What Is Vitamin B12? Functions, Food Sources, and How to Supplement
Vitamin B12 — also called cobalamin — is an essential nutrient your body cannot produce on its own. It plays a fundamental role in three critical processes: building healthy nerve fibres, producing red blood cells, and synthesising DNA. Without enough of it, the consequences range from fatigue and brain fog to irreversible nerve damage — yet millions of people are deficient without knowing it.
Key Facts About Vitamin B12
- B12 is found almost exclusively in animal products — meat, fish, eggs, and dairy. Vegans who do not supplement are at near-certain risk of deficiency over time.
- Adults over 50 often struggle to absorb B12 even when eating enough of it, because absorption requires a protein called intrinsic factor that declines with age.
- If you take metformin or a proton pump inhibitor (omeprazole, lansoprazole), these medications can deplete B12 regardless of your diet — ask your doctor about monitoring your levels.
What Does Vitamin B12 Do in the Body?
B12 is a water-soluble vitamin, meaning the body does not store it in large quantities. You need a consistent supply from food or supplements to keep levels adequate.
It supports three core functions:
- Myelin sheath synthesis: B12 is required to build and maintain the protective coating around nerve fibres. Deficiency gradually damages this coating, causing numbness, tingling, and — in severe cases — irreversible neurological problems. Research published in PMC 8294980 identifies B12 as having the strongest neurological role among the neurotropic B vitamins.
- Red blood cell formation: B12 is needed to produce properly formed red blood cells. Without it, cells become large and immature — a condition called megaloblastic anemia — and cannot carry oxygen efficiently, according to the NIH Office of Dietary Supplements.
- DNA synthesis: B12 acts as a cofactor in DNA production, working alongside folate. This is especially important during pregnancy and any period of rapid cell division.
B12 also works with folate and vitamin B6 to convert homocysteine into methionine. Elevated homocysteine is linked to cardiovascular and neurological risk — another reason adequate B12 matters beyond just avoiding deficiency symptoms.
For a deeper look at the science-backed evidence behind each of these functions, see our full guide to vitamin B12 benefits.
Where Does Vitamin B12 Come From?
Vitamin B12 is naturally present almost exclusively in animal-derived foods. It does not exist in meaningful quantities in unfortified plant foods — this is not a food-preparation issue, it is a biological one.
| Food | Serving Size | Approximate B12 Content |
|---|---|---|
| Clams (cooked) | 3 oz (85g) | ~84 mcg |
| Beef liver (cooked) | 3 oz (85g) | ~70 mcg |
| Salmon (cooked) | 3 oz (85g) | ~4.9 mcg |
| Tuna, light (canned) | 3 oz (85g) | ~2.5 mcg |
| Ground beef (cooked) | 3 oz (85g) | ~2.4 mcg |
| Whole milk | 1 cup (240ml) | ~1.2 mcg |
| Egg (large, whole) | 1 egg | ~0.6 mcg |
| Fortified breakfast cereal | 1 serving | ~1.5–6 mcg (varies by brand) |
Source: NIH ODS Consumer Fact Sheet
For vegans and vegetarians, fortified plant milks, breakfast cereals, and nutritional yeast provide some B12, but amounts vary and bioavailability may differ. A supplement is generally recommended for anyone who avoids animal products.
Worth noting: the adult recommended daily amount is only 2.4 mcg. Most people who eat animal products regularly get enough B12 from food. The bigger issue — especially over 50 — is often absorption, not intake.
How Does the Body Absorb Vitamin B12?
B12 absorption is a two-step process — and understanding it explains why eating plenty of B12-rich food is not always enough.
Step 1 — Release from food: Hydrochloric acid in the stomach separates B12 from the proteins it is bound to in food. This is why low stomach acid — whether from ageing, proton pump inhibitors, or gastric surgery — reduces how much B12 you can extract from what you eat.
Step 2 — Intrinsic factor binding: Once free, B12 binds to a protein called intrinsic factor, produced by specialised cells in the stomach lining. This B12-intrinsic factor complex is then absorbed in the small intestine. People who cannot produce intrinsic factor — due to pernicious anemia or gastric surgery — cannot absorb food-bound B12 regardless of how much they eat.
This is where supplements have a practical advantage over food. Supplement B12 does not need to be released by stomach acid because it is already in free form.
And at high doses (500 mcg or more), a small amount of passive absorption occurs that bypasses the intrinsic factor requirement entirely — roughly 1% of the dose, which is enough to cover daily needs.
Sublingual delivery (drops or dissolving tablets held under the tongue) absorbs B12 through the oral mucosa, also bypassing the intrinsic factor step. A 2025 meta-analysis published in Frontiers in Pharmacology found sublingual B12 delivery comparable to intramuscular injection for correcting deficiency — a significant finding for anyone with absorption concerns.
Who Is Most at Risk of Low B12?
Several groups face a higher likelihood of inadequate B12 — either because they do not eat enough of it or because they struggle to absorb it properly.
- Vegans and vegetarians: B12 is essentially absent from unfortified plant foods. Without a reliable supplement, deficiency is near-certain over time.
- Adults over 50: Stomach acid production declines with age, reducing the ability to separate B12 from food proteins. The NIH recommends adults over 50 meet most of their B12 needs through supplements or fortified foods.
- People with pernicious anemia: An autoimmune condition that destroys intrinsic factor-producing cells. Affected individuals cannot absorb food-bound B12 and require high-dose supplements or injections.
- People with digestive conditions: Crohn's disease, celiac disease, atrophic gastritis, or surgeries affecting the stomach or small intestine can all impair absorption.
- Metformin users: This widely prescribed diabetes medication interferes with B12 uptake in the ileum. A 2025 review in PMC 12562576 found that long-term metformin users have a significantly higher risk of deficiency — particularly at higher doses and after several years of use.
- Proton pump inhibitor (PPI) users: Omeprazole, lansoprazole, esomeprazole, and similar drugs reduce stomach acid, which is needed to free B12 from food. Combined use of metformin and a PPI compounds the risk further.
Important: If you take metformin or a PPI long-term, ask your healthcare provider about testing your B12 levels. Do not self-diagnose or self-treat a potential deficiency — a blood test is the only reliable way to confirm it.
For a full breakdown of deficiency symptoms, causes, and how it is diagnosed and treated, read our guide to vitamin B12 deficiency symptoms and treatment.
How Much Vitamin B12 Do You Need?
The recommended daily amount (RDA) for B12 is straightforward for most adults — but it shifts depending on age, life stage, and whether you are correcting an existing shortfall.
| Group | Recommended Daily Amount |
|---|---|
| Adults (19–50) | 2.4 mcg/day |
| Adults over 50 | 2.4 mcg — but supplementation recommended due to absorption decline |
| Pregnant women | 2.6 mcg/day |
| Breastfeeding women | 2.8 mcg/day |
| Vegans / vegetarians | 25–100 mcg/day supplement (higher dose overcomes passive diffusion variability) |
| Therapeutic (correcting deficiency) | 500–1,000 mcg/day oral, under medical guidance |
Source: NIH ODS Consumer Fact Sheet
No tolerable upper intake level has been established for B12. Because it is water-soluble, excess is excreted in urine rather than stored. High doses are considered safe for most healthy adults.
For adults over 50 or anyone with absorption concerns, a sublingual or liquid B12 supplement offers a practical advantage. Our B12 liquid drops deliver cobalamin sublingually — absorbed through the lining of the mouth, partially bypassing the intrinsic factor requirement.
Supplement Forms: Cyanocobalamin vs. Methylcobalamin
Walk into any supplement aisle and you will see B12 in several different forms. The most common question is: does it matter which one you choose?
| Form | Description | Best For |
|---|---|---|
| Cyanocobalamin | Synthetic; most common in supplements; must be converted by the body to active forms | Cost-effective; well-studied; effective for most people |
| Methylcobalamin | Naturally occurring bioactive form; found in animal foods; ready for immediate use | Those who prefer an active form; may have higher tissue retention |
| Adenosylcobalamin | Mitochondrial form; less common in supplements | Sometimes combined with methylcobalamin |
| Hydroxocobalamin | Long half-life; used primarily in injectable form | Clinical and prescription use |
For most people, both cyanocobalamin and methylcobalamin are effective at correcting deficiency — the practical difference is modest. One important exception: people with Leber's disease (a rare hereditary eye condition) should avoid cyanocobalamin and use methylcobalamin or adenosylcobalamin instead.
The more meaningful question is often about delivery method rather than form. Sublingual (liquid or dissolving tablet) supplements bypass parts of the absorption process — making them a useful option for anyone over 50, or those with pernicious anemia or gut conditions that impair B12 uptake. For more on how B12 affects energy and metabolism specifically, see our article on vitamin B12 and energy.
Try Vast Vitamins Energy Booster Plus B12 Drops
Our B12 liquid drops deliver cobalamin sublingually — absorbed through the lining of the mouth, partially bypassing the intrinsic factor requirement. A practical option for adults over 50 or anyone with absorption concerns.
Frequently Asked Questions
What does vitamin B12 do for the body?
Vitamin B12 serves three primary functions: it helps build and maintain the myelin sheath that protects nerve fibres, it is essential for producing healthy red blood cells, and it acts as a cofactor in DNA synthesis. It also plays a role in converting homocysteine to methionine alongside folate and B6. For a full evidence-based breakdown of these roles, see our guide to vitamin B12 benefits.
What foods are highest in vitamin B12?
The highest natural sources of vitamin B12 are clams (~84 mcg per 3 oz serving), beef liver (~70 mcg), salmon (~4.9 mcg), tuna (~2.5 mcg), beef, dairy products, and eggs. Plant foods contain virtually no B12 unless fortified. Vegans and vegetarians should take a B12 supplement or regularly consume fortified foods such as plant milks and breakfast cereals.
What are the signs that you need vitamin B12?
Common signs of low B12 include persistent fatigue and weakness, numbness or tingling in the hands and feet, difficulty concentrating or memory problems, pale or yellowish skin, and a sore or inflamed tongue. These symptoms can develop gradually and are often mistaken for other conditions. A blood test is the only reliable way to confirm a deficiency — read our full guide to vitamin B12 deficiency symptoms and treatment for more detail.
Who should take a B12 supplement?
People most likely to benefit from a B12 supplement include vegans and vegetarians (B12 is absent from plant foods), adults over 50 (absorption declines with age), people taking metformin or proton pump inhibitors (these medications reduce B12 absorption), and those with pernicious anemia or digestive conditions affecting nutrient absorption. If you fall into one or more of these groups, speak with your healthcare provider about testing your levels.
What is the difference between cyanocobalamin and methylcobalamin?
Cyanocobalamin is the synthetic form of vitamin B12 used in most supplements — the body converts it to the active forms it needs. Methylcobalamin is the naturally occurring, bioactive form found in food. Both are effective for correcting deficiency in most people; the practical difference is modest for the average supplement user. The one exception is people with Leber's disease, who should use methylcobalamin rather than cyanocobalamin.
Can you take too much vitamin B12?
No tolerable upper intake level has been established for vitamin B12. Because it is water-soluble, excess is excreted in urine rather than stored, and high doses are considered safe for most healthy adults. The exception is people with Leber's disease, who should avoid cyanocobalamin specifically. Very high long-term injection doses may warrant periodic monitoring, but standard oral and sublingual supplements do not pose a known toxicity risk.
These statements have not been evaluated by the FDA. This product is not intended to diagnose, treat, cure, or prevent any disease. Always consult a qualified healthcare provider before starting any supplement, especially if you take prescription medications or have an existing health condition.
Sources
- National Institutes of Health, Office of Dietary Supplements. Vitamin B12 — Consumer Fact Sheet. Reviewed 2024.
- National Institutes of Health, Office of Dietary Supplements. Vitamin B12 — Health Professional Fact Sheet. Reviewed 2024.
- Calderón-Ospina CA, Nava-Mesa MO. B vitamins in the nervous system: current knowledge of the biochemical modes of action and synergies of thiamine, pyridoxine, and cobalamin. CNS Neurosci Ther. 2021;26(1):5-13. PMC 8294980.
- Frontiers in Pharmacology. Sublingual vitamin B12 versus intramuscular injection for vitamin B12 deficiency: a systematic review and meta-analysis. Front Pharmacol. 2025. doi: 10.3389/fphar.2025.1602976.
- PMC 12562576. Metformin and proton pump inhibitor combination and risk of vitamin B12 deficiency. Published 2025.
- Harvard T.H. Chan School of Public Health, The Nutrition Source. Vitamin B12.
