Where is vitamin K2 synthesized? Unpacking the microbial origin

November 27, 2025 •

4 min read

While vitamin K1 is famously produced by plants, the vast majority of vitamin K2 is synthesized exclusively by bacteria. This microbial origin is crucial to understanding where is vitamin K2 synthesized and why its sources differ significantly from other vitamins.

Quick Summary

Vitamin K2, or menaquinone, is created by bacteria in the gut microbiome and during food fermentation. This includes fermented soy (natto) and certain cheeses. One form, MK-4, is also converted in animal tissues from vitamin K1.

Key Points

Bacterial Synthesis: The primary synthesis of vitamin K2 is performed by bacteria, both in fermented foods and within the human gut microbiome.
Fermented Food Sources: Foods like natto (fermented soybeans) and aged cheeses are rich sources of different menaquinone (MK-n) subtypes of vitamin K2 due to bacterial fermentation.
Gut Bacteria Contribution: While gut microbes synthesize menaquinones, their contribution to overall human vitamin K status is often less significant than dietary intake due to absorption limitations in the colon.
Animal Tissue Conversion: The MK-4 form of vitamin K2 is an exception, as it is synthesized in animal tissues, including humans, by converting dietary vitamin K1.
Dual Pathways: The presence of different K2 forms (e.g., MK-4 from animal products and MK-7 from fermented foods) stems from these distinct production pathways.
Best Dietary Absorption: Long-chain menaquinones like MK-7 from fermented foods are absorbed more efficiently than the K2 synthesized in the colon.

The Microbial Factories of Vitamin K2

Unlike vitamin K1, the plant-based form, vitamin K2 (menaquinone) is fundamentally a product of bacterial synthesis. This microbial process occurs in two primary locations: within the gastrointestinal tract of humans and animals, and during the fermentation of certain foods. These bacterial powerhouses are key to generating the various subtypes of K2, known as menaquinones (MK-n).

The Role of Your Gut Microbiome

Your intestinal tract is home to trillions of bacteria, a diverse community known as the gut microbiome. Many of these microorganisms are capable of producing menaquinones as part of their metabolic process.

Specific Bacterial Genera: Research has identified several bacterial species within the human gut capable of synthesizing vitamin K2, including Bacteroides and certain strains of Lactobacillus and Bifidobacterium. For instance, Bacteroides species can produce long-chain menaquinones like MK-10 and MK-11.
Location and Absorption: The majority of this bacterial production occurs in the distal colon. However, the bioavailability of this menaquinone is not as efficient as dietary intake because the primary absorption site, the small intestine, is further up the gastrointestinal tract. While gut bacteria contribute to overall vitamin K status, dietary sources are considered the most reliable and primary way to ensure adequate intake.

Vitamin K2 from Fermented Foods

The most potent and bioavailable sources of vitamin K2 come from fermented foods, where specific bacteria are used to create the nutrient during the fermentation process.

Natto: The traditional Japanese food natto, made from fermented soybeans, is famously the richest dietary source of vitamin K2, specifically the MK-7 subtype. The fermentation is performed by the bacterium Bacillus subtilis natto.
Cheeses: The bacteria used to ripen certain types of cheese, especially aged varieties, also produce menaquinones. Gouda, Brie, and Edam are notable examples containing various long-chain menaquinones (MK-5 through MK-9).
Sauerkraut: Fermented cabbage is another source, though the K2 concentration is typically much lower than in natto.

The Unique Case of MK-4

Not all forms of vitamin K2 are produced by the same bacterial synthesis route. The menaquinone subtype MK-4 is an exception. It is created in the tissues of humans and animals via a conversion process. This occurs by modifying dietary vitamin K1 (phylloquinone) or synthetic menadione into MK-4. This conversion happens in various organs, including the brain, kidneys, and pancreas, explaining why animal products contain this specific K2 subtype.

K2 Synthesis in the Body vs. Diet

Determining how much your gut bacteria contribute to your vitamin K status is challenging due to absorption inefficiencies. For most people, dietary intake remains the most crucial source for obtaining functional vitamin K2.


Comparison of Vitamin K2 Synthesis and Sources	Feature	Gut Bacteria (Endogenous)	Fermented Foods (Exogenous)	Animal Tissue (Conversion)
Primary Mechanism	Bacterial metabolic processes	Bacterial fermentation of food	Conversion of vitamin K1 or menadione
Primary Location	Distal colon	Fermentation vats (e.g., natto, cheese)	Various animal tissues (e.g., liver, pancreas)
Key Subtypes	MK-6, MK-7, MK-8, MK-10, MK-11	Primarily MK-7 (natto), MK-8/MK-9 (cheese)	MK-4
Bioavailability	Poorly absorbed, as it's synthesized past the main absorption site	Generally excellent, especially MK-7	Good, as it's stored and utilized in tissues
Primary Source for Humans	Contributes, but major reliance is on diet	Important dietary source for long-chain menaquinones	Major dietary source for MK-4, found in animal products

Dietary Significance and Probiotic Potential

The realization that vitamin K2 is bacterially synthesized has led to increased interest in how food and gut health can be leveraged for better nutrition. High-quality, fermented products are gaining recognition not just for their probiotic content, but also for their direct contribution of bioavailable vitamin K2. In contrast, the MK-4 found in animal products comes from a different pathway, making it important to consume a variety of sources for a full spectrum of menaquinones.

Conclusion: A Tale of Two Pathways

The synthesis of vitamin K2 is a fascinating story involving both microbial and metabolic processes. While bacteria in the gut contribute, their overall impact is debated due to absorption challenges in the colon. The most reliable pathway for humans to obtain bioavailable, long-chain menaquinones (like MK-7) is through the consumption of fermented foods, such as natto and certain cheeses. Simultaneously, the MK-4 subtype is produced within animal and human tissues by converting other forms of vitamin K. This dual synthesis pathway highlights the importance of a varied diet, including both fermented and animal products, to ensure adequate intake of this crucial nutrient for bone and cardiovascular health. Future research into optimal microbial production and absorption may refine our understanding of how to best harness this unique, bacteria-driven vitamin. For more information on dietary sources, visit the Oregon State University's Linus Pauling Institute website.

Frequently Asked Questions

Yes, bacteria in the human gut, particularly in the colon, can synthesize vitamin K2. However, the absorption from this source is poor, so it does not significantly contribute to overall vitamin K requirements compared to dietary sources.

Many bacterial species can synthesize menaquinones. In the human gut, bacteria from the Bacteroides genus are notable producers. For commercial and food production purposes, Bacillus subtilis natto is a key strain used for producing MK-7, especially in natto.

The MK-4 menaquinone is produced in animal tissues (and in humans) by converting dietary vitamin K1. This process is different from the bacterial synthesis that produces other menaquinone subtypes like MK-7.

Fermented foods like natto and certain cheeses are excellent sources of vitamin K2 because they contain bacteria that actively produce menaquinones during the fermentation process. These menaquinones are then consumed and readily absorbed.

Leafy greens contain vitamin K1, not vitamin K2. However, some of this K1 can be converted into the MK-4 form of vitamin K2 in animal and human tissues. For higher levels of other K2 subtypes, fermented foods and animal products are better sources.

The main reason for poor absorption is location. Most menaquinone production by gut bacteria occurs in the large intestine, whereas the optimal absorption of vitamin K happens further up, in the small intestine, and requires bile salts.

Yes, but it's important to distinguish the forms. K2 is found primarily in fermented foods (plant-based) and animal products, while vitamin K1 is concentrated in green leafy vegetables (plant-based).