Menaquinone: The Bacterially Produced Vitamin K
Vitamin K is a family of fat-soluble compounds that are crucial for blood coagulation and bone health. While plants produce vitamin K1 (phylloquinone), bacteria are the primary producers of vitamin K2 (menaquinone), often abbreviated as MK-n, where 'n' represents the number of repeating isoprenoid units in its side chain. This side-chain length is a key differentiator among menaquinones and affects its absorption and function.
The Role of Gut Bacteria in Producing Vitamin K
The human large intestine, or colon, hosts trillions of bacteria that constitute the gut microbiome, and these bacteria produce substantial amounts of vitamins through fermentation, including vitamin K2. The menaquinones produced in the gut are typically longer-chain varieties, such as MK-10 to MK-13.
Some of the specific bacteria found in the gut that are known to produce menaquinones include:
- Bacteroides species: These anaerobic bacteria produce longer-chain menaquinones like MK-10 and MK-11.
- Eubacterium lentum: A bacterium that synthesizes MK-6.
- Enterobacter and Escherichia coli: Known producers of MK-8.
- Veillonella species: These bacteria are involved in the production of MK-7.
While these gut bacteria produce a variety of vitamin K2 forms, the overall contribution to the host's vitamin K status is often considered limited. This is because the majority of this bacterial production occurs in the distal colon, where bile salts needed for optimal absorption are less concentrated.
Fermented Foods as a Source of Bacterial Vitamin K
Beyond our own intestines, a major dietary source of vitamin K2 comes from foods fermented by bacteria. The specific menaquinone type and concentration depend on the bacterial strain and the fermentation process.
Commonly cited fermented sources include:
- Nattō: This traditional Japanese food, made from soybeans fermented with the bacterium Bacillus subtilis natto, is an exceptionally rich source of long-chain menaquinones, particularly MK-7.
- Cheeses: Various cheeses, including Gouda, Swiss, and Edam, contain menaquinones. Bacteria such as Lactococcus lactis and Propionibacteria produce MK-8, MK-9, and MK-9(4H) during the cheese-making process.
- Sauerkraut: Fermented cabbage contains menaquinones, though in lower concentrations than natto.
The Different Roles of Vitamin K1 and Vitamin K2
While both vitamins K1 and K2 are crucial for health, they have different sources and slightly different physiological roles in the body beyond coagulation.
| Feature | Vitamin K1 (Phylloquinone) | Vitamin K2 (Menaquinones) |
|---|---|---|
| Primary Source | Green leafy vegetables, plant oils | Bacteria (gut and fermented foods), animal products |
| Absorption Site | Primarily in the small intestine | Better absorbed from fermented foods in the gut |
| Storage | Stored predominantly in the liver | Found in higher concentrations in extrahepatic tissues, including the brain and kidneys |
| Half-Life | Short half-life of 1–2 hours | Longer half-life, especially for long-chain variants like MK-7 (68 hours) |
| Cardiovascular Health | Indirectly associated via coagulation factors | Linked to preventing vascular calcification |
| Bone Health | Essential for producing bone proteins | May be more effective for bone mineralization and density |
The Biosynthesis Pathway in Bacteria
Bacteria synthesize menaquinones using a complex biochemical pathway. This process involves multiple metabolic pathways, including the shikimate pathway for the naphthoquinone ring and the MEP or MVA pathway for the isoprenoid side chain. The final compound, menaquinone, is formed by combining these two structural components through a series of enzymatic reactions. The biosynthetic process is different from how animals produce MK-4, which is converted from vitamin K1 in tissues and does not involve bacterial action.
Conclusion
Ultimately, the vitamin K that is naturally produced by bacteria is menaquinone (vitamin K2). This occurs in two main ways: by the resident bacteria of the human gut and by specific bacteria used in the fermentation of foods. While gut bacteria contribute a variety of long-chain menaquinones, their bioavailability from this source can be limited. For dietary intake, fermented products like natto and cheese provide a reliable source of specific menaquinone subtypes. Given the distinct health benefits attributed to different forms of vitamin K, understanding these bacterial origins provides valuable insight into optimizing nutritional health. Further research continues to explore the nuances of bacterial menaquinone synthesis and its impact on the human body, particularly concerning extrahepatic health benefits.
