest B12 at 3 months to confirm repletion.

Ongoing management: If the cause is permanent (pernicious anaemia, veganism without intent to change diet), lifelong supplementation or injections are needed. If the cause is reversible (dietary change, medication adjustment), periodic monitoring is still recommended.

Tests to consider through Bloody Good

Tests to order alongside B12

Folate Blood Test — B12 and folate should be tested together. Deficiency of either produces similar haematological changes, and high folate can mask B12 deficiency.

Full Blood Count (FBC) — to check for megaloblastic anaemia (large red blood cells and elevated MCV).

Iron Studies (Including Ferritin) — iron deficiency and B12 deficiency frequently coexist, especially in vegans and vegetarians.

Homocysteine Blood Test — elevated homocysteine can indicate functional B12 deficiency and carries cardiovascular risk implications.

For vegans and vegetarians, also consider

Vitamin D (25-OH) — commonly low in plant-based diets.

Zinc Blood Test — zinc is harder to obtain from plant sources.

If you want broad coverage

The Bloody Good Test covers 100 biomarkers including B12, folate, FBC, iron studies, vitamin D, and more. For vegans and vegetarians, this plus a standalone zinc test provides thorough coverage.

When to retest

Explore more biomarkers

If you want to go deeper into any of the markers covered here, the Bloody Good biomarker directory has detailed pages on what each test measures and how to think about results in general terms.

Browse the Bloody Good Biomarker Directory