The Fundamental Forms of Vitamin K
Vitamin K is a fat-soluble nutrient essential for blood clotting and bone health, but it exists in several forms with distinct origins. The two most important natural types are Vitamin K1 (phylloquinone) and Vitamin K2 (menaquinones).
- Vitamin K1 (Phylloquinone): This form is produced by plants and is abundant in leafy green vegetables like spinach and kale. It is directly involved in photosynthesis.
- Vitamin K2 (Menaquinones): This form is primarily of bacterial origin and consists of a series of subtypes, designated MK-n, where 'n' represents the number of isoprene units in the side chain. Different bacteria produce different menaquinone variants, influencing the nutrient profile of various foods and gut environments.
Bacterial Production in Fermented Foods
Many fermented foods are rich sources of Vitamin K2 due to the activity of specific bacterial starter cultures. These microbes produce menaquinones as part of their metabolic processes, which are then consumed by humans through these foods.
Bacillus subtilis var. natto
Perhaps the most famous example of bacterial Vitamin K2 synthesis comes from the Japanese fermented soybean dish known as natto. The bacterium responsible is Bacillus subtilis var. natto, which produces extremely high concentrations of menaquinone-7 (MK-7) during the fermentation of soybeans. MK-7 is particularly valued for its high bioavailability and long half-life in the body. The pungent flavor and stringy texture of natto are a direct result of this bacterium's activity.
Lactic Acid Bacteria in Dairy and Vegetables
Another significant group of bacteria involved in menaquinone synthesis are lactic acid bacteria (LAB), which are widely used in the production of cheese, yogurt, and other fermented dairy products.
- Lactococcus lactis: Found in many cheeses, this species can produce a range of menaquinones, including MK-7, MK-8, and MK-9. The specific MK profile can vary depending on the strain of bacteria and fermentation conditions.
- Leuconostoc lactis: Also involved in dairy fermentation, this bacterium has been identified as a producer of MK-7 to MK-10.
Intestinal Bacteria and the Gut Microbiome
The human gut is home to a diverse population of bacteria, many of which contribute to our overall vitamin K status by synthesizing menaquinones. However, the bioavailability of these internally produced menaquinones can be limited due to absorption issues in the distal colon, where most of this synthesis occurs.
Escherichia coli (E. coli)
Commonly known as a potential pathogen, E. coli is also a normal and important inhabitant of the human large intestine. Under anaerobic conditions, E. coli can produce menaquinone-8 (MK-8), which functions as a lipid-soluble electron carrier for the bacterium. This contribution adds to the body's supply of Vitamin K2.
Bacteroides species
This dominant genus of anaerobic bacteria in the large intestine is responsible for synthesizing longer-chain menaquinones, primarily MK-10 and MK-11. Though plentiful, the physiological impact of these specific variants produced deep in the colon is less significant than dietary intake.
Other Gut Inhabitants
Other bacterial species found in the gut also play a role, albeit a smaller one, in menaquinone production, including:
- Eubacterium lentum: A species known to produce MK-6.
- Veillonella species: A genus of bacteria that can produce MK-7.
Comparison of Vitamin K-Producing Bacteria
This table summarizes some of the key bacteria, their environment, and the specific menaquinones (MK) they are known to produce, highlighting their distinct roles in contributing to our vitamin K intake through diet and gut flora.
| Bacterium | Environment | Primary MK Type | Role | Significance |
|---|---|---|---|---|
| Bacillus subtilis natto | Fermented soybeans (natto) | MK-7 | Fermentation starter culture | Rich dietary source, high bioavailability |
| Lactococcus lactis | Fermented dairy (cheese) | MK-7, MK-8, MK-9 | Fermentation starter culture | Major dietary source in Western diets |
| Escherichia coli | Human large intestine | MK-8 | Gut flora | Contributes to endogenous supply |
| Bacteroides spp. | Human large intestine | MK-10, MK-11 | Gut flora | Contributes to endogenous supply, but limited absorption |
| Eubacterium lentum | Human large intestine | MK-6 | Gut flora | Minor endogenous contributor |
The Function of Bacterial-Synthesized Vitamin K
Within bacteria, menaquinones serve as essential electron carriers in the electron transport chain, particularly during anaerobic respiration. This process is vital for the microorganism's energy production. When humans consume these bacteria (for instance, in fermented foods) or benefit from their presence in the gut, we acquire the menaquinones they have synthesized. The varying side-chain lengths of menaquinones (MK-n) affect their transport and distribution within the human body, influencing their biological activity. For example, the longer-chain MK-7 has been shown to have a longer half-life than Vitamin K1, suggesting a greater physiological impact on extra-hepatic tissues like bone.
Conclusion
While plants are the source of Vitamin K1, it is exclusively bacteria that create the various subtypes of Vitamin K2, or menaquinones. This process occurs both in fermented foods, where specific starter cultures like Bacillus subtilis natto and Lactococcus lactis produce abundant amounts, and within the human gut, where resident microbes like E. coli and Bacteroides species contribute to the body's supply. While dietary sources are considered the most effective for increasing functional vitamin K2 levels, the intricate relationship between bacteria and vitamin K production underscores the vital role of microbiology in human nutrition. For a deeper understanding of microbial vitamin K production, you may explore the comprehensive review available in Production of Vitamin K by Wild-Type and Engineered Microorganisms.
