Vitamin B12 comes from bacteria. No plant or animal actually manufactures this vitamin on its own. Specific species of bacteria and archaea are the only organisms on Earth that can build the B12 molecule from scratch, and every other living thing gets it secondhand through the food chain or, in some cases, from bacterial partners living inside their bodies.
Bacteria Are the Only True Producers
B12 synthesis is limited to a small number of bacterial and archaeal species. These microbes build the vitamin through one of two biochemical pathways: an oxygen-dependent route used by aerobic bacteria, and an oxygen-free route used by anaerobic bacteria and archaea. The key species include Pseudomonas denitrificans on the aerobic side, and Propionibacterium shermanii and Bacillus megaterium on the anaerobic side. These same organisms form the basis of industrial B12 production for supplements and fortified foods, since chemical synthesis of B12 is impractical due to the molecule’s complexity.
How Animals Accumulate B12
Ruminant animals like cows, sheep, and goats are particularly good at producing B12, but they don’t do it themselves. Bacteria living in the rumen (the first chamber of their multi-compartment stomach) synthesize B12 during fermentation of plant material. There’s a catch: these bacteria need the mineral cobalt to build the vitamin. If cobalt in the animal’s feed drops below about 0.07 mg per kilogram, B12 production stalls. Only 3 to 13 percent of the cobalt a ruminant eats actually gets converted into B12 by rumen bacteria, so adequate cobalt in soil and pasture matters.
Soil cobalt levels directly affect how much B12 ends up in animal tissue. Research in livestock found a significant positive correlation between cobalt concentrations in soil and B12 levels in the animals’ livers. Heavy metals like zinc and lead compete with cobalt for uptake by plants, which means heavily fertilized or contaminated soils can reduce the amount of cobalt available to grazing animals, ultimately lowering B12 in the meat and milk humans consume. Cattle grazing on cobalt-deficient soils develop a condition called “phalaris staggers,” a neurological disorder caused by insufficient B12.
Non-ruminant animals like chickens, pigs, and fish don’t produce B12 in the same way. They accumulate it from their diet, which traces back to bacteria in soil, water, or feed. Fish and shellfish get their B12 through the marine food chain, where bacteria produce the vitamin and it concentrates as it moves up through plankton, small fish, and predators.
B12 in the Ocean
In marine environments, B12 cycles through a web of microbial relationships. Many phytoplankton (the tiny photosynthetic organisms at the base of the ocean food chain) need B12 but cannot make it. They depend on B12-producing bacteria living in the water around them. Species like Ruegeria pomeroyi produce B12 and appear to release it in response to chemical signals from their phytoplankton partners, creating a mutual exchange where bacteria get carbon compounds from algae and algae get B12 from bacteria. This microbial partnership seeds the entire marine food chain with B12, which is why seafood tends to be rich in the vitamin.
Your Gut Makes B12 You Can’t Use
Here’s something that surprises most people: bacteria in your own large intestine produce B12. One estimate suggests that about 42% of human gut bacteria have the genetic machinery to synthesize the vitamin, potentially producing roughly a third of your daily needs. The problem is location. Your body absorbs B12 in the ileum, which is the lower part of the small intestine, well upstream of where these bacteria live in the large intestine. B12 transporters exist only in the small intestine, and passive absorption in the colon accounts for just 1 to 2 percent of a dose. To make matters worse, gut bacteria in the large intestine convert about 80% of any B12 that reaches them into inactive analogs your body can’t use.
This is why you need to get B12 from food or supplements, even though your own body technically hosts B12-producing bacteria. Humans absorb roughly 50% of a 1-microgram oral dose of B12, with absorption rates dropping as the dose increases.
Food Sources That Actually Deliver B12
The richest dietary sources are animal products: organ meats (especially liver), shellfish like clams and mussels, fish, beef, dairy, and eggs. Adults need 2.4 micrograms per day, rising to 2.6 mcg during pregnancy and 2.8 mcg while breastfeeding. Absorption from common foods is reasonably efficient. Studies measuring B12 uptake from milk and fortified bread found absorption rates around 55 to 65 percent, comparable to B12 dissolved in water.
For plant-based eaters, the options are limited but not zero. Most plants contain no B12. The notable exceptions are certain algae and an aquatic plant called Mankai (Wolffia globosa), a type of duckweed. Mankai contains about 3.2 micrograms of authentic B12 per 100 grams of dry weight, and importantly, testing confirmed it contains true B12 rather than inactive analogs. The B12 concentration stays stable across seasons.
Purple laver seaweed, the type used to make nori sheets in sushi, also contains true B12 rather than the inactive pseudovitamin B12 found in many other algae. A clinical trial in vegetarians found that eating just 5 grams of nori daily for four weeks raised serum B12 levels by an average of 59 pmol/L compared to a control group. This is a meaningful improvement, though the effect plateaued at that low dose. Spirulina, by contrast, contains roughly three parts pseudo-B12 for every one part real B12, making it an unreliable source.
How Supplements Are Made
Every B12 supplement on the market is produced by bacterial fermentation, not chemical synthesis or animal extraction. The industrial process grows specific bacterial strains in large fermentation tanks, then extracts and purifies the B12 they produce. Pseudomonas denitrificans and Propionibacterium shermanii are the two workhorses of commercial production. This means B12 supplements are inherently vegan, regardless of what form (cyanocobalamin, methylcobalamin, or others) appears on the label. The same fermentation process supplies the B12 used to fortify cereals, plant milks, nutritional yeast, and other enriched foods.
Why Cobalt Ties the Whole Story Together
B12 is sometimes called cobalamin because it contains a single atom of cobalt at its core. This is the only known biological role of cobalt in human nutrition. The entire chain of B12 production, from soil bacteria to ruminant stomachs to your dinner plate, depends on cobalt being available in the environment. Soils naturally vary in cobalt content, and modern agriculture can deplete it or introduce competing heavy metals that reduce cobalt availability. In regions with cobalt-poor soils, livestock require cobalt supplementation in their feed to maintain adequate B12 levels in their meat and milk. This is a routine practice in commercial farming worldwide.