The role of bacteria in vitamin K2 production
Vitamin K2, or menaquinone (MK), is a fat-soluble vitamin primarily of bacterial origin. While its more common relative, vitamin K1 (phylloquinone), is synthesized by plants, K2 is largely produced by microorganisms. This occurs in two main ways: within the human digestive system and through food fermentation processes.
Gut microbiota synthesis
The bacteria in our large intestine, such as Bacteroides and Eubacterium, are capable of synthesizing menaquinones. While this internal production occurs, its contribution to our overall vitamin K2 status is often considered limited. The primary reason for this is inefficient absorption, as the vitamin is largely produced in the colon, which is beyond the main absorption site in the small intestine. Furthermore, much of the K2 produced is bound within the bacterial membranes themselves. Still, the presence of a healthy gut microbiome is a key factor in endogenous K2 production, and a disrupted gut flora, often from broad-spectrum antibiotics, can impact this process.
Fermentation by specific bacteria
Outside of our bodies, bacteria are used deliberately to produce vitamin K2 through fermentation. This method is responsible for some of the most potent dietary sources of K2. For example, the Japanese dish natto, made from fermented soybeans, is an exceptionally rich source of MK-7 thanks to the bacterium Bacillus subtilis natto. Other fermented foods like certain cheeses and sauerkraut also contain K2, produced by bacteria such as lactic acid bacteria (Lactococcus lactis). The specific bacterial strain used and the fermentation conditions can influence the amount and type of menaquinones produced.
Animal products as a source
Vitamin K2 is also found in animal products, where it exists primarily as the menaquinone-4 (MK-4) subtype. MK-4 is not produced by gut bacteria but is instead formed in animal tissues through a conversion process from dietary vitamin K1. This conversion occurs in organs like the pancreas and testes, and in arterial walls. The concentration of MK-4 in animal products can vary depending on the animal's diet, with grass-fed animals often having higher levels due to greater intake of K1 from fresh pasture.
Rich animal food sources
- Goose liver: One of the most concentrated animal sources of MK-4.
- Egg yolks: Especially from pasture-raised chickens, egg yolks are a good source of MK-4.
- Meat: The MK-4 content is highest in organ meats, such as beef and chicken liver, as well as fatty cuts of meat like chicken legs.
- Dairy: High-fat dairy products like butter and some cheeses contain MK-4, as well as other menaquinones depending on the fermentation.
Comparison of K2 sources: Fermented foods vs. animal products
| Feature | Fermented Foods (e.g., Natto) | Animal Products (e.g., Liver, Eggs) |
|---|---|---|
| Primary MK Subtype | Longer-chain menaquinones (e.g., MK-7, MK-8) | Shorter-chain menaquinone (MK-4) |
| Production Process | Bacterial fermentation | Conversion from dietary K1 in animal tissues |
| Bioavailability | MK-7 is known for excellent bioavailability and a long half-life in the body | MK-4 is readily absorbed but has a shorter half-life compared to MK-7 |
| Dietary Availability | Primarily from specific, often traditional, fermented dishes; less common in Western diets | Can be obtained more easily in a Western diet through consumption of high-fat dairy, eggs, and certain meats |
| Bioactive Form | The bioactive form is a result of the fermentation process | The bioactive form is a result of metabolic conversion |
Conclusion
Vitamin K2 is a versatile fat-soluble nutrient produced through several biological mechanisms. Its synthesis is largely bacterial, occurring naturally within the gut microbiome and more predictably through the fermentation of certain foods like natto and cheese. Furthermore, animal tissues convert dietary vitamin K1 into the highly bioavailable MK-4 subtype, making fatty animal products like egg yolks and liver important dietary sources. While the body can produce small amounts, obtaining sufficient vitamin K2 from diverse dietary sources remains the most effective strategy for ensuring adequate levels for optimal bone and cardiovascular health. A diet incorporating both fermented and animal-based products can provide a broad spectrum of menaquinones for maximum benefit.
What are the specific types of bacteria that produce vitamin K2?
Different types of bacteria produce different menaquinone subtypes. Bacillus subtilis is responsible for the high MK-7 content in natto, while lactic acid bacteria like Lactococcus lactis and Propionibacteria are known for producing MK-8, MK-9, and MK-9(4H) found in certain cheeses. In the human gut, Bacteroides species produce long-chain menaquinones like MK-10 and MK-11.
Can I get enough vitamin K2 just from what my gut bacteria produce?
No, relying solely on production from gut bacteria is not sufficient for meeting vitamin K2 needs. Absorption of bacterially produced menaquinones in the colon is limited, making dietary intake of fermented foods and animal products essential for ensuring functionally available vitamin K2.
How does the MK-4 found in animal products differ from the MK-7 in fermented foods?
MK-4 is a short-chain menaquinone formed by the conversion of vitamin K1 in animal tissues, with a relatively short half-life. MK-7 is a long-chain menaquinone produced by bacterial fermentation, particularly in natto, and has a significantly longer half-life in the bloodstream, making it highly bioavailable.
Does heating or cooking affect vitamin K2 levels in food?
Vitamin K2 is generally stable to heat and air. Standard cooking procedures may result in only a slight decrease in levels, meaning most cooked animal and fermented foods will retain their vitamin K2 content.
Are vegan diets typically lacking in vitamin K2?
Vegan diets often lack sufficient vitamin K2, as most potent sources come from animal and specific fermented products. Natto is a notable exception as a vegan source rich in MK-7, but other fermented vegetable products contain much lower amounts. Vegans may need to consider supplementation if their diet does not include specific fermented foods rich in K2.
How can I increase my intake of vitamin K2?
You can increase your vitamin K2 intake by incorporating certain foods into your diet. Excellent sources include natto (for MK-7), goose liver, egg yolks, chicken, and high-fat dairy products like hard and soft cheeses (for MK-4 and other menaquinones). Pairing K2-rich foods with healthy fats can also enhance absorption.
What is the difference between vitamin K1 and vitamin K2 production?
Vitamin K1 is produced by plants, specifically in green leafy vegetables, as a key component of photosynthesis. Vitamin K2 is produced by bacteria, both in fermented foods and in the animal and human gut. Animals can also convert K1 into the MK-4 variant of K2. The different origins and forms lead to different functions and bioavailability in the human body.
