The Forms of Vitamin K: K1 vs. K2
There are two main naturally occurring forms of vitamin K: vitamin K1 (phylloquinone) and vitamin K2 (menaquinone). A third synthetic form, vitamin K3 (menadione), is not naturally occurring and has toxicity concerns.
- Vitamin K1 (Phylloquinone): Primarily found in green leafy vegetables, phylloquinone is the main dietary source of vitamin K for most people. It is essential for the activation of blood-clotting factors produced in the liver.
- Vitamin K2 (Menaquinone): Synthesized by bacteria, menaquinone is found in fermented foods like natto and certain cheeses, as well as in animal products. It exists in various subtypes, referred to as MK-n, where 'n' represents the number of isoprenoid units in its side chain.
Menaquinones: The Bacterial Contribution
The synthesis of menaquinones (MKs) by intestinal bacteria is a critical, though often misunderstood, aspect of human nutrition. While vitamin K1 from the diet is primarily absorbed in the small intestine, the bacteria in the large intestine (colon) are responsible for producing a range of menaquinone subtypes. These menaquinones are used by bacteria for their own cellular functions, such as anaerobic respiration.
Key bacterial producers of menaquinones include members of the following genera:
- Bacteroides: Major inhabitants of the human intestinal tract known to synthesize long-chain menaquinones such as MK-10 and MK-11.
- Enterobacter: Another producer, primarily associated with synthesizing MK-8.
- Veillonella: A genus of bacteria that principally produces MK-7.
- Eubacterium lentum: Synthesizes MK-6.
- Lactobacillus and Bifidobacterium: Research has shown these probiotic bacteria, prominent in the gut microbiome, are also active producers of menaquinones, particularly MK-4 and MK-7.
The Challenge of Absorption and Bioavailability
Despite the significant production of menaquinones by gut flora, the bioavailability and absorption of this bacterially synthesized vitamin K2 remain subjects of research. The primary challenge is that most menaquinone production occurs in the large intestine, but efficient fat-soluble vitamin absorption typically requires bile salts present in the small intestine. Some studies suggest that the bioavailability of menaquinones synthesized in the colon may be limited, while others point to evidence that some absorption does occur, contributing meaningfully to overall vitamin K status, especially in the long-chain forms. The exact contribution is likely influenced by the specific bacterial strains, the composition of the gut microbiota, and the individual's diet. Recent findings, however, suggest some bacteria produce water-soluble forms of vitamin K2, which may be more readily absorbed in the colon.
Comparison of Vitamin K Forms
| Feature | Vitamin K1 (Phylloquinone) | Vitamin K2 (Menaquinone) | Vitamin K3 (Menadione) |
|---|---|---|---|
| Source | Plants, especially green leafy vegetables | Intestinal bacteria, fermented foods, animal products | Synthetic form |
| Primary Role | Liver-based blood clotting | Extrahepatic functions: bone mineralization, vascular health | Historically used; no longer recommended due to toxicity |
| Synthesized by Bacteria? | No, exclusively produced by plants | Yes, produced by gut microbiota | No |
| Absorption Site | Primarily small intestine, requires bile salts | Small and large intestine; absorption from colon is debated | Absorbed in small intestine |
| Subtypes | One main form | Multiple forms (MK-4 to MK-13), varying side-chain lengths | One main form; converted to MK-4 in tissues |
The Broader Role of Gut Bacteria and Vitamin K
The symbiotic relationship between our gut bacteria and our body's vitamin K supply is more complex than just a simple source of nutrients. A balanced and diverse gut microbiota is essential for effective menaquinone production. Factors like antibiotic use, certain medications, or inflammatory bowel diseases can disrupt the microbial balance and potentially affect vitamin K levels, increasing the risk of deficiency. For newborns, whose gut flora is not yet established, a vitamin K injection is standard procedure to prevent hemorrhagic disease due to their inability to synthesize menaquinones. The ongoing investigation into how the gut microbiota influences vitamin K bioavailability and its role in extrahepatic functions continues to shed new light on the critical connection between gut health and overall well-being.
Conclusion
In summary, the form of vitamin K synthesized by intestinal bacteria is menaquinone (Vitamin K2). This vital fat-soluble vitamin, produced by a variety of gut microbes including Bacteroides, Lactobacillus, and Eubacterium, contributes to extrahepatic functions like bone health and vascular health. While its absorption from the large intestine is not as straightforward as dietary vitamin K1, recent evidence suggests that bacterially-produced menaquinones, potentially in water-soluble forms, can be absorbed and utilized by the body. Understanding this process underscores the importance of a healthy and diverse gut microbiome in maintaining adequate vitamin K status for optimal health.
