Vitamin B12
Vitamin B12 (cobalamin) is required for DNA synthesis, red blood cell production, and nervous system function. Deficiency causes macrocytic anemia, fatigue, glossitis, peripheral neuropathy, and cognitive changes, and is common in older adults, vegans, and people taking metformin or proton pump inhibitors. Serum B12 may miss mild deficiency, so MMA and homocysteine add value.
Units: pg/mL; some labs report pmol/L (pmol/L = pg/mL × 0.738). Check the units on your report before comparing to any range here.
Optimal range: who says what
Institutions, researchers and practitioners draw the line in different places. Each position is listed with who holds it and, where a document states it, a link. How we verify ranges.
- Institution or study≥300 pmol/L (about 400 pg/mL)Smith, Warren & Refsum 2018 review (Adv Food Nutr Res)The authors call serum B12 below 300 pmol/L suboptimal and note there is no sharp cutoff for deficiency
- Institution or studyNo reliable single cutoff; >148 pmol/L (200 pg/mL) misclassifiesNHANES analysis (Bailey et al. 2011, Am J Clin Nutr)Common B12 and methylmalonic acid cutoffs gave different prevalence estimates; the authors classed clear deficiency as low B12 together with elevated MMA
- Institution or studySubnormal below 200-250 pg/mLNIH Office of Dietary Supplements fact sheetODS calls methylmalonic acid the most sensitive marker and uses it to confirm deficiency when serum B12 is 150-399 pg/mL
- Named practitioner>400 pg/mLLife Extension · Lab Testing Protocol (Suggested Optimal Ranges)Disclosure: Life Extension sells lab panels that appear in our blood-test comparisons.
- Lab reference interval299-1054 pg/mL (MedlinePlus)What a lab report flags against. A lab interval is not an optimal target.
Practitioner targets are individual positions, often tighter than guideline ranges. Each was checked against its article before publishing.
16 of 21 itemized panels
Vitamins & Nutrients
What is Vitamin B12?
Vitamin B12 (cobalamin) is a water-soluble vitamin essential for DNA synthesis, red blood cell formation, neurological function, and the methylation cycle. Unlike most vitamins, B12 is found almost exclusively in animal products (meat, fish, eggs, dairy), making vegetarians and vegans particularly susceptible to deficiency. B12 is absorbed in the small intestine with the help of intrinsic factor (a protein produced by stomach parietal cells), and deficiency can result from either dietary insufficiency or malabsorption.
B12 deficiency is insidious and often misdiagnosed. It can take 5-10 years to develop after B12 intake stops, because the liver stores 2-5 mg (enough for 3-5 years). Early symptoms—fatigue, brain fog, mood changes—are nonspecific and often attributed to aging or stress. But if left untreated, B12 deficiency causes irreversible neurological damage: peripheral neuropathy, cognitive decline, dementia, and spinal cord degeneration (subacute combined degeneration). Nerve damage can appear before anemia does, and some patients with neurologic symptoms have only moderately low serum B12 (Healton 1991).
Serum B12 measures total B12 in blood, but it's not always reliable. Some people have normal serum B12 but functional deficiency at the cellular level. Methylmalonic acid (MMA) and homocysteine are more sensitive functional markers: they rise when B12 is insufficient for cellular metabolism. If serum B12 is borderline (200-400 pg/mL) and you have symptoms, check MMA and homocysteine to confirm deficiency.
Why Vitamin B12 Matters for Longevity
- Neurological health: B12 is essential for myelin sheath formation around nerves. Deficiency causes irreversible nerve damage, peripheral neuropathy, cognitive decline, dementia.
- DNA synthesis and cell division: B12 is required for synthesis of nucleotides. Deficiency impairs rapidly dividing cells (bone marrow, gut lining) causing megaloblastic anemia.
- Methylation cycle: B12 (as methylcobalamin) is a cofactor for methionine synthase, converting homocysteine→methionine. Deficiency raises homocysteine, a risk factor for cardiovascular disease, stroke, Alzheimer's.
- Red blood cell formation: B12 deficiency causes macrocytic anemia (large, immature red blood cells) leading to fatigue, weakness, shortness of breath.
- Energy production: B12 (as adenosylcobalamin) is required for conversion of methylmalonyl-CoA to succinyl-CoA in the Krebs cycle. Deficiency impairs mitochondrial energy production.
- Mood and mental health: B12 supports neurotransmitter synthesis. Deficiency linked to depression, anxiety, cognitive decline, and increased dementia risk.
Interpretation bands (curated, PMID-backed)
Holder: OptimizeBiomarkers editorial synthesis of: The origin of vitamin B12 levels and risk of all-cause, cardiovascular and cancer specific mortality: A systematic review and dose-response meta-analysis (Liu et al., 2024); Association between serum vitamin B12 and risk of all-cause mortality in elderly adults: a prospective cohort study (Xu et al., 2021); Nonlinear associations of serum cobalamin with risk of all-cause and cardiovascular mortality in hypertensive adults (Zhu et al., 2023). No single paper sets these cutoffs; for attributed targets see who says what.
Optimal (Longevity)500-1000 pg/mL▼
- Target range for optimal brain health, energy, and methylation
- Many functional medicine doctors target >600 pg/mL
Adequate (Standard)400-500 pg/mL▼
Meets standard guidelines but may be suboptimal for some individuals, especially if symptoms present.
Suboptimal200-400 pg/mL▼
- Low-normal range
- May have subclinical deficiency with elevated MMA/homocysteine
- Consider supplementation if symptomatic
Deficient<200 pg/mL▼
- Frank deficiency
- High risk of anemia, neuropathy, cognitive impairment
- Requires immediate B12 replacement (sublingual or injection)
- The origin of vitamin B12 levels and risk of all-cause, cardiovascular and cancer specific mortality: A systematic review and dose-response meta-analysis.
- Association between serum vitamin B12 and risk of all-cause mortality in elderly adults: a prospective cohort study.
- Nonlinear associations of serum cobalamin with risk of all-cause and cardiovascular mortality in hypertensive adults.
What raises or lowers Vitamin B12
Causes of high Vitamin B12
- Liver disease: Hepatitis, cirrhosis (impaired B12 clearance).
- Kidney disease: Chronic kidney failure (reduced excretion).
- Myeloproliferative disorders: Leukemia, polycythemia vera (increased B12-binding proteins).
- Excessive B12 supplementation: >1000 mcg/day (generally harmless, water-soluble).
Causes of low Vitamin B12
- Dietary insufficiency: Vegetarian/vegan diet without B12 supplementation. No reliable plant sources of B12.
- Pernicious anemia: Autoimmune destruction of gastric parietal cells → no intrinsic factor → B12 malabsorption. Most common in elderly, Northern Europeans.
- Atrophic gastritis: Age-related or H. pylori-induced stomach atrophy reduces acid and intrinsic factor. Common in elderly.
- Medications: Metformin (long-term use lowers B12), PPIs (omeprazole, pantoprazole), H2 blockers (ranitidine).
- Malabsorption: Celiac disease, Crohn's disease, gastric bypass, pancreatic insufficiency, bacterial overgrowth (SIBO).
- Alcohol abuse: Damages stomach lining, impairs absorption.
- Aging: Reduced stomach acid and intrinsic factor production after age 50.
- Nitrous oxide exposure: Inactivates B12 (anesthesia, recreational use).
Symptoms when Vitamin B12 is high or low
When high
High B12 (>1000 pg/mL) from supplementation is generally harmless. Very high B12 (>2000 pg/mL) without supplementation may indicate:
- Liver disease (impaired B12 clearance)
- Kidney disease (reduced excretion)
- Myeloproliferative disorders (leukemia, polycythemia vera)
- Rarely, excessive supplementation
Very high unsupplemented B12 should prompt evaluation for underlying disease, but is not directly harmful.
When low
- Fatigue, weakness, low energy
- Shortness of breath, palpitations (anemia)
- Pale or jaundiced skin
- Glossitis (sore, red, smooth tongue)
- Mouth ulcers
- Peripheral neuropathy: Tingling, numbness in hands/feet (starts distally)
- Balance problems, difficulty walking, ataxia
- Memory problems, brain fog, poor concentration
- Depression, mood changes, irritability
- Dementia, cognitive decline (in severe chronic deficiency)
- Vision problems (optic neuropathy, rare)
- Subacute combined degeneration (spinal cord damage, irreversible if untreated)
Deficiency <200 pg/mL causes anemia and neurological symptoms. Levels 200-400 pg/mL may cause subclinical deficiency (check MMA/homocysteine).
How to move Vitamin B12
Effect sizes (how much something moves the marker) appear only where the paper cited under the item states them. Talk to your clinician before starting or changing any medication or supplement.
B12 Supplementation (Dose and Form)
- Methylcobalamin or hydroxocobalamin: Preferred forms. Methylcobalamin is the active form used in methylation; hydroxocobalamin is converted to both methyl- and adenosyl-cobalamin.
- Cyanocobalamin: Synthetic form, most common in supplements. Requires conversion to active forms. Generally effective but methylcobalamin may be superior for neurological issues.
- Sublingual B12: 1000-5000 mcg daily. Bypasses stomach absorption issues. Effective for most people with mild-moderate deficiency.
- B12 injections (IM): 1000 mcg weekly or monthly for severe deficiency, pernicious anemia, or malabsorption. Most effective for rapid repletion.
- Maintenance dose: 500-1000 mcg/day sublingual or 1000 mcg/month injection after repletion.
Identify and Treat Underlying Cause
- Pernicious anemia (autoimmune): Antibodies destroy stomach parietal cells → no intrinsic factor → can't absorb B12. Requires lifelong B12 injections or very high-dose oral (1000-2000 mcg/day can overcome lack of intrinsic factor via passive diffusion). Check anti-intrinsic factor antibodies, anti-parietal cell antibodies.
- Atrophic gastritis: Reduced stomach acid impairs B12 release from food. Common in elderly, H. pylori infection, long-term PPI use. May need B12 supplementation.
- Metformin: Long-term use impairs B12 absorption. Check B12 annually if on metformin; supplement if needed.
- Dietary insufficiency: Vegetarians/vegans must supplement B12 (no reliable plant sources except fortified foods).
- Malabsorption: Celiac disease, Crohn's disease, gastric bypass, pancreatic insufficiency impair B12 absorption.
Optimize Methylation Cofactors
- B12 works synergistically with folate in the methylation cycle. Deficiency of either causes elevated homocysteine.
- Folate (methylfolate): 400-800 mcg/day. Required for methylation cycle. Use methylfolate (5-MTHF) if MTHFR gene variant.
- Vitamin B6: 25-50 mg/day. Cofactor for homocysteine metabolism via transsulfuration pathway.
- Betaine (TMG): 500-2000 mg/day. Alternative methyl donor; can lower homocysteine if B12/folate insufficient.
Address Elevated Homocysteine (if >10 mcmol/L)
- Elevated homocysteine indicates functional B12 or folate deficiency even if serum levels appear normal.
- Homocysteine targets differ by source; the homocysteine page lists who says what. Levels above 15 mcmol/L are associated with higher cardiovascular risk.
- B12 + folate + B6 supplementation effectively lowers homocysteine. Retest in 3 months.
Check MMA if Borderline B12
- Methylmalonic acid (MMA): Most sensitive marker of B12 deficiency. Rises before serum B12 drops.
- Normal MMA: <250 nmol/L. Elevated MMA confirms functional B12 deficiency.
- If serum B12 is 200-400 pg/mL but you have symptoms (fatigue, neuropathy, cognitive issues), check MMA and homocysteine.
When to retest
- Baseline: Check serum B12 if symptoms of deficiency (fatigue, neuropathy, cognitive issues) or risk factors (vegetarian/vegan, elderly, on metformin/PPIs).
- If borderline (200-400 pg/mL): Check MMA and homocysteine to confirm functional deficiency.
- After starting supplementation: Retest in 3 months to ensure adequate repletion. Life Extension targets above 400 pg/mL (see who says what above).
- Annual screening: For vegetarians/vegans, elderly (>60), patients on metformin or PPIs.
- If treating pernicious anemia: Monitor B12 and MMA to ensure adequacy of injection schedule (may need monthly injections lifelong).
- If elevated homocysteine: Retest 3 months after B12/folate/B6 supplementation to confirm normalization.
Scientific Evidence
B12 Deficiency Prevalence
In large US and UK surveys, about 6% of adults aged 60 and older are B12 deficient (plasma B12 below 148 pmol/L, about 200 pg/mL), and close to 20% have marginal status (148-221 pmol/L, about 200-300 pg/mL). Low intake of animal foods is the main cause in younger adults; in older adults, malabsorption of food-bound B12 linked to gastric atrophy predominates.
B Vitamins and Brain Atrophy
In the VITACOG trial, 271 adults over 70 with mild cognitive impairment took folic acid (0.8 mg), B12 (0.5 mg), and B6 (20 mg) daily or placebo for 24 months. Brain atrophy was 0.76% per year with treatment versus 1.08% with placebo. In those with baseline homocysteine above 13 µmol/L, the atrophy rate was 53% lower.
Homocysteine and Cardiovascular Disease
A meta-analysis of genetic (MTHFR) and prospective studies found each 5 µmol/L rise in homocysteine associated with higher odds of ischemic heart disease (1.32 in prospective studies) and stroke (1.59). The authors estimated that lowering homocysteine by 3 µmol/L would cut heart disease risk by 16% and stroke by 24%. These estimates come from observational and genetic data, not treatment trials.
Source: Homocysteine and cardiovascular disease: evidence on causality from a meta-analysis.
Neurologic Effects of B12 Deficiency
In 153 episodes of B12 deficiency affecting the nervous system, paresthesias or ataxia were usually the first symptoms, and the hematocrit was normal in 27% of episodes. All patients improved with B12 treatment and 47% recovered completely; residual disability was tied to how long symptoms had lasted before treatment.
Which Providers Test Vitamin B12?
Cheapest: Empirical Health at $190/year. Most comprehensive: Function Health, 160+ biomarkers for $365/year.
16 panels that include it, cheapest first
| Provider | Annual Cost | Total Biomarkers | $/Biomarker |
|---|---|---|---|
| $190 | 100+ | $1.90 | |
| $297 | 100+ | $2.97 | |
 Superpower | $349 | 100+ | $3.49 |
| $349 | 75+ | $4.65 | |
 WHOOP Advanced Labs | $349 | 65 | $5.37 |
 Function Health | $365 | 160+ | $2.28 |
| $365 | 100+ | $3.65 | |
| $375 | 108 | $3.47 | |
| $449 | 83 | $5.41 | |
| $495 | 80+ | $6.19 | |
| $499 | 100+ | $4.99 | |
| $700 | 138 | $5.07 | |
| $829 | 54 | $15.35 | |
| $895 | 100+ | $8.95 | |
| $996 | 59 | $16.88 | |
| $1950 | 130+ | $15.00 |
$/Biomarker is annual cost divided by the panel's advertised biomarker count, not the price of this one test. We track 23 blood test providers (32 plans); 21 of those plans are itemized marker by marker. Compare all of them.
5 itemized panels do not include Vitamin B12
Labcorp OnDemand · Life Extension Elite · Marek Health Base · Oura Health Panels · Healthspan
11 providers not itemized in our matrix
We have not itemized these marker by marker (some sell tests one at a time), so this page cannot say whether they offer Vitamin B12. Check with them directly.
Life Extension Standard · Quest Health · Ultrahuman Blood Vision · Lifeforce · Hundred Health · Personalabs · Walk-In Lab · HealthLabs · WHOOP Specialized Panels · Marek Health Essential · Vitality Blueprint Upload
Experts discussing Vitamin B12
Podcast episodes where clinicians and researchers cover this marker in depth. Curated by hand; an episode is listed only when the marker is a substantial topic, not a passing mention.
Frequently Asked Questions
What is the optimal range for Vitamin B12?
Published positions differ: ≥300 pmol/L (about 400 pg/mL) (Smith, Warren & Refsum 2018 review (Adv Food Nutr Res)); No reliable single cutoff; >148 pmol/L (200 pg/mL) misclassifies (NHANES analysis (Bailey et al. 2011, Am J Clin Nutr)); Subnormal below 200-250 pg/mL (NIH Office of Dietary Supplements fact sheet). Named practitioners target: >400 pg/mL (Life Extension). Lab reference interval: 299-1054 pg/mL (MedlinePlus). That is what a lab report flags against, not an optimal target.
Which blood test providers include Vitamin B12?
16 of the 21 blood testing panels we itemize marker by marker include Vitamin B12 in their panels. This biomarker is widely available across major providers.
Related: YouBoost rates lifestyle and supplement interventions by evidence.
Medical Disclaimer
This information is for educational purposes only and is not medical advice. Always consult with a qualified healthcare provider about your specific health needs.
Last reviewed Sep 23, 2026 · First published Sep 30, 2025