Natto: The Foremost Natural Source of K2 MK7
Natto, a traditional Japanese food consisting of fermented soybeans, is by far the richest known natural source of menaquinone-7 (MK-7). This powerful nutrient is produced during the fermentation of soybeans by the bacterium Bacillus subtilis natto. While natto's strong smell and slimy texture can be an acquired taste for Western palates, its nutritional density is unmatched. A small serving can provide several hundred micrograms of MK-7, exceeding the recommended daily intake.
The Science Behind Bacterial Synthesis
Vitamin K2, including the MK-7 subtype, is primarily synthesized by bacteria, which distinguishes it from vitamin K1 found in leafy greens. Different bacterial strains produce different lengths of menaquinones. The Bacillus subtilis natto strain is uniquely effective at producing the long-chain MK-7, which is particularly beneficial due to its longer half-life in the bloodstream compared to shorter-chain forms like MK-4. The long half-life means MK-7 stays active in the body for a longer period, allowing it to reach and activate key proteins in extrahepatic tissues like bones and blood vessels.
Other Dietary Sources of K2 MK7
While natto is the clear winner, other fermented foods and animal products contain varying levels of K2, including some MK-7. The amount depends heavily on the specific bacteria involved and the fermentation process.
Fermented Dairy and Cheeses
Certain cheeses, especially hard and soft fermented varieties, are known to contain K2 menaquinones, including MK-7, MK-8, and MK-9. Gouda and Brie are examples of cheeses that offer moderate amounts of K2. The bacterial cultures used in cheese production are responsible for synthesizing these menaquinones. This makes cheese a significant, albeit less concentrated, source of K2 MK-7 in many Western diets.
Fermented Vegetables
Some fermented vegetables, like sauerkraut, contain smaller amounts of MK-7. This is again a result of bacterial fermentation. The quantity can be considerably lower than in natto, but it still contributes to overall vitamin K2 intake. Other fermented vegetable products like kimchi may also contain some K2 from their bacterial content.
Animal Products
While animal products are more typically associated with the MK-4 form of vitamin K2, some cured and fermented meats may contain modest amounts of MK-7. However, the primary source of MK-7 remains bacterial fermentation, especially that which occurs with specific strains like Bacillus subtilis natto.
Comparison of K2 MK7 Dietary Sources
| Source | Primary K2 Form | K2 MK7 Content | Notes |
|---|---|---|---|
| Natto | MK-7 | Very High (up to 1,100+ mcg/100g) | Richest known source. Product of Bacillus subtilis natto fermentation. |
| Hard Cheeses (e.g., Gouda) | MK-8, MK-9, MK-7 | Moderate (e.g., 76 mcg/100g total K2) | Contains a mix of menaquinones synthesized by cheese cultures. |
| Soft Cheeses (e.g., Brie) | MK-8, MK-9, MK-7 | Moderate (e.g., 56 mcg/100g total K2) | Lower concentration than hard cheeses, but still a source. |
| Sauerkraut | MK-4, MK-5, MK-6, MK-7 | Low (e.g., 4.8 mcg/100g total K2) | Fermentation provides a mix of K2 forms, but at lower levels. |
The Role of Supplements
For those who do not consume natto or other specific fermented foods regularly, supplements derived from natural fermentation are a popular option to ensure adequate intake of K2 MK7. High-quality, naturally fermented K2 MK7 supplements are often derived from Bacillus subtilis natto and provide a reliable dosage. Synthetic K2 MK7 also exists but may contain a higher percentage of the less effective cis isomer, whereas naturally fermented MK-7 typically has over 99% of the more bioactive trans isomer.
K2 MK7 Absorption and Bioavailability
MK-7 is known for its excellent bioavailability and long half-life, meaning it is absorbed efficiently and stays in the body longer than other forms like vitamin K1 or even shorter-chain MK-4. The absorption is further enhanced when consumed with a source of dietary fat, as vitamin K is a fat-soluble vitamin. This is one reason why fatty fermented dairy products and egg yolks, which naturally contain fat, are also good sources. Research indicates that dietary MK-7 effectively increases serum levels of vitamin K2, benefiting bone and cardiovascular health.
The Importance of a Balanced Diet
While natto offers a concentrated dose of K2 MK7, a balanced diet including a variety of fermented foods and some animal products is beneficial for getting a spectrum of vitamin K2 types. The modern Western diet is often low in K2 because many of these traditional fermented foods have become less common. Incorporating diverse sources, or considering supplementation, can help ensure you meet your nutritional needs for this important vitamin.
Conclusion
In summary, the natural source of K2 mk7 is overwhelmingly fermented foods, with natto standing out as the most potent source. It is produced by the Bacillus subtilis natto bacteria during the fermentation of soybeans. Lesser amounts can be found in certain fermented cheeses and other cultured products. The bacterial origin of MK-7 sets it apart from plant-derived K1, offering superior bioavailability and a longer-lasting effect in the body. For those unable to consume these specific foods, naturally fermented supplements provide a highly effective alternative to support bone and cardiovascular health.
Visit the NIH website for more detailed information on Vitamin K
