The Microbial Connection to Vitamin K2
Yes, vitamin K can be obtained from bacteria, though the specifics are more nuanced than a simple 'yes' or 'no'. The story revolves around the two main forms of naturally occurring vitamin K: vitamin K1 (phylloquinone) and vitamin K2 (menaquinone). While plants produce K1, bacteria are the primary producers of K2. Bacteria synthesize menaquinones primarily for their own cellular functions, specifically as crucial electron carriers within their respiratory chains, which are essential for generating energy in the form of ATP. The length of the side chain on the menaquinone molecule can vary depending on the bacterial species.
Gut Microbiota: A Natural Source
A human's own gut microbiota is a significant source of vitamin K2. The large intestine houses trillions of bacteria that produce substantial amounts of vitamin K, particularly K2. Some of the main types of bacteria involved include Bacteroides (producing MK-10 and MK-11), Veillonella (MK-7), and Enterobacter (MK-8). Escherichia coli, a facultatively anaerobic bacterium common in the lower intestine, also synthesizes menaquinone, primarily MK-8, under oxygen-limited conditions. For a long time, it was assumed that this endogenous production was enough to meet some of our vitamin K needs, especially during periods of low dietary intake. However, this is more complex than it appears, mainly due to the absorption paradox, as much of the production occurs far down the digestive tract where absorption is less efficient.
Fermented Foods: A Direct Dietary Source
Beyond our internal production, we can also obtain vitamin K2 from bacteria in fermented foods. This is because the fermentation process relies on specific bacterial strains that are potent producers of menaquinones. This is particularly relevant for the long-chain forms of K2, such as MK-7, MK-8, and MK-9.
- Natto: This traditional Japanese fermented soybean dish is one of the richest dietary sources of K2, specifically MK-7, which is produced by the bacterium Bacillus subtilis natto.
- Cheeses: Many types of cheese, especially aged varieties, contain menaquinones produced by lactic acid bacteria used in the fermentation process. Specific strains of Lactococcus lactis, for example, are known K2 producers.
- Sauerkraut: This fermented cabbage product can also contain various forms of menaquinone, depending on the bacteria involved in its production.
The Absorption Paradox: Why Dietary Intake Matters
While gut bacteria synthesize K2, the bioavailability of this endogenous supply is limited, and relying solely on it is not enough for most people. Most K2 is produced in the large intestine (colon), where the concentration of bile salts needed for fat absorption is low. This means much of the bacterially synthesized vitamin K2 remains bound within the bacterial membranes and is ultimately excreted. In contrast, dietary K1 (from leafy greens) and K2 (from fermented foods and animal products) are absorbed more efficiently in the small intestine, where bile salts are plentiful. For this reason, a diet rich in vitamin K foods remains crucial, and endogenous production from gut flora acts more as a supportive, but often insufficient, secondary source.
The Broader Role of Vitamin K in Health
Both K1 and K2 are essential for human health, playing critical roles beyond just bacterial functions. Vitamin K is a key cofactor for proteins involved in blood clotting and bone metabolism. Emerging research suggests that K2 may offer unique benefits for cardiovascular health and bone mineralization that differ from K1. A varied diet that includes both K1-rich vegetables and K2-rich fermented foods can help ensure adequate levels of all forms.
Comparing Vitamin K1 and K2
| Feature | Vitamin K1 (Phylloquinone) | Vitamin K2 (Menaquinone) |
|---|---|---|
| Primary Source | Plants (e.g., leafy greens, vegetable oils) | Bacteria (e.g., gut flora, fermented foods) |
| Primary Function in Source | Photosynthesis in plants | Electron transport in bacteria |
| Forms | One form | Multiple subtypes (MK-4 to MK-13) |
| Best Dietary Sources | Kale, spinach, broccoli, vegetable oils | Natto, cheese, eggs, meat |
| Absorption Site | Primarily small intestine | Small intestine (dietary) and large intestine (limited from bacteria) |
| Bioavailability | Variable (often low from plants) | Often higher than K1, especially longer chain forms like MK-7 |
| Plasma Half-Life | Short (1-2 hours) | Longer, especially MK-7 (around 68 hours) |
Conclusion
Ultimately, while bacteria produce and are a source of vitamin K, relying solely on this endogenous production is not a reliable strategy for meeting nutritional requirements. The body benefits most from a combination of dietary sources. Leafy greens provide vitamin K1, while fermented foods and animal products contribute vitamin K2, which is uniquely produced by bacteria and has distinct benefits. Understanding this dual-source nature of vitamin K helps clarify its importance in maintaining proper blood coagulation and supporting long-term bone and cardiovascular health.
For more technical information on the microbial production of vitamin K2, consult this resource: New aspects of microbial vitamin K2 production by expanding the product spectrum and membrane engineering.
