Vitamin B12
Vitamin essential for nerve function and red blood cells
15 of 22 providers
Vitamins &Nutrients
fatigue
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.
Here's the critical insight: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). Standard reference ranges (>200 pg/mL) are too low;functional medicine practitioners target >400-500 pg/mL to prevent subclinical deficiency.
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.
Optimal vs Standard Ranges
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)
Scientific Evidence
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 (30% of long-term users develop deficiency), 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).
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).
B12 Deficiency Prevalence
B12 deficiency affects 10-15% of adults >60 due to atrophic gastritis and reduced intrinsic factor. Vegetarians/vegans have 50-90% prevalence if not supplementing. Metformin users have 30% risk. Subclinical deficiency (B12 200-400 pg/mL with elevated MMA) is much more common.
Source:Allen LH. How common is vitamin B-12 deficiency? Am J Clin Nutr. 2009;89(2):693S-696S. (PubMed)
B12 and Cognitive Decline
B12 deficiency causes brain atrophy, cognitive decline, and dementia. Observational studies show low B12 (<300 pg/mL) associated with 2-3x higher Alzheimer's risk. B12 supplementation slows brain atrophy by 30-50% in elderly with high homocysteine and prevents progression from MCI to dementia.
B12 and Cardiovascular Disease (via Homocysteine)
B12 deficiency raises homocysteine, which damages endothelium and increases CVD risk. Meta-analyses show homocysteine >10 mcmol/L associated with 50% higher CVD risk. B12/folate/B6 supplementation lowers homocysteine by 25-30% but CVD benefit in RCTs is modest (likely because started too late in disease process).
Source:Wald DS, et al. Homocysteine and cardiovascular disease:evidence on causality from a meta-analysis. BMJ. 2002;325(7374):1202.
B12 and Peripheral Neuropathy
B12 deficiency is a common cause of peripheral neuropathy (tingling, numbness, pain in hands/feet). Early treatment with B12 injections can reverse neuropathy, but delayed treatment may result in permanent nerve damage. Methylcobalamin form may be superior to cyanocobalamin for neurological issues.
Source:Healton EB, et al. Neurologic aspects of cobalamin deficiency. Medicine. 1991;70(4):229-245.
Which Providers Test Vitamin B12?
Full Provider Comparison
| Provider | Includes | Annual Cost | Biomarkers |
|---|---|---|---|
 Superpower | ✓ | $199 | 100+ (150 with ratios) |
 WHOOP Advanced Labs | ✓ | $349 | 65 |
 Labcorp OnDemand | — | $398 | 30+ |
| — | $486 | 40+ | |
| ✓ | $444 | 288 | |
| ✓ | $349 | 100+ | |
| — | $761 | 54 | |
 Function Health | ✓ | $365 | 160+ |
| — | $250 | 65 | |
| ✓ | $495 | 70+ | |
| ✓ | $895 | 100+ | |
| ✓ | $1950 | 150+ | |
| ✓ | $375 | 80+ | |
| — | $Varies | 75+ | |
| ✓ | $190 | 100+ | |
| — | $99 | 50 | |
| ✓ | $124 | 60 | |
| ✓ | $199 | 50 | |
| ✓ | $499 | 120+ | |
| — | $4188 | 70-80+ | |
| ✓ | $375 | 85 | |
| ✓ | $700 | 128 |
Frequently Asked Questions
What is Vitamin B12?
Vitamin essential for nerve function and red blood cells
What is the optimal range for Vitamin B12?
The standard reference range for Vitamin B12 is fatigue. Optimal ranges may differ based on individual health goals and expert recommendations.
Which blood test providers include Vitamin B12?
15 out of 22 blood testing providers include Vitamin B12 in their panels. This biomarker is widely available across major providers.
What category does Vitamin B12 fall under?
Vitamin B12 is categorized under Vitamins &Nutrients. This category includes biomarkers that help assess related aspects of health and wellness.
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:2026-02-20