What form of vitamin K is made by bacteria in the large intestine?

January 4, 2026 •

3 min read

While most people get a significant amount of vitamin K from their diet, experts believe that gut bacteria also contribute by synthesizing a crucial form of the nutrient. The specific form of vitamin K is menaquinone, also known as vitamin K2, which is produced by the microbiome in the large intestine. This process provides a supplemental, but important, source of the vitamin for the human body.

Quick Summary

The specific form of vitamin K produced by bacteria in the large intestine is menaquinone (vitamin K2). The intestinal microbiome synthesizes various subtypes of menaquinone, although the amount produced alone does not satisfy the body's full vitamin K requirement. This bacterial synthesis complements dietary intake of both K1 and K2.

Key Points

Menaquinone (K2): The form of vitamin K produced by bacteria in the large intestine is menaquinone (vitamin K2), which complements dietary vitamin K intake.
Microbiome Synthesis: Gut bacteria, such as E. coli and Bacteroides, synthesize menaquinone as part of their metabolic processes.
Multiple Subtypes: Different bacterial species produce various menaquinone subtypes (MK-n), which differ in their side-chain length and bioavailability.
Beyond Coagulation: Menaquinone plays a key role in bone health by activating osteocalcin and preventing arterial calcification by activating matrix Gla-protein.
Vulnerable Populations: Newborns, who lack sufficient gut bacteria, are particularly susceptible to vitamin K deficiency, a risk addressed by prophylactic injections at birth.
Maintaining Levels: Long-term antibiotic use can disrupt menaquinone-producing gut bacteria, potentially leading to a deficiency.

The Role of Menaquinones (Vitamin K2)

Vitamin K is a fat-soluble vitamin crucial for several physiological processes, most notably blood coagulation and bone health. The body obtains this vital nutrient from two primary sources: the diet and the gut microbiome. Plant-based sources provide phylloquinone (vitamin K1), whereas bacteria in the large intestine synthesize menaquinones (vitamin K2).

How Gut Bacteria Create Menaquinone

The process of menaquinone synthesis by gut bacteria involves a complex biosynthetic pathway. These bacteria break down substances in the chyme that reaches the large intestine, and in doing so, produce menaquinone as a byproduct of their metabolism. The intestinal wall can then absorb this menaquinone. Some of the most prolific menaquinone-producing bacteria include species like Bacteroides and E. coli, which are common inhabitants of the human gut.

The synthesis is not a simple, single-step process. It involves multiple enzymatic reactions performed by the bacteria.
Different bacterial species synthesize different subtypes of menaquinone, characterized by varying lengths of their isoprenoid side chains. For example, B. subtilis (found in the fermented food natto) is known for producing MK-7, while other bacteria in the large intestine might produce longer-chain menaquinones like MK-10 and MK-11.
While the absorption of bacterially produced menaquinone from the colon is confirmed, its overall contribution to a person's total vitamin K needs is still a subject of scientific debate.
Unlike dietary vitamin K1, which is primarily absorbed in the small intestine, menaquinone produced in the large intestine has to be absorbed from a more distal part of the gut.

Menaquinone vs. Phylloquinone: A Comparison

While both menaquinone (K2) and phylloquinone (K1) are essential forms of vitamin K, they differ in their sources and physiological roles. This comparison helps clarify the importance of both dietary intake and bacterial synthesis.


Feature	Phylloquinone (Vitamin K1)	Menaquinone (Vitamin K2)
Primary Source	Plant-based foods, especially green leafy vegetables.	Synthesized by bacteria in the gut; also found in fermented foods and some animal products.
Chemical Structure	Has a phytyl side chain attached to the naphthoquinone ring.	Contains a varying number of isoprenoid units in its side chain (e.g., MK-4, MK-7).
Absorption Site	Primarily absorbed in the small intestine.	Mostly produced and absorbed in the large intestine, though some dietary K2 is absorbed earlier.
Main Function	Best known for its crucial role in the liver for synthesizing blood clotting factors.	Plays important roles in bone health by activating osteocalcin and in cardiovascular health by preventing arterial calcification.
Bioavailability	Can be low unless consumed with fat.	Subtypes vary in half-life and bioavailability; MK-7 often noted for its high bioavailability.

The Importance of a Healthy Microbiome

For the body to benefit from the menaquinone produced in the large intestine, a healthy and balanced gut microbiome is essential. Long-term antibiotic use can significantly disrupt the populations of menaquinone-producing bacteria, potentially contributing to vitamin K deficiency. Similarly, newborns are particularly susceptible to vitamin K deficiency because their intestinal tracts are not yet fully colonized with these bacteria. This is why a vitamin K injection is routinely administered to newborns. Maintaining a healthy gut through a diet rich in fiber and fermented foods is important for supporting the bacterial populations responsible for synthesizing menaquinone.

Conclusion

The form of vitamin K synthesized by bacteria in the large intestine is menaquinone, also known as vitamin K2. This bacterial production complements the intake of phylloquinone (vitamin K1) from dietary sources, contributing to the body's overall vitamin K status. While the amount produced by gut bacteria may not meet all of a person's needs, it plays a supportive role, especially in maintaining long-chain menaquinone stores. A healthy and diverse gut microbiome is therefore an important, though often overlooked, factor in ensuring adequate vitamin K levels for functions ranging from blood clotting to bone and cardiovascular health.

For more detailed information on vitamin K's functions and different forms, see this comprehensive review from the NIH Office of Dietary Supplements: Vitamin K - Health Professional Fact Sheet.

Frequently Asked Questions

Vitamin K1 (phylloquinone) is found primarily in green leafy vegetables. Vitamin K2 (menaquinone) is produced by gut bacteria and is also found in fermented foods and some animal products. While K1 is important for blood clotting, K2 plays a more prominent role in bone health and preventing arterial calcification.

No, the amount of menaquinone synthesized by bacteria in the large intestine is generally insufficient to meet the body's total vitamin K needs. Dietary intake of both K1 and K2 is also required for optimal vitamin K status.

Newborn babies are at risk because they have not yet developed the intestinal bacteria necessary to produce vitamin K. Additionally, vitamin K is poorly transferred across the placenta and is low in breast milk.

Yes, prolonged use of broad-spectrum antibiotics can kill the beneficial bacteria in the gut responsible for producing menaquinone, potentially contributing to a vitamin K deficiency.

Excellent food sources of menaquinone include fermented products like natto (fermented soybeans) and certain cheeses. Modest amounts are also found in meat, eggs, and dairy products.

To support your gut bacteria, consume a varied diet rich in fiber, which feeds beneficial bacteria. Additionally, consuming fermented foods containing live cultures can help promote a healthy microbiome.

Severe vitamin K deficiency can lead to defective blood coagulation, causing excessive bleeding and bruising. In the worst cases, it can cause life-threatening hemorrhage, especially intracranial bleeding in infants.