Does the Human Body Produce Vitamin B12?
No, the human body does not produce vitamin B12—it must be obtained entirely from dietary sources or supplementation. 1, 2, 3
Why Humans Cannot Synthesize B12
Vitamin B12 (cobalamin) is synthesized exclusively by prokaryotic organisms (bacteria and certain microorganisms), not by any eukaryotic organisms including humans, animals, plants, or fungi. 1, 2, 3
Humans are completely dependent on exogenous dietary intake because we lack the enzymatic machinery required for cobalamin synthesis. 1, 4
The vitamin is produced by bacteria in the gastrointestinal tracts of ruminant animals (dependent on soil cobalt content), which is why animal products become our primary dietary sources. 1, 4
Dietary Sources Required
Since the body cannot manufacture B12, humans must obtain it from:
Animal-based foods: ruminant meat, organs, milk and dairy products, fish, shellfish, and eggs 1, 5, 3
Fortified foods: cereals and nutritional yeast for those avoiding animal products 1
Supplementation: particularly critical for vegetarians, vegans, and those with malabsorption conditions 5, 3
Clinical Implications of This Dependency
Vegetarians and vegans are at high risk for deficiency because edible plants and mushrooms rarely contain considerable amounts of cobalamin. 1, 3
The body can store B12 reserves (approximately 2,500 μg, mainly in the liver) that last 12-36 months without sufficient intake, which can delay recognition of dietary deficiency. 1
Daily requirements are relatively low (2.4 μg/day per DRI, or 4 μg/day per EFSA) compared to other vitamins, but the requirement for external sources is absolute. 1, 4
Common Pitfall to Avoid
Do not confuse the presence of bacteria in the human gut with endogenous B12 production. While bacteria in our intestines can synthesize B12, this occurs in the colon—beyond the terminal ileum where B12 absorption takes place—making this bacterial production unavailable for human use. 1
Human B12 status depends entirely on dietary intake, proper absorption mechanisms (gastric acid, intrinsic factor, and intact terminal ileum), and adequate transport proteins. 1, 4, 6