Vitamin B12 is a unique vitamin: it's the only one containing a metal (cobalt), the only one that neither plants nor animals can synthesize, and one of the rare ones whose deficiency can develop silently for years before affecting the blood, then the nerves. It contributes to red blood cell formation, nervous system function, energy metabolism, and fatigue reduction: four roles, among eight, recognized by EFSA.
This guide provides comprehensive evidence-based information: what B12 does, how it is absorbed (and why this mechanism is so fragile), which foods truly contain it, who is at risk of deficiency, what symptoms should raise concern, what dose to take depending on your situation, and what methylcobalamin and cyanocobalamin are really worth. It extends our article on B vitamins, which examines all eight B-group vitamins through the lens of science.
All eight B-group vitamins combined in a daily vegetable capsule, including B12 in methylcobalamin (2.5 µg, 100% of NRV), B9 Quatrefolic®, and B6 in pyridoxal-5-phosphate. Free from allergens, manufactured in France.
Vitamin B12 (cobalamin) contributes to red blood cell formation, nervous system function, energy metabolism, and fatigue reduction (EFSA claims). Requirements are low at 4 µg per day according to ANSES, but its absorption depends on intrinsic factor, a fragile mechanism: vegans, people over 50 years old, patients taking metformin or PPIs are most at risk of deficiency. Only foods of animal origin and fortified foods reliably provide it; spirulina is not a reliable source. In case of confirmed deficiency, 1,000 µg per day orally works as well as injections (Cochrane, 2018). The form (methyl or cyanocobalamin) matters less than the dose and consistency.
- What is vitamin B12?
- What is vitamin B12 for? The 8 EFSA claims
- How it is absorbed: the intrinsic factor
- How much vitamin B12 per day?
- Where to find vitamin B12: food sources
- Spirulina, algae, tempeh: the pseudo-B12 trap
- Vitamin B12 deficiency: symptoms and diagnosis
- Who is at risk? Causes of deficiency
- Supplementation: what dose depending on your situation?
- Methylcobalamin, cyanocobalamin: which form to choose?
- High Vitamin B12: Should You Worry?
- Safety, Side Effects and Interactions
- Choosing the Right Supplement: B12 Alone or B Complex?
- Frequently Asked Questions
1. What is Vitamin B12?
The cobalamine is the largest and most complex of all vitamins. At the center of its structure lies a cobalt atom, hence its name. Its history is linked to pernicious anemia: in the 1920s, this then-fatal disease was treated for the first time by feeding patients large quantities of raw liver, before the active principle was isolated in 1948.
Its most important characteristic for your diet: B12 is produced only by bacteria and archaea. Animals obtain it from their diet or microbiota (ruminants produce it thanks to their digestive bacteria) and concentrate it in their tissues, especially the liver. The bacteria in our own colon produce it as well, but too low in the digestive tract for it to be absorbed: in humans, it must absolutely come from food or a supplement.
Another singularity: unlike other water-soluble vitamins, B12 is stored in quantity, 2 to 5 mg in an adult, mainly in the liver, and recycled by bile. With daily requirements around 4 µg, these reserves cover several years. Good news, but also a trap: a person who stops absorbing it remains symptom-free for a long time, then develops fatigue, anemia or tingling. To place B12 among its seven sisters, see our comprehensive guide to B vitamins.
2. What is Vitamin B12 For? The 8 Claims Recognized by EFSA
Everything is based on two enzymatic reactions. The first, methionine synthase, transformshomocysteine into methionine using folate (vitamin B9) as a methyl donor. Without B12, folate remains trapped in an unusable form, a phenomenon called the "folate trap": DNA synthesis slows down, rapidly renewing cells (bone marrow especially) produce abnormally large and fragile red blood cells, and homocysteine accumulates in the blood. The second, methylmalonyl-CoA mutase, acts in the mitochondria on the metabolism of certain fatty acids and amino acids; when it lacks B12,methylmalonic acid accumulates and the myelin sheath that insulates nerves becomes fragile.
These mechanisms explain the eight health claims authorized by EFSA (EU regulation no. 432/2012). Vitamin B12 contributes to:
- the normal formation of red blood cells ;
- the normal functioning of the nervous system ;
- normal energy metabolism ;
- à reducing fatigue ;
- to the normal homocysteine metabolism ;
- to normal psychological functions (memory, concentration, mood) ;
- to normal immune system functioning ;
- to the process of cell division.
It does not provide a "boost" to someone who is not deficient: the claims above describe the maintenance of normal functions, not a stimulating effect. It neither causes weight loss nor weight gain. And in non-deficient people, trials of B vitamin supplementation have not shown measurable benefits on memory, even when they lowered homocysteine. Conversely, correcting a true deficiency improves fatigue, anemia, and often neurological disorders, gradually, over several weeks.
3. How it is absorbed: the intrinsic factor, a fragile link
Few nutrients follow such a winding path. In foods, B12 is bound to proteins. Thegastric acid and pepsin release it in the stomach, where it attaches to a salivary protein, haptocorrin. In the duodenum, pancreatic enzymes detach it, and it then binds to intrinsic factor, a glycoprotein manufactured by the parietal cells of the stomach. This complex travels to the terminal ileum, at the bottom of the small intestine, where specific receptors allow it to enter the bloodstream. There, only a minority of B12 (approximately 20%, bound to transcobalamin) is actually available to cells: this isholotranscobalamin.
Two figures change everything in practice. First, the intrinsic factor pathway saturates quickly : its receptors only accept approximately 1.5 to 2 µg per dose. It is therefore better to spread intake than to take large occasional doses. Second, approximately 1% of an oral dose passes through passive diffusion, without intrinsic factor: a dose of 1,000 µg provides absorption of around ten micrograms, enough to meet needs even when the normal pathway is out of service. This is the basis of high-dose oral treatments.
Each link can break: less acidic stomach (age, antacids) no longer releases B12 from food, while still absorbing "free" B12 from supplements; an autoimmune disease can destroy the cells that produce intrinsic factor; damage to the ileum eliminates the entry point; metformin disrupts uptake. These flaws explain why deficiency is so widespread, far beyond vegetarians alone.
Depending on the definition used, subclinical vitamin B12 deficiency affects 2.5 to 26% of the general population, with elderly people being most affected; deficiency results from insufficient intake, reduced bioavailability, or malabsorption.
Green R, et al. Nat Rev Dis Primers 2017;3:17040. DOI: 10.1038/nrdp.2017.40
4. How much vitamin B12 per day? Requirements by age
Requirements are tiny in absolute value, a few millionths of a gram, but they do not tolerate approximation over time. The most recent references:
| Population | Reference (µg / day) | Source |
|---|---|---|
| Adults | 4 | ANSES (RNP) and EFSA (AI) |
| Pregnant women | 4.5 | EFSA |
| Nursing women | 5 | EFSA |
| Children aged 1 to 10 years | 1.5 to 2.5 | EFSA |
| Adolescents aged 11 to 17 years | 3.5 to 4 | EFSA |
| NRV labeling reference (all adults) | 2.5 | EU Regulation No. 1169/2011 |
Two clarifications. These values assume normal absorption: a diet providing 4 µg spread throughout the day actually absorbs about half of it, which is sufficient to maintain reserves. And the NRV of 2.5 µg on labels dates from old recommendations: a supplement at "100% of the NRV" is a coherent addition for someone eating animal products, not a specific supplementation for a vegan or someone with poor absorption (see the dosage section).
5. Where to find vitamin B12: foods that really contain it
Here are the orders of magnitude from composition tables (Ciqual, USDA). Contents vary depending on species, farming and cooking, hence the ranges:
| Food | Content (µg / 100 g) | Reference |
|---|---|---|
| Liver (beef, veal, lamb) | 50 to 100 | The absolute champion: one slice covers weekly needs |
| Shellfish (clams, mussels, oysters) | 15 to 90 | Clams are among the richest known foods |
| Fatty fish (mackerel, sardine, herring) | 8 to 19 | A 100 g fillet covers 2 to 4 days |
| Salmon, trout, tuna | 2 to 7 | A 120 g serving covers approximately one day |
| Red meat (beef, lamb) | 2 to 3 | A 120 g steak covers one day |
| Hard cheese (Emmental, Comté) | 1.5 to 3 | 30 g provide 0.5 to 0.9 µg |
| Eggs | 0.9 to 1.1 | One egg provides approximately 0.5 µg, mainly in the yolk |
| Milk, yogurt | 0.3 to 0.5 (per 100 ml) | A 250 ml bowl provides approximately 1 µg, well absorbed |
| Poultry, pork | 0.3 to 0.7 | Modest sources: do not rely on them alone |
| Fortified foods (plant-based drinks, cereals, enriched nutritional yeast) | Variable, read the label | Often 0.4 to 2.5 µg per serving: the only reliable plant-based source |
Indicative content before absorption. Bioavailability also varies depending on the food: it is good for dairy products, meat and fish (approximately 40 to 60% at these doses), significantly lower for eggs (less than 10% in some studies).
Two questions come up repeatedly: which fruit contains vitamin B12? Which vegetables? The honest answer is: none. Neither fruits, nor vegetables, nor legumes, nor grains provide active B12, since only bacteria produce it; traces found on unwashed root vegetables come from soil and have no reliable value. A vegan diet, however varied, therefore does not meet requirements without fortified food or supplementation.
For those who eat animal products, the goal is simple: regular and distributed intake rather than a huge liver meal once a month, since absorption plateaus with each dose. Finally, be careful with yeast: regular brewer's yeast does not provide B12; only enriched nutritional yeast contains it, and this is mentioned on the label. Our vitamins and minerals, including the B complex, are brought together in the vitamins and minerals.
6. Spirulina, algae, tempeh: the pseudo-B12 trap
This is one of the most persistent misconceptions in plant-based nutrition. Early analyses of spirulina, performed using microbiological methods, counted all molecules in the corrinoid family without distinguishing true cobalamin from its analogs. Now spirulina contains essentially pseudo-vitamin B12, in which an adenine replaces the group that allows the vitamin to attach to our enzymes. Result: no activity in humans, a risk of competing with true B12 on transport proteins, and falsely reassuring blood levels with certain tests (Watanabe, Nutrients, 2014). ANSES moreover points out that spirulina does not constitute a reliable source of B12 for vegans.
The other plant candidates fare little better. Tempeh and fermented foods contain variable traces, with no guarantee from batch to batch; chlorella contains some true B12, at highly unequal levels. Only dried nori (the algae used in makis) has shown significant amounts of active cobalamin, but the necessary quantities and product variability prevent recommending it as a sole source. For a vegan, the only safe choice remains fortified food or supplementation.
This takes nothing away from the true benefits of organic spirulina : complete proteins, iron, carotenoids and phycocyanin, which we detail in our article on The benefits of spirulina. Simply, don't ask it to do what it cannot do.
7. Vitamin B12 Deficiency: Symptoms and Diagnosis
Because liver reserves are substantial, deficiency advances gradually, often over months or years. Signs fall into three categories:
Fatigue and Shortness of Breath
Often attributed to stress or lack of sleep.
Persistent fatigue, reduced endurance, shortness of breath on exertion, palpitations, paleness, dizziness. These reflect the beginning of anemia or simply an early cellular deficiency.
Macrocytic Anemia and Painful Tongue
Visible on a simple blood count.
Abnormally large red blood cells (MCV above 100 fL), decreased hemoglobin, sometimes reduced white blood cells and platelets. The tongue becomes smooth, red, and sensitive (glossitis), canker sores or a slight yellowish tint may appear.
Tingling, Balance, Memory, Mood
Can occur without any anemia, and should be a priority warning sign.
Tingling or numbness in the hands and feet, loss of vibratory sensation, unsteady gait, muscle weakness, memory and concentration problems, irritability, depressed mood, even confusion in elderly people. The longer they persist, the more likely they are to leave lasting effects: prompt medical advice is essential.
A little-known point: high folic acid intake can correct anemia while allowing neurological damage to progress, because the two vitamins share the DNA synthesis pathway but not the myelin pathway. All the more reason not to self-medicate haphazardly in the face of fatigue, and to distinguish iron, folate, and B12 deficiency—the three major nutritional causes of anemia (on iron, see our comparison bis-glycinate iron and other forms of iron).
Total Serum B12 : below approximately 148 pmol/L (200 pg/mL), deficiency is likely; between 148 and 221 pmol/L (200 to 300 pg/mL), you're in a gray zone; above that, deficiency is unlikely but not excluded, since only a fraction of this B12 is active.
To Clarify in the Gray Zone : holotranscobalamin (the active fraction, low below approximately 35 pmol/L) and especially methylmalonic acid, which rises as soon as cells lack B12. Homocysteine also increases, but it also depends on folates, B6, and kidney function.
Interpretation Pitfalls : a recent supplement, a course of spirulina, pregnancy, or oral contraception alter the result. These thresholds are guidelines, not verdicts: interpretation is the doctor's responsibility, in light of symptoms, blood count, and treatments.
8. Who is At Risk? Causes of Vitamin B12 Deficiency
Vegetarians and Vegans The figures speak for themselves. A review of 18 studies measuring deficiency with reliable markers (methylmalonic acid or holotranscobalamin) found it in 62% of pregnant vegetarian women, 25 to 86% of children, 21 to 41% of adolescents, and 11 to 90% of elderly people, even more so among vegans and those who had followed this diet since childhood (Pawlak, Nutr Rev, 2013). The authors' conclusion: vegetarians develop deficiency regardless of their age, country, or diet type, and regular supplementation is necessary. This also applies to ovo-lacto-vegetarians: two eggs and one yogurt provide barely more than one microgram.
People Over Age 50 With age, the stomach lining thins (atrophic gastritis in 10 to 30% of those over 60) and produces less acid: B12 is no longer released from food proteins. "Free" B12 from supplements and fortified foods, however, remains absorbed, hence the recommendation by several health authorities to obtain part of your intake from these sources after age 50.
Patients Taking Metformin The standard treatment for type 2 diabetes interferes with B12 uptake in the ileum. In the long-term follow-up of the American Diabetes Prevention Study, the risk of deficiency increased by 13% per year of treatment, and neuropathies were more frequent in metformin patients with low B12 (Aroda, J Clin Endocrinol Metab, 2016). Diabetes societies now recommend regular dosing.
After 5 years, low or borderline B12 was found in 19.1% of participants on metformin versus 9.5% on placebo, and anemia was more frequent in the metformin group.
Aroda VR, et al. J Clin Endocrinol Metab 2016;101(4):1754-1761. DOI: 10.1210/jc.2015-3754
Long-term antacids. Proton pump inhibitors (omeprazole, esomeprazole, pantoprazole) and, to a lesser extent, H2-blockers reduce the acidity necessary for the release of dietary B12. In a study of nearly 26,000 cases of deficiency, taking PPIs for at least two years multiplied the risk by 1.65, even more at high doses (Lam, JAMA, 2013). Not a reason to stop a beneficial treatment, but a good reason to discuss it with your doctor and monitor it.
Other situations to know about. The Biermer disease (pernicious anemia), autoimmune, destroys cells that produce intrinsic factor and requires lifelong treatment. The bariatric surgery (bypass, sleeve) makes supplementation systematic and permanent. The ileal diseases (Crohn's, resection), the celiac disease, bacterial overgrowth, pancreatic insufficiency and chronic alcoholism also impair absorption. Finally, nitrous oxide ("laughing gas"), diverted for recreational use, oxidizes the cobalt in B12 and inactivates it: it causes sometimes severe neurological damage even with normal intake, a public health problem on the rise among young adults.
9. Supplementation: what dose of vitamin B12 for your situation?
Why such a gap between 2.5 µg and 1,000 µg? Because of intrinsic factor saturation: small daily doses are sufficient when absorption works, whereas you must rely on the 1% passive diffusion, therefore on large doses, when absorption is failing or when intake has been zero for a long time.
Level 1, maintenance. You regularly consume meat, fish, eggs or dairy products, you are under 50 years old and have no malabsorption: your intake normally covers your needs. A supplement of 2.5 to 10 µg per day serves as a safety net, ideally in a complex that brings together all eight B vitamins, including B9 and B6 which work with B12 on homocysteine. This is the case with B Vitamin Complex : 2.5 µg of methylcobalamine, or 100% of the NRV, alongside the seven other B vitamins.
Level 2, strict vegans and vegetarians. Recommendations from vegetarian nutrition societies converge: 25 to 100 µg per day, or 1,000 µg two to three times per week, or 2,000 µg in a single weekly dose. A randomized trial in vegans and vegetarians with marginal deficiency compared 50 µg per day sublingually to 2,000 µg per week: after 12 weeks, both regimens had increased serum B12 and holotranscobalamin and lowered methylmalonic acid and homocysteine, with no difference between them, leading the authors to recommend the daily dose of 50 µg, six times more economical, rather than the 2,000 µg weekly dose (Del Bo', Clin Nutr, 2019). The moderate daily dose is therefore a validated strategy, provided it is taken every day. In this case, a B complex is a useful foundation for the other vitamins, but it must be supplemented with a specifically dosed B12.
Level 3, confirmed deficiency. A Cochrane review compared oral and intramuscular injection routes: at 1,000 to 2,000 µg per day, tablets normalize serum B12 as well as injections, at lower cost (Wang, Cochrane, 2018). A dose-finding trial in elderly people with mild deficiency clarifies the order of magnitude: 647 to 1,032 µg per day was needed to normalize methylmalonic acid, whereas 2.5 or 100 µg only reduced it by 16% (Eussen, Arch Intern Med, 2005). In other words, a multivitamin does not correct an established deficiency. Hydroxocobalamine injections retain their place in cases of neurological signs, severe malabsorption, or Biermer's disease, according to a schedule determined by your doctor, with follow-up at 3 or 6 months.
Morning or evening? The timing matters little: B12 is water-soluble and passes with or without food. Breakfast remains most practical to avoid forgetting; some avoid evening due to concern about a slight stimulating effect, which is unproven. Tablet, sublingual, drops or spray? No formulation has proven its superiority: the sublingual route works (it's the one used in the cited Italian trial), but not better than the swallowed tablet at equal dose. On metformin or PPIs? No need to space out doses; the goal is to compensate for reduced absorption with regular intake and biological monitoring.
The B Vitamin Complex brings together all eight B vitamins in active forms: B12 as methylcobalamine (2.5 µg, 100% NRV), B9 Quatrefolic®, B6 as P-5-P. Vegetable capsule, allergen-free, made in France.
Discover the B Vitamin Complex →Vegan or confirmed deficiency? This complex is a daily foundation: combine it with a B12 dosed for your situation, and discuss it with your doctor.
10. Methylcobalamine, cyanocobalamine, hydroxocobalamine: which form to choose?
The methyl versus cyano debate occupies a disproportionate place in online discussions. Here's what chemistry and trials tell us:
| Form | Nature | Strengths | Limitations | Common use |
|---|---|---|---|---|
| Cyanocobalamin | Synthetic, the most stable (heat, light resistant) | The most studied (majority of clinical trials), the least expensive, effective at all doses | Must be converted within the cell; releases a negligible trace of cyanide (approximately 20 µg per 1,000 µg, well below food safety thresholds); avoided in rare cases (severe kidney failure, Leber's optic neuropathy) | Supplements, fortified foods, high-dose tablets |
| Methylcobalamin | Coenzyme form of methionine synthase, present in plasma | Cyanide-free, directly usable by one of the two enzymes | Light-sensitive, therefore less stable; no demonstrated clinical superiority; slightly more expensive | B-complex and premium supplements (this is the form in our B Vitamin Complex) |
| Adenosylcobalamin | Coenzyme form of methylmalonyl-CoA mutase (mitochondria) | Natural active form, often combined with methylcobalamin | Limited clinical data when used alone | Combined supplements "active forms" |
| Hydroxocobalamin | Natural form produced by bacteria, present in foods | Prolonged retention in the body, reference form for injections in France, also an antidote to cyanide | Limited oral availability in France | Intramuscular injections for severe deficiencies and Biermer's disease |
Coenzyme forms (methyl- and adenosylcobalamin) are probably not superior to cyano- and hydroxocobalamin for preventing or treating a deficiency: all forms lose their group upon entering the cell before being reconverted as needed, and methylcobalamin is furthermore more light-sensitive.
Obeid R, Fedosov SN, Nexo E. Mol Nutr Food Res 2015;59(7):1364-1372. DOI: 10.1002/mnfr.201500019
In other words, no one needs to switch supplements because it contains cyanocobalamin. We selected methylcobalamin in our complex for its nature as a coenzyme form and the absence of a cyanide group, for consistency with the other active B vitamins in the formula (B9 Quatrefolic®, B6 P-5-P), not as a superiority argument. What will make the difference in your test results is the dose adapted to your profile and the consistency of your intake.
11. B12 vitamin too high: should you be concerned?
Two very different situations hide behind a number above the normal range. The first, the most common: you take B12, as a supplement, in multivitamins or as injections, sometimes without thinking about it (energy drink, enriched yeast, B complex). Since B12 is water-soluble and has no upper safety limit, this level simply reflects recent intake and requires no action, except to mention it to your doctor to avoid unnecessary tests.
The second deserves more attention. In a person who is not taking supplements, elevated B12 can indicate excessive release by damaged liver cells or overproduction of its carrier proteins. A literature review links this "hypercobalaminemia" to diseases of the liver (acute hepatitis, cirrhosis, fatty liver, tumors), the kidneys (kidney failure) and the blood (certain bone marrow diseases), as well as inflammatory conditions; for the authors, this is a warning signal to investigate, not a laboratory curiosity (Andrès, QJM, 2013). Paradoxically: in these situations, B12 can be elevated in the blood but poorly usable by cells.
In practice: elevated levels while supplementing, note it and move on; elevated levels without supplementation, discuss it with your doctor, who will decide whether to check liver function, kidney function, and blood count.
12. Safety, side effects and interactions
Because excess is eliminated in the urine and absorption itself is limited, no toxicity has been described with B12, even at several milligrams per day for years: EFSA, like the American authority, has not established a maximum dose. A few adverse effects are reported: acne or rosacea flare-ups at high doses (they regress upon discontinuation) and very rare allergic reactions, especially with injections. At the start of treatment for severe anemia, the doctor also monitors blood potassium, which can drop when the bone marrow suddenly resumes producing red blood cells.
Medications that reduce absorption : metformin, proton pump inhibitors and anti-H2 agents taken long-term, colchicine, certain antibiotics (neomycin). They do not contraindicate B12, quite the opposite: they justify monitoring and often supplemental intake, decided with your doctor.
High-dose folic acid : it can mask anemia from B12 deficiency while allowing nerve damage to progress. Never take high doses of folates alone for unexplained fatigue without prior testing.
Nitrous oxide : including during anesthesia in a person already deficient, it inactivates B12; inform your anesthesiologist of any supplementation or vegan diet.
Pregnancy, breastfeeding, children, chronic disease : needs and doses differ; seek professional health advice before any supplementation. Respect the indicated daily dose, keep supplements out of children's reach and do not use them as substitutes for varied and balanced nutrition.
13. Choosing the right supplement: B12 alone or B complex?
B12 does not work alone. Methionine synthase needs folates (B9), homocysteine metabolism also mobilizes B6, and cellular energy production involves B1, B2, B3 and B5. All water-soluble and, except for B12, poorly stored, B vitamins must be supplied daily. This is the logic of the B complex : cover the entire group at nutritional doses, in a single daily capsule. This is the most coherent answer for maintenance profiles.
It is not for situations requiring 50, 100 or 1,000 µg of B12: no balanced complex can contain such doses of a single vitamin. In this case, a highly dosed B12 is added, alone or in addition to the complex for the other seven vitamins. A few selection criteria, valid for either one:
- The dose of B12 per serving, in µg, clearly readable, and the percentage of NRV it represents (100% NRV = 2.5 µg).
- The named form (methylcobalamin, cyanocobalamin, hydroxycobalamin) rather than a vague "vitamin B12".
- The forms of other B vitamins : methylfolate (Quatrefolic® or equivalent) rather than folic acid, B6 as pyridoxal-5-phosphate, B2 as riboflavin-5-phosphate.
- A clean capsule : vegetable capsule (HPMC), without titanium dioxide, without colorant, without major allergen, traceable manufacturing.
- Reasonable doses for other vitamins : high-dose B6 over the long term is not without risk and niacin can cause hot flashes; a well-designed complex stays close to daily requirements.
Methylcobalamine, Quatrefolic®, pyridoxal-5-phosphate: the B Vitamin Complex provides each day the eight vitamins from the group at nutritional doses, without allergen or titanium dioxide. Thirty capsules, one per day at breakfast.
Discover the B Vitamin Complex →Find all of our vitamins and minerals and our formulas dedicated to fatigue.
Choose the situation that best matches yours: the answer appears just below.
Your diet does not provide reliable B12, regardless of its quality. Aim for 25 to 100 µg per day (or 1,000 to 2,000 µg per week) with a B12 specifically dosed, have your levels checked, and also monitor iron, vitamin D, zinc and iodine. The B Vitamin Complex is a good foundation for the other seven vitamins, in addition, not instead. Also read: creatine and the brain, another nutrient that vegetarians have in lower reserves.
Your stomach may release B12 from food less effectively, but it still absorbs B12 from supplements. Ask for a B12 test (and methylmalonic acid if the result is borderline) at your next checkup. Daily intake via the B Complex Vitamins serves as a safety net; if your level is low, your doctor will prescribe a corrective dose. Never modify a treatment on your own initiative.
In an omnivore under 50 years old, fatigue rarely comes from B12 alone: iron, vitamin D, sleep, thyroid, and training load are more frequent suspects (see chronic fatigue: 11 validated plants and nutrients). A blood test will sort it out. In the meantime, one daily capsule of B Complex Vitamins covers all eight B vitamins, and magnesium bisglycinate usefully complements during periods of stress.
A confirmed deficiency or neurological signs require corrective treatment prescribed by your doctor: approximately 1,000 µg daily orally or injections depending on the cause, with monitoring at 3 or 6 months. The maintenance doses in a complex are insufficient at this stage. Once reserves are restored and the cause identified, a B complex can resume its place in your daily routine.
This test provides guidance; it does not replace a blood test or professional medical advice.
Frequently asked questions about vitamin B12
What is vitamin B12 for?
Vitamin B12 (cobalamin) contributes to the normal formation of red blood cells, normal nervous system function, normal energy metabolism, homocysteine metabolism, normal psychological functions, immune system function, fatigue reduction, and cell division: eight claims authorized by EFSA. At the cellular level, it is a cofactor for two enzymes essential for DNA synthesis and maintenance of the myelin sheath around nerves.
Where to find vitamin B12? What is the richest food source?
Vitamin B12 is found almost exclusively in foods of animal origin. The richest sources are liver (50 to 100 µg per 100 g) and shellfish such as clams and mussels (15 to 90 µg), well ahead of fatty fish (mackerel, sardine, herring: 8 to 19 µg), red meat (2 to 3 µg), hard cheeses (1.5 to 3 µg), eggs (approximately 1 µg), and milk (0.3 to 0.5 µg per 100 ml). Fortified foods (plant-based drinks, enriched nutritional yeast) are the only reliable plant-based source.
Which fruit or vegetable contains vitamin B12?
None. Neither fruits, nor vegetables, nor grains, nor legumes naturally contain active vitamin B12: it is produced solely by bacteria and stored in animal tissues. Algae such as spirulina contain mainly inactive analogues (pseudo-B12). Only fortified plant foods and supplements provide reliable B12 to vegetarians and vegans.
What are the reasons for vitamin B12 deficiency?
Two main mechanisms: insufficient intake (vegan diet or very poor in animal products) and impaired absorption. The latter is most common after age 50: decreased gastric acidity, Biermer disease (autoimmune), prolonged use of metformin or proton pump inhibitors (PPIs), bariatric surgery, Crohn's disease or celiac disease, nitrous oxide consumption. A reference review (Nat Rev Dis Primers, 2017) estimates that subclinical deficiency affects 2.5 to 26% of the population depending on the definition used.
What are the symptoms of vitamin B12 deficiency?
Persistent fatigue, pallor, shortness of breath on exertion and heart palpitations (signs of anemia), smooth and painful tongue, but also tingling in the hands and feet, balance problems, decreased memory and concentration, irritability or depressive mood. Neurological signs can appear without anemia and become lasting if the deficiency persists: they warrant prompt medical attention.
What vitamin B12 level is concerning?
Usual thresholds: serum B12 below 148 pmol/L (approximately 200 pg/mL) suggests a deficiency; between 148 and 221 pmol/L (200 to 300 pg/mL), you are in a gray zone and the doctor may request holotranscobalamin or methylmalonic acid to decide. Interpretation is the doctor's responsibility, taking into account symptoms, blood count, and current treatments.
How much vitamin B12 per day?
ANSES nutritional reference is 4 µg per day for an adult (4.5 µg during pregnancy and 5 µg during breastfeeding according to EFSA); the reference nutritional value used on labels is 2.5 µg. As a supplement, a maintenance dose of 2.5 to 10 µg is sufficient for people who consume animal products. Vegans need much higher doses (25 to 100 µg per day or 1,000 to 2,000 µg per week), and correcting a confirmed deficiency is the doctor's responsibility (approximately 1,000 µg per day or injections).
When to take vitamin B12, morning or evening?
Timing matters little for absorption: B12 is water-soluble and is absorbed with or without food. In the morning, at breakfast, remains the most practical choice to avoid forgetting, and some people prefer to avoid evening due to fear of a slight stimulating effect, which is not proven however. For high doses, splitting the dose in two improves the amount absorbed, as the intrinsic factor saturates around 1.5 to 2 µg per dose.
Methylcobalamin or cyanocobalamin: which is the best form?
Both correct a deficiency. Cyanocobalamin is the most studied and most stable form; methylcobalamin is a coenzyme form naturally present in plasma, without cyanide group, but more sensitive to light. A reference review (Obeid, Mol Nutr Food Res, 2015) concludes that all forms go through the same transformation pathway in the cell and that no superiority of methylcobalamin is demonstrated. Dose and regularity matter more than form.
Is spirulina a source of vitamin B12?
No. Spirulina contains essentially pseudo-vitamin B12, analogues that the human body cannot use and which may even distort blood tests (Watanabe, Nutrients, 2014). It remains interesting for its proteins and iron, but a vegan should not rely on it to meet B12 needs.
Tablets or injections: what to choose in case of deficiency?
A Cochrane review (2018) shows that 1,000 to 2,000 µg daily orally normalize serum B12 as well as intramuscular injections, at lower cost, because approximately 1% of the dose passes through passive diffusion even without intrinsic factor. Injections retain their place in case of neurological signs, severe malabsorption, or Biermer disease, according to the doctor's decision.
Is excessively high vitamin B12 dangerous?
If you take a supplement or have recently received injections, a high level is expected and harmless: excess is eliminated in urine and EFSA has set no upper safety limit. On the other hand, a high level without supplementation is not trivial: it may reflect liver, kidney, or blood disease (Andrès, QJM, 2013) and deserves to be explored with the doctor, without alarm but without neglecting it.
Does vitamin B12 give you energy?
Not in the way a stimulant does. B12 contributes to normal energy metabolism and fatigue reduction, but this benefit applies to people whose intake or reserves are insufficient. In someone who isn't deficient, an extra dose won't provide a measurable energy boost. Correcting a true deficiency, however, does improve fatigue, progressively, over a few weeks.
- Cobalamin
- The chemical name of vitamin B12, which designates the family of active molecules built around a cobalt atom (cyano-, hydroxo-, methyl- and adenosylcobalamin).
- Intrinsic factor
- A glycoprotein secreted by the parietal cells of the stomach. It binds to B12 in the small intestine and allows its absorption in the terminal ileum. Its absence (Biermer's disease, gastrectomy) requires high oral doses or injections.
- Holotranscobalamin (holoTC)
- The fraction of blood B12 bound to transcobalamin, the only form truly available to cells (approximately 20% of the total). Its measurement is more early and more specific than total serum B12.
- Methylmalonic acid (MMA)
- A metabolite that accumulates in the blood and urine when cells lack B12. It is the reference functional marker for confirming a deficiency in the gray zone.
- Homocysteine
- An intermediate amino acid recycled into methionine through B12 and folates. It rises in cases of B12 deficiency, but also in B9, B6 deficiency, or renal insufficiency, making it less specific.
- Megaloblastic (macrocytic) anemia
- Anemia characterized by abnormally large red blood cells (MCV greater than 100 fL), caused by slowed DNA synthesis due to lack of B12 or folates. Pernicious anemia (Biermer's disease) is its autoimmune form.
- Pseudo-vitamin B12
- A cobalamin analog produced by certain cyanobacteria such as spirulina. Inactive in humans, it can skew certain blood tests and has no nutritional value.
- Green R, Allen LH, Bjørke-Monsen AL, et al. Vitamin B12 deficiency. Nat Rev Dis Primers. 2017;3:17040. doi:10.1038/nrdp.2017.40
- Pawlak R, Parrott SJ, Raj S, Cullum-Dugan D, Lucus D. How prevalent is vitamin B12 deficiency among vegetarians? Nutr Rev. 2013;71(2):110-117. doi:10.1111/nure.12001
- Watanabe F, Yabuta Y, Bito T, Teng F. Vitamin B12-containing plant food sources for vegetarians. Nutrients. 2014;6(5):1861-1873. doi:10.3390/nu6051861
- Obeid R, Fedosov SN, Nexo E. Cobalamin coenzyme forms are not likely to be superior to cyano- and hydroxyl-cobalamin in prevention or treatment of cobalamin deficiency. Mol Nutr Food Res. 2015;59(7):1364-1372. doi:10.1002/mnfr.201500019
- Eussen SJ, de Groot LC, Clarke R, et al. Oral cyanocobalamin supplementation in older people with vitamin B12 deficiency: a dose-finding trial. Arch Intern Med. 2005;165(10):1167-1172. doi:10.1001/archinte.165.10.1167
- Del Bo' C, Riso P, Gardana C, et al. Effect of two different sublingual dosages of vitamin B12 on cobalamin nutritional status in vegans and vegetarians with a marginal deficiency: a randomized controlled trial. Clin Nutr. 2019;38(2):575-583. doi:10.1016/j.clnu.2018.02.008
- Aroda VR, Edelstein SL, Goldberg RB, et al. Long-term metformin use and vitamin B12 deficiency in the Diabetes Prevention Program Outcomes Study. J Clin Endocrinol Metab. 2016;101(4):1754-1761. doi:10.1210/jc.2015-3754
- Lam JR, Schneider JL, Zhao W, Corley DA. Proton pump inhibitor and histamine 2 receptor antagonist use and vitamin B12 deficiency. JAMA. 2013;310(22):2435-2442. doi:10.1001/jama.2013.280490
- Wang H, Li L, Qin LL, Song Y, Vidal-Alaball J, Liu TH. Oral vitamin B12 versus intramuscular vitamin B12 for vitamin B12 deficiency. Cochrane Database Syst Rev. 2018;3:CD004655. doi:10.1002/14651858.CD004655.pub3
- Andrès E, Serraj K, Zhu J, Vermorken AJ. The pathophysiology of elevated vitamin B12 in clinical practice. QJM. 2013;106(6):505-515. doi:10.1093/qjmed/hct051
- EFSA NDA Panel. Scientific Opinion on Dietary Reference Values for cobalamin (vitamin B12). EFSA Journal. 2015;13(7):4150. doi:10.2903/j.efsa.2015.4150
- ANSES. Nutritional reference values for vitamins and minerals. anses.fr. Health claims: Regulation (EU) No. 432/2012; Nutritional reference values: Regulation (EU) No. 1169/2011.
