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food source · 10
←Beef
B12 comes almost only from animal foods. Cattle cannot make it themselves; the microbes in their rumen do, and the cow absorbs it and stores it in liver and muscle, so one serving of beef covers a good part of a day's need.
Mechanism · The cow concentrates it
Only bacteria and archaea make B12. The microbes in the rumen make it, and the cow absorbs it and stores it in liver and muscle, so beef does not produce B12; it concentrates it. The lower human gut also has bacteria that make B12, but they sit downstream of where it is absorbed, so what they make arrives too late. From plate to blood there are three steps: stomach acid frees B12 from food protein, intrinsic factor (an escort protein made by the stomach) picks it up, and the end of the small intestine takes it in.
Clinical · Eating meat does not always prevent it
If any of the three steps fails, the amount on the plate does not count. People with less stomach acid in older age, people on long-term acid-suppressing drugs, and people whose stomach lining can no longer make intrinsic factor can run short of B12 even while eating meat. The liver holds a store of B12, so symptoms appear long after intake stops; sometimes numbness in the hands and feet or unsteady walking comes first, while the blood count is still normal.
Animal liver is extremely high in B12, and one serving covers several days. But liver has almost no fiber, little calcium and not much vitamin C, so the rest of the plate still has to round out the meal.
Mechanism · Nutrients mounted on protein
The B12, iron, copper and riboflavin in liver are not loose powder but finished parts already mounted on protein: iron sits in the heme ring, copper in copper-containing enzymes, riboflavin on its coenzymes. They are taken apart and absorbed step by step through ordinary digestion rather than released in the gut all at once, which may be one reason food is often gentler than a supplement at the same dose.
In practice · A serving of liver is not a meal
One serving of liver covers several days of B12. The more impressive the nutrient list, the easier it is to think this one meal is enough, when the opposite is true: liver has almost no fiber, little calcium and not much vitamin C, so the meal still needs vegetables and whole grains.
←Tuna
B12 comes almost only from animal foods, and tuna is very high in it: one serving usually exceeds a day's need. B12 maintains the insulating sheath around nerve fibers and lets the bone marrow make red blood cells of normal size.
Mechanism · Three steps to absorption
Absorbing B12 takes three steps: stomach acid frees it from food protein, intrinsic factor made by the stomach escorts it, and receptors at the end of the small intestine take it in. If any step fails, you can eat it and still not absorb it.
Clinical · Eating enough can still fall short
One serving of tuna usually supplies far more B12 than a day's need. But atrophic gastritis, stomach surgery, long-term strong acid suppression and disease at the end of the small intestine can all leave someone eating enough and still not absorbing it. Whether to test for B12 is a decision for a doctor; do not match your own symptoms against a list.
Salmon is a good source of B12: farmed Atlantic salmon has about 2.5–3 μg per 100 g, enough to cover most adults' recommended amount for a day (2.4 μg).
Mechanism · The fish does not make B12
Only bacteria and archaea make B12. Microbes in the sea make it and it builds up step by step along the food chain, so salmon concentrates B12 rather than producing it. In people short of stomach acid or intrinsic factor, none of the amount on the plate gets absorbed.
Numbers · One serving covers a day
Salmon has about 2.5–3 μg of B12 per 100 g, while the US adult Recommended Dietary Allowance (RDA) is only 2.4 μg a day. It comes in the same piece of fish as the omega-3 and vitamin D, not as a separate item.
←Eggs
B12 comes almost only from animal foods, and an egg's B12 is in the yolk, about 0.6 μg per egg. For lacto-ovo vegetarians it is one source among several, but one egg falls well short of a day's need.
Mechanism · Plants do not make B12
Plants do not make B12. The yolk carries a share, alongside the choline and vitamin D in the same yolk. Lacto-ovo vegetarians can count it as a source; people who eat no animal foods at all still need fortified foods or a supplement.
Numbers · One egg is a share, not a day
One egg has about 0.6 μg of B12, while the US adult Recommended Dietary Allowance (RDA) is 2.4 μg a day, so an egg is only a share and should be counted together with milk, fish or liver.
←Pork
B12 comes almost only from animal foods. Lean pork carries a share, in the same lean meat as its protein, thiamin and zinc.
Mechanism · It follows the lean
The B12 in lean pork sits in the muscle tissue together with thiamin, zinc and protein. Choose a fatty cut, and fat dilutes all of these micronutrients at once.
In practice · An everyday share
Pork is not as high in B12 as liver or clams; it is one share on an everyday plate with meat. People who eat no animal foods at all get none of it and need fortified foods or a supplement.
Dried nori contains real, active B12, and one review judges it the most suitable natural source currently available to vegetarians; but the amount varies widely with species and processing, so it cannot be the only source. What is measured in spirulina, by contrast, is mostly inactive look-alikes.
Mechanism · Real B12, not a look-alike
Watanabe 1999 measured it two ways and found that dried nori contains a substantial amount of active B12, not the look-alikes in spirulina that the body cannot use. Plants do not make B12 themselves, and the B12 in nori is thought to come from bacteria associated with the alga.
Evidence · Containing it is not covering it
The amount can differ severalfold between species and between ways of drying and storing, and in some samples the B12 breaks down into inactive look-alikes; the UK Vegan Society and other bodies still list seaweeds, nori included, as unreliable B12 sources. A small open-label trial in 2024, with 30 vegetarians over 4 weeks, found that eating roasted nori improved serum B12 and related markers. That is a positive signal, but it is one small, short trial that still needs repeating, and it is not enough to justify stopping a supplement. So nori is a bonus, not a plan.
←Lamb
B12 comes almost only from animal foods, and lamb is a reliable source; people who eat no animal foods at all need to take it separately.
Mechanism · Animals do not make B12 themselves
Only microbes can make B12. Sheep, too, get it from the microbes in their rumen and store it in meat and liver, so lamb is a source, not a producer; that is why strict vegetarians need to take B12 separately. Once eaten, B12 still needs stomach acid and intrinsic factor to be absorbed, and in people short of acid or of intrinsic factor, no amount eaten gets in.
Myth · Grass-fed lamb is not an omega-3 source
Grass-fed lamb fat has slightly more omega-3 than grain-fed lamb fat, but the absolute amounts are tiny, so do not count lamb as an omega-3 source; that is the job of fatty fish such as salmon. For B12, iron and zinc, lamb is much like beef.
Seafood is generally a good source of B12. White fish is not the richest seafood, but it is mild, cheap and the easiest to eat every day, and one serving meets more than half of a day's need.
Mechanism · Only microbes make it
Only bacteria and archaea can make B12. The sea is full of B12-making microbes, and B12 passes up the food chain step by step, so seafood is generally a good source while plant foods have almost none. Once eaten, it still has to pass the three steps of stomach acid, intrinsic factor and the end of the small intestine.
Numbers · The B12 in a serving of cod
Raw cod has about 0.9 µg of B12 per 100 g, more by weight once cooking drives off water, and one serving meets more than half of a day's need. It is lower than shellfish such as oysters and clams, but it is mild, cheap and easy to eat every day, and something you can keep eating is worth more than the highest number.
←Milk
B12 comes almost only from animal foods. A 250 ml glass of milk supplies about 45% of an adult's recommended daily amount, which makes milk an important source for lacto-ovo vegetarians, who eat no meat.
Mechanism · Three steps after you drink it
Only bacteria and archaea make B12; cows get it from the microbes in their rumen and store it in meat and milk. The B12 in milk is dissolved in the watery part, so skimming does not remove it. After you drink it, B12 still has three steps to pass before it reaches the blood: stomach acid frees it from protein, intrinsic factor (an escort protein made by the stomach) picks it up, and the last part of the small intestine takes the pair in.
Numbers · The B12 in one glass
Milk has about 0.45 μg of B12 per 100 g; a 250 ml glass supplies about 45% of the adult Recommended Dietary Allowance (RDA). Milk supplies B12, calcium and protein, but not iron: it has almost no fiber, very little vitamin C and little iron, so adults cannot rely on milk for iron.
synergy · 3
Homocysteine in the blood is managed by three B vitamins together: B12 and folate turn it back into methionine, and B6 sends it toward making cysteine. A shortage of any one of the three can raise homocysteine.
Betaine (TMG) is the methyl donor on the backup route. When the main route is limited by low B12 or folate, or by an MTHFR gene variant, betaine can take over and turn homocysteine back into methionine. In trials, betaine supplements lowered fasting homocysteine in healthy people.
In the methylation cycle that B12 takes part in, the step that turns methionine into the methyl donor SAM uses magnesium. The mechanism predicts that long-term magnesium shortage could make the metabolic effects of low B12 more pronounced; this has not been measured directly in people.
cofactor · 4
B12 and folate work together in one reaction: turning homocysteine back into methionine, which feeds methylation throughout the body and supplies material for making DNA. A shortage of either one makes red blood cells large and few. High-dose folic acid can correct that anemia yet cannot stop the nerve damage of B12 deficiency, so the cause of the anemia has to be found first.
Vitamin B12 deficiency damages the myelin sheath that wraps nerve fibers, and the spinal cord pathways for position sense and movement are hit first; this is called subacute combined degeneration. The sooner B12 is replaced, the better the chance the nerves recover; left too long, the damage can become permanent. Methylmalonic acid (MMA) in the blood is a very sensitive marker for catching deficiency early.
Once folate has turned a one-carbon unit into the methyl form, the only way that methyl group can be passed on is through an enzyme that depends on vitamin B12. Without enough B12, methyl-folate gets stuck: folate in the blood looks fine, yet the building blocks for DNA run out. This is called the methyl trap, and it is why folate can hide the anemia of B12 deficiency but cannot prevent the nerve damage.
←Zinc
Making stomach acid uses a zinc-containing enzyme, and B12 in food depends on stomach acid to be freed from protein. The mechanism predicts that severe zinc deficiency could indirectly affect B12 release; this step has not been measured directly in people.
antagonism · 1
Most of the B12 in spirulina is pseudo-B12 (an analogue): it looks much like real B12 but the body cannot use it, and it may interfere with using real B12. So relying on spirulina for B12 on a vegan diet is dangerous and, left long enough, can lead to nerve damage; use a B12 supplement or fortified foods instead.
depletes · 3
Long-term metformin use goes with low B12; one explanation is that it interferes with the calcium-dependent step of B12 uptake in the ileum. In DPPOS the risk of B12 deficiency was about 1.13 times higher for each year of use (about 13% higher), so periodic B12 checks are routine monitoring.
Metformin is a first-line drug, cheap, with decades of safety data; one of its costs is that long-term use lowers vitamin B12, generally thought to be by interfering with calcium-dependent absorption at the end of the small intestine. In a large follow-up study, 4.3% of people on metformin had low B12 at 5 years, versus 2.3% on placebo. That is not a reason to stop the drug; it is a reason to monitor.
↮GERD
Long-term acid suppression has a real cost: the vitamin B12 in food is bound to protein, and stomach acid and pepsin have to free it first. With acid suppressed, that step struggles; in a large case-control study, people who had used proton pump inhibitors for two years or more had a higher risk of B12 deficiency, so long-term users should have their B12 monitored.
regulates · 4
→Blood Work · Specialized Markers
B12 is a helper in the step that turns homocysteine back into methionine; without it, homocysteine backs up in the blood, which makes low B12 one of the most common and most easily fixed causes of a high level, especially in older adults and long-term vegetarians. But the marker is sensitive, not specific: low folate or B6, or reduced kidney function, raise it too, so it has to be read with methylmalonic acid (MMA). And check B12 before taking folate, or the anemia is hidden while nerve damage carries on.
Plant-based eating is a spectrum, and the further toward vegan it goes, the less B12 has a reliable food source: plants contain no reliable B12, and vegans who do not supplement will sooner or later run short. Eating healthily does not make it enough on its own; it takes supplements or fortified foods, with periodic checks of methylmalonic acid (MMA) or homocysteine.
Vitamin B12 deficiency is a routine item in a fatigue work-up, and the high-risk groups are easy to name: older adults, vegans, and people on long-term metformin or proton pump inhibitors (PPIs, a class of acid-suppressing drugs). Once found and corrected, it is a reversible cause and should not be counted as chronic fatigue.
With age the stomach lining often wastes away and makes less acid, so the B12 in food cannot be stripped free: a review reports that more than 20% of older people are short of B12, and in more than six cases in ten it was eaten but never released from the food. Long-term use of acid-reducing medicines is also linked to low B12.
contrast · 1
Chlorella is often sold to vegans as a source of B12. Testing shows that the cobalamin in most chlorella tablets is mainly genuine B12, but the amount varies enormously between products, from almost none to a great deal, and some also contain inactive look-alikes. So it cannot replace a B12 supplement: people eating mostly plants are safer with a B12 supplement that states its dose, or with fortified foods.