Where does B12 come from in your body?

December 28, 2025 •

3 min read

Unlike many other vitamins, the human body cannot produce its own vitamin B12. The definitive answer to where does B12 come from in your body lies not within our own cells, but in a fascinating process that involves microorganisms, animal products, and fortified foods.

Quick Summary

The human body cannot synthesize its own B12; it must be obtained from external sources derived from bacteria, such as animal products or fortified foods. Effective absorption relies on a multi-step process involving a specific protein called intrinsic factor.

Key Points

Bacterial Synthesis: Vitamin B12 is produced exclusively by certain bacteria and archaea, not by human cells.
External Sources Required: Humans must obtain B12 from dietary sources like animal products, which accumulate the vitamin from bacteria.
Intrinsic Factor is Key: A protein called intrinsic factor, made in the stomach, is essential for absorbing B12 in the small intestine.
Internal Production is Wasted: Though B12-producing bacteria exist in the human large intestine, the production site is too far down the digestive tract for effective absorption.
Risk Factors for Deficiency: Vegans, vegetarians, the elderly, and individuals with digestive disorders are at higher risk of deficiency.
Liver Stores: The body has a substantial storage capacity for B12 in the liver, which can delay the onset of deficiency symptoms for years.
Modern Sources: Fortified cereals, nutritional yeast, and supplements are reliable B12 sources, especially for those on plant-based diets.

The Bacterial Origins of B12

While humans cannot synthesize vitamin B12 (cobalamin), it is produced naturally by certain bacteria and archaea. This production forms the basis of the entire food chain's B12 supply. Humans acquire this essential nutrient either directly or indirectly from these microorganisms, as it accumulates in other animals and fortified products.

The Role of Animals and Bacteria

Many animals, particularly herbivores like cows and sheep, have a symbiotic relationship with B12-synthesizing bacteria in their stomach or foregut. These bacteria produce B12, which the animals then absorb and store in their muscles and liver, or pass into their milk. When humans consume these animal products, we, in turn, acquire the B12 that was originally made by the bacteria. In aquatic environments, bacteria also produce B12, which is consumed by phytoplankton. This B12 then moves up the food chain to fish and shellfish.

The Human Gut and Unusable B12

Some B12-producing bacteria also live in the human large intestine (colon). However, this internal production is not a reliable source for humans because absorption of B12 occurs much earlier in the digestive tract, primarily in the small intestine. By the time the B12 is produced in the colon, it is too late to be absorbed and instead is excreted. This anatomical disconnect is why humans must consistently consume B12 from external sources.

The Journey of B12 Absorption

For dietary B12 to be used by the body, it must navigate a complex series of steps within the digestive system.

The B12 absorption pathway:

Oral Stage: When B12 is ingested, it is bound to food proteins. In the mouth, saliva contains haptocorrin, a carrier protein that binds to B12.
Stomach Stage: In the stomach, the B12 is released from its food protein by hydrochloric acid and pepsin. This free B12 then binds to haptocorrin. The stomach's parietal cells also secrete another crucial protein called intrinsic factor.
Duodenum and Ileum Stage: As the food moves to the duodenum, pancreatic enzymes break down the B12-haptocorrin complex. This frees the B12, which then immediately binds to intrinsic factor. The newly formed B12-intrinsic factor complex travels to the final section of the small intestine, the ileum.
Absorption and Transport: Specialized receptors in the ileum recognize the B12-intrinsic factor complex and absorb it into the bloodstream. The B12 is then transported throughout the body via another protein, transcobalamin.

B12 Sources: Food and Supplements

Since our bodies cannot create B12, it must be obtained from the diet. The most reliable sources are animal products. However, with modern agricultural practices and diets, fortified foods and supplements have become equally important, especially for those following plant-based diets.

B12 sources comparison


Source Type	Examples	Key Takeaways	Bioavailability (as per various studies)
Animal Products	Meat (especially liver and clams), fish, poultry, eggs, dairy	Excellent natural source, B12 is stored in the animal's tissue.	Varies by source; e.g., dairy often more bioavailable than meat/poultry.
Fortified Foods	Cereals, nutritional yeast, plant-based milks	Reliable for vegans and vegetarians. Contain free B12, often absorbed more easily.	Absorption may be higher than from food, especially in older individuals.
Supplements	Oral tablets, injections, sublingual forms	Crucial for those with dietary restrictions or malabsorption issues.	Absorption from high doses relies on passive diffusion and is inefficient.

The Criticality of Absorption

Even with sufficient dietary intake, absorption issues can lead to B12 deficiency. Conditions affecting the stomach or small intestine, such as atrophic gastritis (reducing stomach acid) or pernicious anemia (lacking intrinsic factor), can impede the absorption process. This makes regular intake of B12 from supplements or fortified foods essential for at-risk groups, including the elderly and those with certain medical conditions.

Conclusion: The External Necessity

The short answer to "where does B12 come from in your body?" is that it doesn't. Our reliance on external, bacterially-derived sources highlights an interesting evolutionary quirk. From the symbiotic relationship between bacteria and ruminants to the targeted fortification of modern foods, understanding this process is key to preventing deficiency. By consuming a varied diet rich in animal products or fortified plant-based alternatives and addressing any underlying absorption issues, we can ensure a steady supply of this vital nutrient.

For more detailed information on vitamin B12, consult the National Institutes of Health Office of Dietary Supplements fact sheet.

Frequently Asked Questions

No, the human body cannot produce its own vitamin B12. It must be obtained from external sources, primarily through diet.

While some gut bacteria in the large intestine produce B12, the absorption of this vitamin occurs in the small intestine. The B12 produced in the colon is too far down the digestive tract to be effectively absorbed.

Intrinsic factor is a protein secreted by the stomach's parietal cells that binds with B12. This complex is then absorbed in the small intestine. Without intrinsic factor, B12 cannot be properly absorbed.

For those who consume animal products, the best natural sources include meat, fish (especially clams and salmon), eggs, and dairy products like milk, yogurt, and cheese.

Reliable sources for vegans and vegetarians include fortified foods like breakfast cereals, nutritional yeast, and plant-based milks. Supplements are also a crucial and reliable source.

B12 deficiency can lead to a range of symptoms, including fatigue, headaches, neurological problems, and a type of anemia called megaloblastic anemia. Severe deficiency can cause permanent nerve damage.

Because the body stores a significant amount of vitamin B12 in the liver, it can take several years for a deficiency to manifest after intake stops or is reduced.