In this article
- The scope of the problem
- What neurology says about B12 and nerves
- Normal is not optimal
- What B12 actually does
- Symptoms that suggest deficiency
- Who has higher risk?
- Understanding test results
- MMA and active B12: the tests that catch what serum B12 misses
- What if you are deficient?
- Australian context: cost, access, and current advice
- Bottom line
- Where to from here
MMA: the B12 test standard panels miss, your nerves don't
Serum B12 can read "normal" while methylmalonic acid (MMA) and active B12 (holotranscobalamin) reveal a functional deficiency that is already damaging peripheral nerves. AAN, European Academy of Neurology, and NICE 2024 evidence all converge on the same message: when nerves are involved, do not stop at total B12.
52%
of vegans are B12 deficient—and neurological damage can occur even at 'normal' levels
Source: [1]
The Short Answer
The evidence increasingly supports broader testing when nerve symptoms are present. Total serum B12 misses functional deficiency in the 150-300 pmol/L grey zone, and an American Academy of Neurology 2024 abstract reported elevated methylmalonic acid (MMA) with about 99.8% sensitivity for B12-deficiency neuropathy, even when serum B12 looks normal.[9] A 2021 European Journal of Neurology meta-analysis of nine studies (827 patients with neuropathy vs 1492 without) found MMA was the metabolic marker most consistently raised in the neuropathy group.[10] NICE Guideline NG239 (2024) now accepts either total B12 or active B12 (holotranscobalamin) as the first-line test and adds MMA when results are indeterminate or symptoms persist.[11] If you have peripheral neuropathy (tingling, numbness, balance loss) or you are vegan, vegetarian, over 60, on long-term metformin or a PPI, MMA plus active B12 will catch what serum B12 alone misses.[1][2][9][10][11]
The Scope of the Problem
B12 deficiency is more common than most people realise. The data is stark:
52%
of vegans are B12 deficient (<156 pmol/L) compared to just 1% of omnivores[1]
20-25%
of Australians over 60 may be B12 deficient or have suboptimal levels[3]
3-5 years
Your liver stores this much B12, so deficiency develops slowly and is often missed until symptoms become significant
B12 is unique among vitamins. Your body can store years worth in the liver, so deficiency develops slowly and is often too gradual to notice until symptoms become significant. Many people attribute symptoms to just getting older rather than recognising them as deficiency.
What Neurology Says About B12 and Nerves
AAN, European Academy of Neurology and NICE 2024 Converge
The neurology evidence base on B12 deficiency and peripheral neuropathy now points in one direction: when nerves are involved, total serum B12 is not enough.
- American Academy of Neurology (AAN), 2009 PN evaluation guideline (still current): routine evaluation of distal symmetric peripheral neuropathy should include B12 testing, alongside fasting glucose and serum protein electrophoresis. Despite this, real-world audits show many clinicians do not consistently order it.
- AAN 2024 abstract "Provider B-ware" (Neurology): methylmalonic acid (MMA) is presented as the most reliable marker for B12-deficiency neuropathy, with a reported 99.8% sensitivity for B12 deficiency in this context. Authors flag that patients with neurological symptoms but only borderline-low serum B12 frequently have elevated MMA, i.e. genuine functional deficiency that needs treating.[9]
- European Journal of Neurology, Stein et al. 2021 (published by the European Academy of Neurology member journal): a systematic review and meta-analysis of nine studies (827 patients with neuropathy vs 1492 without) found that elevated MMA was the B-vitamin marker most consistently associated with neuropathy. Folate and B6 were less consistent. Total serum B12 alone was a weaker discriminator than MMA.[10]
- NICE Guideline NG239 (2024): either total B12 or active B12 (holotranscobalamin) can be used as the first-line test for suspected B12 deficiency. If the result is indeterminate or symptoms persist, measure MMA. The guideline explicitly accepts that serum B12 alone misses functional deficiency.[11]
One caveat the neurology literature is also clear about: a small subset of patients with B12-responsive neuropathy have normal MMA and only mildly low serum B12. MMA is the most sensitive single marker, but it is not infallible. Where neurology suspicion is high, a B12 treatment trial may be appropriate even if biochemistry is borderline.[9]
Normal Is Not Optimal: The 2025 UCSF Finding
Beyond peripheral neuropathy, a 2025 UCSF study challenged the fundamental assumption that normal serum B12 levels mean healthy B12 status:
Brain Changes at "Normal" B12
Older adults with B12 levels averaging 414 pmol/L (well above the deficiency cutoff of 148 pmol/L and firmly in the "normal" range) still showed measurable signs of neurological and cognitive change:[2]
- Slower processing speed on cognitive tests
- Significant delays responding to visual stimuli
- Higher volume of white matter lesions on MRI scans (lesions associated with cognitive decline, dementia risk, and stroke)
This is consistent with the neurology view that the lower reference limit for serum B12 is set too low and that functional deficiency exists across a much broader band than the laboratory range suggests.
Japan sets its lower B12 limit at 500-550 pmol/L, nearly 4 times higher than many Western laboratories.[4] Some researchers argue this better reflects truly optimal status, particularly for brain and nerve health.
What B12 Actually Does
Vitamin B12 (cobalamin) is essential for fundamental processes throughout your body:
- Nerve function: B12 maintains the protective myelin sheath around nerves. Deficiency causes demyelination, which strips the insulation from peripheral and central nerves. This explains tingling, numbness, balance loss, and (in advanced cases) subacute combined degeneration of the spinal cord.
- DNA synthesis: Required for cell division throughout the body, affecting red blood cells, gut lining, and skin.
- Red blood cell formation: Deficiency leads to megaloblastic anaemia (large, immature red cells that do not carry oxygen effectively). This explains fatigue and breathlessness.
- Energy metabolism: Converts food into cellular energy via mitochondrial pathways.
- Cognitive function: Essential for neurotransmitter production and brain health, which is why the UCSF study saw white matter changes at apparently normal B12.
Symptoms That Suggest Deficiency
B12 deficiency symptoms often overlap with other conditions or are dismissed as normal ageing. This is precisely why testing can be valuable, it provides objective data when symptoms are vague.
Neurological (priority for testing)
- Tingling or numbness in hands and feet
- Burning or pins-and-needles peripheral neuropathy
- Balance problems, unsteady gait
- Difficulty walking or proprioception loss
- Memory problems, brain fog, cognitive decline
Energy and Mood
- Persistent fatigue
- Weakness
- Depression or mood changes
- Difficulty concentrating
Physical
- Pale or yellowish skin
- Sore, red tongue (glossitis)
- Mouth ulcers
- Breathlessness on exertion
Why Early Detection Matters for Nerves
Neurological damage from prolonged B12 deficiency may not be fully reversible. Myelin can regenerate with treatment, but if demyelination has been severe or prolonged, some damage becomes permanent. This is why the AAN evaluation pathway prioritises B12 measurement in peripheral neuropathy, and why functional markers (MMA, active B12) matter when serum B12 is in the grey zone.[2][9]
Early detection and treatment offer the best chance of full recovery.
Who Has Higher Risk?
Anyone can develop B12 deficiency, but certain groups have significantly elevated risk. If you fit multiple categories, testing (and adding MMA or active B12 if nerve symptoms are present) becomes more compelling:
| Risk Factor | Why It Matters |
|---|---|
| B12 is only naturally present in animal products. With 52% of vegans deficient, supplementation is essential. Regular testing confirms adequacy. | |
| Dairy and eggs contain B12, but 50-70% of vegetarians across multiple studies have suboptimal levels. Do not assume you are getting enough. | |
| Any diet limiting animal products increases risk, including many traditional plant-forward diets. |
| Risk Factor | Why It Matters |
|---|---|
| Stomach acid decreases with age, reducing B12 absorption from food. Affects 20-25% of older Australians. Supplements bypass this issue. | |
| Autoimmune destruction of intrinsic factor (required for B12 absorption). Requires lifelong B12 injections; oral supplements alone do not work. | |
| Removes cells producing intrinsic factor. Annual B12 monitoring is standard post-operative care. | |
| Intestinal damage impairs absorption in the terminal ileum where B12 is absorbed. Regular monitoring is recommended. |
| Risk Factor | Why It Matters |
|---|---|
| Reduces B12 absorption. 10-30% of long-term metformin users develop deficiency, and metformin-associated B12 deficiency is a well-recognised cause of peripheral neuropathy in type 2 diabetes. Periodic B12 (and MMA if symptoms) is reasonable.[7] | |
| Long-term omeprazole, pantoprazole, esomeprazole, and similar reduce stomach acid needed to release B12 from food. Risk increases with duration of use. | |
| Famotidine and similar medications have the same acid-reducing effect as PPIs, typically less pronounced. | |
| Inactivates B12 by oxidising cobalt. Acute and recurrent N2O exposure is a recognised cause of subacute B12-deficiency myeloneuropathy even with normal serum B12. |
Understanding B12 Test Results
Serum B12 sounds straightforward, but interpretation has nuances that most people, and many GPs, do not fully appreciate. The neurology and laboratory-medicine evidence is consistent: in the grey zone, add a functional marker.
Generally considered deficient. Supplementation typically recommended.
Where MMA and active B12 add the most information. AAN and NICE pathways recommend functional markers here, especially with neurological symptoms.[9][11]
Considered adequate by Western reference ranges, but the UCSF 2025 study saw brain changes at average 414 pmol/L. Consider MMA or active B12 if symptoms persist.[2]
Japan lower limit. May better represent optimal B12 status for brain and nerve health.[4]
MMA and Active B12: The Tests That Catch What Serum B12 Misses
Standard serum B12 measures total B12. Only about 20% of that total is the active form (holotranscobalamin, or active B12) that cells actually use. This means serum B12 can read normal while functional deficiency is already in progress, which is exactly what the UCSF study and the AAN and European Academy of Neurology evidence describe.
What Each Marker Tells You
Total serum B12: cheap, widely available, but combines active and inactive fractions. Misses functional deficiency in the grey zone.
Active B12 (holotranscobalamin / Holo-TC): the fraction bound to transcobalamin that cells can use. Reported to be more sensitive than total B12 for early deficiency, particularly in neuropsychiatric contexts.[11] NICE 2024 accepts active B12 as a first-line test in place of total B12.[11]
Methylmalonic acid (MMA): a metabolic by-product that accumulates when B12 is functionally deficient at the cellular level, even if serum B12 reads normal. The AAN 2024 abstract reports about 99.8% sensitivity for B12 deficiency in the neuropathy context, and the 2021 Stein meta-analysis (European Journal of Neurology) found MMA to be the most consistently elevated B-vitamin marker in patients with neuropathy across 9 studies.[9][10]
Homocysteine: rises in both B12 and folate deficiency. Useful in combination with MMA. Elevated MMA plus elevated homocysteine = strong evidence of functional B12 deficiency. Elevated homocysteine with normal MMA points more to folate deficiency.[5]
MMA Reference Ranges (General Guide)
MMA is typically reported in nmol/L:
- <270 nmol/L: generally normal
- 270-400 nmol/L: borderline. Interpret in context of serum B12 and symptoms.
- >400 nmol/L: suggests functional B12 deficiency at the cellular level. Stronger evidence the higher the value.
Reference ranges vary slightly by laboratory. Renal impairment can also raise MMA independently of B12 status, so MMA should be interpreted alongside kidney function.
The Folate Trap
High folate intake (common with fortified foods and supplements) can mask the blood changes of B12 deficiency (megaloblastic anaemia) while neurological damage continues silently. Since mandatory folic acid fortification of bread began in Australia in 2009, this interaction has become more clinically relevant.[6]
This is one of the strongest arguments for measuring MMA and active B12 alongside total B12 in anyone with neurological symptoms.
What If You Are Deficient?
This section summarises what published clinical sources describe about treatment of confirmed B12 deficiency. Specific treatment decisions are made by your health practitioner. The Australian Prescriber 2026 discussion (Forsyth, haematologist) outlines the scenarios where intramuscular replacement is preferred over oral, and the scenarios where oral replacement is generally appropriate:[12]
- Intramuscular B12 is the preferred route described by Forsyth for: subacute combined degeneration, neurological involvement (including peripheral neuropathy), cognitive change or psychosis, megaloblastic anaemia with pancytopenia, and irreversible causes such as pernicious anaemia, gastrectomy, or terminal ileal disease. Typical maintenance after loading doses is described as 3-monthly. Once intramuscular replacement is established, ongoing retesting is generally not required.[12]
- Oral B12 (typical dose described in clinical sources is 1000-2000 micrograms daily) is described as appropriate for reversible causes such as dietary deficiency or some coeliac patients on gluten-free diets. About 1% of oral B12 is absorbed passively regardless of intrinsic factor.
- Metformin users: Oral supplementation is generally described as sufficient in the clinical literature. Some clinicians describe routine supplementation rather than waiting for deficiency to develop.
- Follow-up on oral therapy: Forsyth describes symptom review at 2-3 months and retesting at 3-6 months if adherence is uncertain or symptoms persist. MMA and homocysteine are described as normalising by 3-6 months on adequate replacement.[12]
- Neuropathic symptoms: Clinical sources describe earlier and higher-intensity replacement, with neurological recovery typically slower than haematological recovery (6-12 weeks before patients notice improvement) and not always fully reversible if damage was prolonged.[9][10][12]
Forsyth summarises the pragmatic position: in indeterminate cases, "it is better to treat these patients than to miss B12 deficiency, because B12 replacement isn't particularly expensive and it isn't dangerous."[12]
This is educational information. Treatment decisions should be made with your health practitioner.
Australian Context: Cost, Access, and Current Advice
What Australian Prescriber (2026) Says
The most recent Australian Prescriber guidance (Episode 215, 26 May 2026, host David Liew with Dr Cecily Forsyth, haematologist) lays out the AU-specific picture:[12]
- Total serum B12 above ~250 pmol/L makes deficiency unlikely. Below that, results fall into clearly low, indeterminate (grey zone), or unlikely categories.
- Active B12 (holotranscobalamin) is particularly valuable in pregnancy and in oral contraceptive users, because oestrogen suppresses haptocorrin and can distort total B12 results. It is standard in many pregnancy panels.
- Cost barrier in Australia: active B12 testing carries an out-of-pocket cost while the Medicare rebate sits well below the test cost. This is the main reason it is not routine in Australian primary care.
- MMA is more specific but is slower, more expensive, and not universally available. Useful in indeterminate cases but constrained by access.
- Homocysteine is often run automatically alongside B12 but is non-specific (renal disease, hypothyroidism, ageing, and inflammation all raise it).
- Medicare cadence: B12 rebate is generally once every 11 months.
- Neurologists frequently see neuropathy without blood-count abnormality, consistent with the AAN and European Academy of Neurology evidence that elevated MMA can be present in B12-related nerve disease while serum B12 and FBC look fine.[9][10][12]
- Pregabalin paradox: pregabalin is used to treat neuropathy but can itself contribute to B12 deficiency. Worth checking B12 status in patients on long-term pregabalin with persistent or progressive symptoms.[12]
The Bottom Line
When nerve symptoms are involved, the published evidence describes total serum B12 as insufficient on its own. The American Academy of Neurology, the European Academy of Neurology member journal, NICE 2024, and the 2026 Australian Prescriber Podcast (Forsyth) all describe methylmalonic acid (MMA) as a more sensitive functional marker, and active B12 (holotranscobalamin) as a more sensitive first-line option than total B12 when total B12 sits in the grey zone or when peripheral neuropathy is suspected.[9][10][11][12]
Total serum B12 can miss functional deficiency. MMA is described as identifying it at the cellular level with about 99.8% sensitivity in the neuropathy context, and active B12 is described as identifying it earlier than total B12.[9][11] The UCSF 2025 study suggests this gap matters for the brain as well as the nerves.[2]
People who are vegan, vegetarian, over 60, on long-term metformin or a PPI, or who experience tingling, numbness, balance problems, or unexplained cognitive change may want to discuss the broader panel (serum B12 plus MMA plus, where available, active B12) with their health practitioner.
The barrier in Australia is not the science. It is awareness and cost: active B12 carries an out-of-pocket cost against a much lower Medicare rebate, and MMA is less accessible again.[12] Most GPs are still trained on total B12 alone. The 2024 NICE position, the AAN evidence, and the 2026 Australian Prescriber discussion together describe a reasonable basis for raising the broader panel with a health practitioner, particularly where nerve symptoms are present.
This article is for general education. Testing and treatment decisions are made with your health practitioner.
Where to from here
If you would like your active B12 and MMA tested, Clarity Labs offers them as part of our Functional B12 panel and as individual add-ons. If not, we hope this gave you something evidence-based to discuss with your regular doctor.
Frequently Asked Questions
Disclaimer:This information is educational only and not medical advice. Results should be interpreted by your health practitioner in the context of your symptoms and health history. Treatment decisions should be made with your doctor or specialist.