- Vitamin B12 is essential for nerve function, red blood cell production, and DNA synthesis; deficiency affects multiple body systems
- Plant-based diets, older age, certain medications (particularly metformin), and conditions affecting stomach acid all increase deficiency risk
- The body stores several years' worth of B12; meaning deficiency develops slowly and symptoms often appear well after the problem began
- Neurological symptoms from B12 deficiency (tingling, numbness, balance problems) can be partially or fully irreversible if deficiency is prolonged
- A blood test is the only reliable way to assess B12 status; symptoms alone are insufficient and overlap with many other conditions
What does vitamin B12 do?
Vitamin B12 (cobalamin) is a water-soluble vitamin involved in some of the body's most fundamental processes:
- Red blood cell production: B12 is essential for the production of healthy red blood cells. Deficiency causes large, misshapen red blood cells that cannot carry oxygen efficiently (megaloblastic anaemia)
- Nerve function and myelin maintenance: B12 is required for the maintenance of the myelin sheath, the protective coating around nerve fibres. Deficiency disrupts nerve signalling and can cause neurological symptoms that, if left untreated, may be irreversible
- DNA synthesis: B12 is a co-factor in DNA replication, affecting rapidly dividing cells throughout the body
- Homocysteine metabolism: B12 (alongside folate and B6) converts homocysteine to methionine. Elevated homocysteine is associated with cardiovascular risk
Unlike most water-soluble vitamins that the body cannot store, the liver can store several years' worth of B12 reserves. This is why deficiency typically takes 2–5 years to develop after dietary intake or absorption becomes insufficient, and why many people are significantly depleted before symptoms appear.
Symptoms of vitamin B12 deficiency
B12 deficiency can present across multiple body systems. Symptoms are often non-specific and easy to attribute to other causes:
Haematological (blood-related):
- Fatigue and weakness
- Pallor or slightly yellowish skin tone
- Breathlessness with exertion
Neurological:
- Tingling or numbness in hands and feet ("pins and needles")
- Balance and coordination problems
- Difficulty walking
- Muscle weakness
Cognitive and mood:
- Brain fog and difficulty concentrating
- Memory problems
- Mood changes, low mood, or irritability
Other:
- Glossitis (sore, swollen, smooth tongue)
- Mouth ulcers
The neurological symptoms are the most concerning. Unlike the haematological effects of deficiency (which typically resolve with treatment), neurological damage from prolonged B12 deficiency can be only partially reversible. Early detection matters.
Overlap with other conditions: Fatigue, brain fog, and mood changes overlap with dozens of other conditions, including iron deficiency, thyroid dysfunction, vitamin D deficiency, sleep disorders, and more. This is why a blood test panel covering multiple markers simultaneously is more informative than testing a single marker based on a symptom.
Who is most at risk?
Several groups face significantly higher risk of B12 deficiency:
People following vegan or strict vegetarian diets B12 is found almost exclusively in animal products: meat, fish, dairy, and eggs. Plant foods do not provide bioavailable B12. Without supplementation or consistent consumption of fortified foods, vegans will develop deficiency given the years-long timeline described above.[2]
Older adults As people age, stomach acid production typically decreases. Stomach acid is essential for releasing B12 from food proteins so it can be absorbed. Many older adults absorb significantly less dietary B12 than they consume. Australian research suggests deficiency rates are meaningfully higher in people over 65.
People taking metformin Metformin (the most commonly prescribed medication for type 2 diabetes in Australia) is associated with reduced B12 absorption in the gut. Long-term metformin use can lead to B12 depletion, and B12 testing is recommended for anyone on long-term metformin therapy.
People with pernicious anaemia or gastric conditions Pernicious anaemia is an autoimmune condition that destroys the stomach cells responsible for producing intrinsic factor, a protein essential for B12 absorption. Without intrinsic factor, dietary B12 cannot be absorbed regardless of intake. People with this condition require B12 injections rather than oral supplementation.
Other gastric conditions (gastric surgery, long-term use of proton pump inhibitors, and Helicobacter pylori infection) can also impair B12 absorption.
How is B12 measured?
B12 status is measured via a standard blood test reporting serum cobalamin levels, measured in pmol/L (picomoles per litre) in Australia.
Reference ranges (approximate):
- Below 148 pmol/L: deficient; treatment is indicated
- 148–221 pmol/L: low-normal; may be subclinical deficiency, particularly with symptoms present
- Above 221 pmol/L: within the normal range
The "grey zone" between 148 and 221 pmol/L is clinically meaningful. Many people with results in this range, particularly those with neurological symptoms, risk factors, or a plant-based diet, benefit from further assessment. In some cases, additional markers (methylmalonic acid, homocysteine) may be tested to confirm functional deficiency.[1]
Vitamin B12 is not part of a standard full blood count. It must be specifically requested in a blood panel. A standard annual health check typically does not include B12 unless specifically added.
How is B12 deficiency treated?
Treatment depends on the cause and severity of deficiency:
Oral supplementation: appropriate for dietary deficiency (e.g., vegan diet) where absorption is intact. High-dose oral B12 can be effective even in some absorption-related conditions because passive diffusion absorbs a small fraction of oral B12 without intrinsic factor.
Intramuscular injections: required for pernicious anaemia and significant absorption disorders where oral supplementation cannot raise levels adequately. Injections bypass the gut entirely.
Dietary changes: increasing intake of animal products (meat, fish, eggs, dairy) for those whose deficiency is primarily dietary.
Managing the underlying cause: addressing H. pylori infection, reviewing PPI use, or adjusting metformin dose in consultation with a prescribing doctor.
The appropriate treatment should be determined by a healthcare professional based on the severity of deficiency, the underlying cause, and your individual circumstances.
B12 and methylmalonic acid: confirming functional deficiency
In some cases, serum B12 can appear borderline or low-normal while functional deficiency is already present at the cellular level. Methylmalonic acid (MMA) accumulates when B12 is functionally insufficient and can be measured in blood or urine to confirm deficiency in ambiguous cases.
If your B12 is in the grey zone and symptoms are present, ask your GP whether MMA testing is appropriate.
FAQ
What are the symptoms of vitamin B12 deficiency?
Symptoms include fatigue, weakness, brain fog, tingling or numbness in the hands and feet, mood changes, and pale or yellowish skin. Neurological symptoms, particularly tingling and numbness, can persist even after treatment if deficiency has been prolonged.
Who is most at risk of vitamin B12 deficiency in Australia?
Those at highest risk include people following vegan or strictly plant-based diets, older adults, people taking metformin, anyone with gastric surgery or conditions affecting stomach acid production, and those with pernicious anaemia.
Is vitamin B12 testing included in a standard blood test?
Not automatically. B12 is not part of a standard full blood count and must be specifically requested. It is included as standard in the Honed Health Essential Health Panel.
Can I get enough B12 from a vegan diet?
Not without supplementation or consistent use of B12-fortified foods. Vitamin B12 is found almost exclusively in animal products. Vegans should supplement B12 and test their levels periodically.
How quickly does B12 deficiency develop?
Slowly. The body stores several years' worth of B12 in the liver. Deficiency typically takes 2 to 5 years to develop after dietary intake or absorption becomes insufficient. This delay means symptoms often appear well after the underlying problem began.
