Can Vitamin B12 Be Synthesized by the Human Body? The Scientific Answer

November 16, 2025 •

3 min read

According to the National Institutes of Health, vitamin B12 is a crucial nutrient that the human body does not make, relying instead on external sources. This essential vitamin, also known as cobalamin, is vital for nerve function, red blood cell formation, and DNA synthesis.

Quick Summary

Humans cannot synthesize vitamin B12 due to a lack of necessary genes, depending entirely on dietary intake or supplements for this vital nutrient.

Key Points

Synthesis is Bacterial: The human body does not produce vitamin B12; it is synthesized exclusively by certain bacteria and archaea found in nature.
Absorption is Limited: While some B12-producing bacteria live in the human colon, the primary site for absorption is the small intestine, meaning most of this B12 is not utilized.
Intrinsic Factor is Crucial: The stomach produces a protein called intrinsic factor, which is necessary to bind and absorb dietary vitamin B12 in the small intestine.
Deficiency Causes Damage: Untreated B12 deficiency can cause serious health issues, including megaloblastic anemia and irreversible neurological damage.
Dietary Sources are Key: Humans must obtain B12 from external sources, predominantly animal products, fortified foods, and supplements.
Risk Groups Require Attention: Individuals at higher risk of deficiency, such as vegans, vegetarians, and older adults, should ensure adequate B12 intake through fortified products or supplements.

A Biological Limitation: Why Humans Can't Produce Vitamin B12

Unlike many plants and animals, the human body lacks the genetic and enzymatic pathways required to synthesize vitamin B12, a complex molecule with a central cobalt atom. This intricate process, involving up to 30 enzymatic steps, is a capability restricted to certain microorganisms, specifically bacteria and archaea. Our evolutionary lineage diverged from relying on internal production, shifting dependence to external, dietary sources.

The Role of Bacteria in B12 Production

Vitamin B12 is exclusively produced in nature by certain bacteria, which are then consumed by animals. For instance, ruminant animals like cattle and sheep have specialized gut bacteria in their forestomachs that produce copious amounts of B12, which the animal then absorbs. Humans also have B12-producing bacteria within their large intestine (colon), but a critical flaw exists in our anatomy. The primary site for B12 absorption is the small intestine (ileum), located upstream from the bacteria in the colon, meaning most of the vitamin synthesized there is passed as waste and not absorbed.

The Intrinsic Factor: A Key to Absorption

For the small amount of B12 that we do consume from food, a specialized protein called intrinsic factor is necessary for absorption. Produced by parietal cells in the stomach, intrinsic factor binds to B12 in the stomach. This new complex travels to the small intestine, where it is absorbed. This dependence on intrinsic factor explains why even individuals who consume enough dietary B12 can develop a deficiency if their body fails to produce this crucial protein, a condition known as pernicious anemia.

Consequences of B12 Deficiency

Without adequate vitamin B12, the body's metabolic pathways, including DNA synthesis and the metabolism of homocysteine, are severely impaired. The liver can store a significant amount of B12, sometimes enough for several years, which is why a deficiency can take a long time to manifest. When stores are depleted, symptoms can range from fatigue and weakness to more severe and irreversible neurological problems.

Common Symptoms of B12 Deficiency

Extreme fatigue and weakness
Pins and needles sensations (peripheral neuropathy)
Shortness of breath and dizziness
A sore and red tongue (glossitis)
Memory loss and cognitive impairment
Mood changes, such as depression or irritability
Pale or yellowish skin

Sources of Vitamin B12 for the Human Body

Since internal synthesis is not an option, humans must actively seek B12 through their diet. Animal products are the richest natural sources, but fortified foods and supplements are also widely available and effective.

Comparison of B12 Sources


Source Type	Examples	Key Takeaways
Animal Products	Meat (beef, liver), fish (salmon, tuna, clams), poultry, eggs, milk, cheese	The most direct and bioavailable natural source. High concentration of B12.
Fortified Foods	Breakfast cereals, plant-based milks (soy, almond), nutritional yeast, meat substitutes	Excellent option for vegans and vegetarians. Offers a reliable and consistent intake.
Supplements	Oral tablets, sublinguals, injections	Critical for those with malabsorption issues (like pernicious anemia) or strict vegan diets. High doses can be absorbed passively even without intrinsic factor.
Certain Plants/Algae	Nori, shiitake mushrooms, tempeh	Contain some B12, but levels can be low or contain B12 analogues that are inactive in the human body. Not considered reliable standalone sources.

Who Is at Risk of Deficiency?

While anyone can develop a deficiency, certain groups are at higher risk. Vegetarians and vegans are vulnerable if they do not use fortified foods or supplements, as plant-based foods do not naturally contain B12. Older adults may have difficulty absorbing B12 due to decreased stomach acid or intrinsic factor production. Additionally, individuals with gastrointestinal conditions like Crohn's disease, celiac disease, or those who have had gastric surgery may experience impaired absorption. Certain medications, such as metformin for diabetes and proton pump inhibitors for acid reflux, can also interfere with B12 absorption. You can read more about B12 from authoritative sources like the NIH Office of Dietary Supplements.

Conclusion

The inability of the human body to synthesize its own vitamin B12 is a fundamental biological reality. We depend on a food chain that relies on bacteria as the ultimate producers of this vital nutrient. Whether through the consumption of animal products, fortified foods, or dietary supplements, understanding our need for external B12 sources is key to preventing deficiency and maintaining long-term health. Given the severe and sometimes irreversible consequences of a deficiency, proactive management of B12 intake is essential, particularly for those following restrictive diets or facing absorption challenges.

Frequently Asked Questions

Most plants and algae do not contain vitamin B12, and those that do, like some seaweeds and mushrooms, often have very low or unstable levels, or B12 analogues that are inactive in humans. They are not considered reliable sources, and experts recommend supplements or fortified foods for those on vegan diets.

The bacteria that produce B12 in humans reside primarily in the large intestine (colon), which is located after the ileum, the section of the small intestine where B12 absorption occurs. This physical separation prevents most of the bacterially-produced B12 from being absorbed into the bloodstream.

Intrinsic factor is a protein made in the stomach that is essential for vitamin B12 absorption. It binds to B12 in the stomach and carries it to the small intestine, where it is absorbed into the bloodstream. Without enough intrinsic factor, B12 cannot be absorbed properly.

Early signs of a B12 deficiency can include fatigue, weakness, a sore tongue, and pins and needles sensations in the hands and feet. Because the body stores B12, it can take years for a deficiency to manifest.

Vegans can obtain sufficient B12 by consuming fortified foods, such as certain breakfast cereals, plant-based milks, and nutritional yeast. Many also opt for a reliable vitamin B12 supplement to ensure adequate intake.

Vitamin B12 is water-soluble, and excess amounts are typically excreted through urine, making it less toxic than fat-soluble vitamins. However, high doses may lead to side effects like headaches or rashes in some individuals.

Pernicious anemia is an autoimmune condition in which the body attacks the cells that produce intrinsic factor in the stomach. This leads to impaired vitamin B12 absorption, resulting in a B12 deficiency and, eventually, a type of anemia.

Yes, some medications that reduce stomach acid, such as proton pump inhibitors and H2 blockers, can hinder the absorption of B12 from food over time. Stomach acid is needed to release B12 from the food proteins it is bound to.