What Vitamin Comes From Human Feces? Unveiling the Facts

December 28, 2025 •

4 min read

Approximately 30-50% of human feces is composed of bacteria, viruses, and other microorganisms that reside in the gut. While these gut microbes are known to synthesize a variety of vitamins, particularly B vitamins and vitamin K, the location of this synthesis and the human digestive process prevent them from becoming a reliable source for human health.

Quick Summary

Gut bacteria produce vitamins, but this synthesis occurs mainly in the colon, preventing absorption in the small intestine. The vitamins are mostly used by the bacteria themselves or excreted, making feces an unviable and dangerous source of nutrients for humans.

Key Points

Gut Bacteria Produce Vitamins: Gut microbes, primarily in the large intestine, are known to synthesize vitamins, including vitamin B12 (cobalamin) and vitamin K2 (menaquinone).
Absorption Location is Key: Most bacterial vitamin synthesis occurs in the colon, but the body absorbs vitamin B12 mainly in the small intestine (ileum).
Vitamins are Excreted: Due to the separation of synthesis and absorption sites, most bacterially produced vitamins are excreted in feces and are not bioavailable to the human host.
Analogue Forms are Common: A large percentage of the corrinoids produced by gut bacteria are inactive forms of B12, rendering them useless for human metabolism.
Consuming Feces is Dangerous: Engaging in coprophagy (feces consumption) to obtain vitamins is extremely dangerous and exposes individuals to a host of harmful bacteria and pathogens.
Diet is the Safe Source: The reliable and safe way for humans to acquire vitamin B12 and vitamin K is through diet (animal products, fortified foods) and supplements.
Humans Differ from Ruminants: Unlike ruminants with different digestive systems, humans cannot rely on their gut bacteria for a meaningful vitamin supply.

The Gut Microbiome: A Vitamin Factory

The human digestive tract contains trillions of microbes, collectively known as the gut microbiome, which performs a wide array of functions essential for health. Some of these microorganisms, including certain species of Lactobacillus and Bifidobacterium, are capable of synthesizing vitamins. This microbial vitamin production is significant, and researchers have estimated that it could contribute a notable percentage of the daily recommended intake for some B vitamins in adults. However, the crucial distinction lies in where this production takes place and how the human body is designed to absorb nutrients.

The Vitamin Connection: B12 and Vitamin K

Among the vitamins produced by gut bacteria, vitamin B12 and vitamin K are the most prominent examples. Vitamin B12, or cobalamin, is exclusively synthesized by certain bacteria and archaea; neither plants nor animals can produce it. The synthesis of vitamin K2 (menaquinone) is also a function of gut bacteria. While this might sound like a symbiotic win-win, the human digestive system's architecture presents a critical problem for absorption.

The Anatomy of Non-Absorption

Location, Location, Location: The vast majority of bacterial vitamin synthesis happens in the large intestine (colon).
The Problem: The primary absorption site for vitamin B12 is the small intestine, specifically the ileum, which comes before the colon in the digestive tract.
The Result: The B12 and other vitamins synthesized by bacteria in the colon are excreted in feces before the body can absorb and utilize them effectively.

Inactive Analogues and Microbial Competition

Even if the location wasn't a barrier, a significant portion of the corrinoids produced by gut bacteria are inactive analogues, also known as pseudovitamin B12, rather than the active form the human body needs. A 2008 study found that in people with a consistent B12 intake, over 98% of the B12 analogue in their stool was inactive. Furthermore, there is fierce competition for these nutrients within the microbiome itself, as other non-producing bacteria rely on them for their own metabolism.

Why Feces Are Not a Nutritional Source

Beyond the physiological inability to absorb these vitamins, deliberately consuming feces, a practice known as coprophagy, is extremely dangerous. It is not an alternative source of nutrition and carries significant health risks, including exposure to pathogens.

Comparison: Human vs. Ruminant Digestion

The human digestive process differs significantly from that of ruminant animals, such as cows and sheep, which are able to utilize the vitamins produced by their gut bacteria. This is primarily due to differences in digestive anatomy.


Feature	Human	Ruminant (e.g., Cow)
Primary Digestion	Stomach and small intestine	Rumen (a specialized stomach chamber)
Microbial Vitamin Synthesis	Occurs mainly in the large intestine (colon)	Occurs in the forestomachs (e.g., rumen)
Location of Absorption	Small intestine (before vitamin synthesis)	Intestine (after microbial synthesis)
Vitamin Availability to Host	Low due to synthesis location and excretion	High; synthesized vitamins are absorbed downstream
Risk of Coprophagy	Extremely high (exposure to pathogens)	Natural behavior for nutrient recycling (though not a primary vitamin source)

The True Dietary Sources of B12 and Vitamin K

Given that human feces are not a viable source of these essential vitamins, a healthy diet remains the most reliable method for obtaining them. For vitamin B12, the primary sources are animal products or fortified foods. For vitamin K, dietary intake, especially through fermented foods for K2, is vital.

Vitamin B12 (Cobalamin)

Animal Sources: Meat (especially liver), poultry, eggs, fish, and dairy products naturally contain active vitamin B12.
Fortified Foods: Many cereals, nutritional yeast products, and plant-based milk alternatives are fortified with B12, making them reliable sources for vegans and vegetarians.
Supplements: Oral supplements or injections are available for those with dietary restrictions or absorption issues like pernicious anemia.

Vitamin K2 (Menaquinone)

Fermented Foods: Fermented foods like natto (fermented soybeans) and some hard cheeses are rich sources of vitamin K2.
Animal Products: Organ meats, meat, and high-fat dairy products contain vitamin K2.

Conclusion

In summary, while it is a fascinating biological fact that our gut microbiome produces vitamins like B12 and K, the digestive system's architecture makes them largely unavailable to the human body. The vitamins are synthesized in the colon, beyond the main site of absorption in the small intestine, and are subsequently excreted. The notion that human feces could be a source of vitamins is a dangerous misconception due to the significant risk of consuming harmful pathogens. For these vital nutrients, humans must rely on a healthy diet of animal products, fermented foods, and fortified products, or turn to supplements when necessary. It is crucial to understand the limitations of our own digestive physiology and to rely on safe, proven methods to meet our nutritional needs.

VeganHealth.org provides additional detailed information on this topic, particularly concerning vitamin B12 sources for individuals on plant-based diets.

Frequently Asked Questions

Bacteria in the human colon synthesize several B vitamins and vitamin K2 (menaquinone). The most notable is vitamin B12, as humans cannot produce it, but it is not absorbed from the colon.

No, not effectively. While gut bacteria produce vitamins, the production location in the large intestine is past the primary absorption site for most vitamins, which is the small intestine.

The primary reason is the mismatched location of production and absorption. The vitamins are synthesized in the colon, while the main absorption occurs upstream in the small intestine, leading to excretion.

Attempting to get vitamins from feces is extremely dangerous and poses a high risk of exposure to pathogenic bacteria, viruses, and other microorganisms that can cause severe illness or death.

Ruminants have a different digestive system with a specialized chamber (the rumen) where bacteria produce vitamins. Absorption happens later in their digestive tract, allowing them to utilize the bacterially-produced nutrients.

The safest sources of vitamin B12 are animal products such as meat, fish, and dairy, as well as fortified foods like cereals and nutritional yeast. Supplements are also a reliable option.

While vegan gut bacteria can produce B12, the synthesis location prevents meaningful absorption by the host. Therefore, vegans must obtain B12 from fortified foods or supplements to avoid deficiency.

Yes, studies have shown that a very high percentage of the B12-like compounds found in human stool are inactive analogues, which cannot be used by the human body for metabolic functions.