Vitamin K is a fat-soluble vitamin essential for various bodily functions, including blood clotting. While Vitamin K1, or phylloquinone, is abundant in green leafy vegetables, Vitamin K2, or menaquinone, is less common in a typical Western diet and serves a different, though equally vital, purpose. While K1 concentrates in the liver for coagulation factors, K2 is active in extrahepatic tissues like bones and arteries, where it activates proteins that regulate calcium. Specifically, K2 helps direct calcium into the bones and teeth and prevents its harmful buildup in soft tissues like arteries and kidneys. Understanding the distinct sources of K2 is key to obtaining its full spectrum of health benefits.
Fermented Foods: The Richest Sources of MK-7
The most bioavailable and long-acting form of Vitamin K2, menaquinone-7 (MK-7), is produced by bacteria and primarily found in fermented foods. The fermentation process naturally enriches these foods with beneficial menaquinones. The undisputed champion of dietary K2 is a Japanese staple.
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Natto (Fermented Soybeans): The richest known source, with up to 1000 micrograms (mcg) of MK-7 per 100 grams, natto is an exceptional way to obtain a potent dose of K2. Its unique strong smell and sticky texture can be an acquired taste, but even small, regular servings can have a significant impact.
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Cheeses: Certain hard and soft cheeses, particularly aged varieties, are also excellent sources of Vitamin K2, predominantly in the form of longer-chain menaquinones like MK-8 and MK-9. The specific bacteria used in cheese production directly influence the final K2 content.
- High-K2 Cheeses: Aged Gouda, Brie, Munster, and Jarlsberg are notable examples, with Dutch-made Gouda often cited for its high K2 concentration.
- Lower-K2 Cheeses: Many common US-made cheeses like American or some Cheddar varieties contain less K2.
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Sauerkraut: This fermented cabbage product is a valuable plant-based source of K2, containing smaller amounts of various menaquinones, including MK-7. The exact K2 content varies depending on the bacterial strains and fermentation process used.
Animal Products: Good Sources of MK-4
The other prominent form of K2, menaquinone-4 (MK-4), is found exclusively in animal products. Mammals produce MK-4 by converting other forms of Vitamin K, such as K1 from green plants, within their tissues.
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Goose Liver Pâté: This is one of the densest animal sources of K2, providing exclusively MK-4.
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Egg Yolks: The yolks from pastured or grass-fed chickens contain significantly higher levels of MK-4 than those from grain-fed hens. A typical egg yolk from a fortified hen contains around 30 mcg of MK-4 per yolk.
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Grass-Fed Butter and Dairy: Like eggs, the quality of dairy depends on the animal's diet. Butter and other dairy products from cows that graze on grass contain higher concentrations of MK-4 because they consume K1 from plants and convert it.
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Organ and Dark Meats: Organ meats such as beef and chicken liver, along with dark chicken meat, are good sources of MK-4.
The Grass-Fed Advantage
For animal-derived sources, the feed of the animal is paramount. Animals that consume a diet rich in vitamin K1 from grass will convert it to higher levels of K2 (MK-4) in their fat and tissues. Choosing grass-fed options for meat and dairy can thus be a strategy to increase MK-4 intake over conventionally raised products.
Optimizing K2 Absorption
Since Vitamin K2 is fat-soluble, its absorption is significantly enhanced when consumed with dietary fats. Incorporating K2-rich foods into meals that contain some fat, such as adding cheese to a salad or cooking with grass-fed butter, can maximize absorption. This is particularly important for obtaining MK-7 from fermented vegetables, which are often low in fat naturally.
Comparison of K2 Types (MK-4 vs. MK-7)
| Feature | Menaquinone-4 (MK-4) | Menaquinone-7 (MK-7) |
|---|---|---|
| Primary Source | Animal products (organ meats, egg yolks, grass-fed butter) | Fermented foods (natto, aged cheese, sauerkraut) |
| Half-Life | Short (clears within 6-8 hours) | Long (up to 3 days or more) |
| Bioavailability | Poor at nutritional levels; requires high, frequent doses | High, leading to sustained blood levels with smaller doses |
| Concentration | Highest in specific organ meats and high-fat dairy | Highest in natto |
| Origin | Converted by animal tissue from other K vitamins | Produced directly by bacteria during fermentation |
Conclusion
For a truly comprehensive diet, obtaining Vitamin K2 from both fermented foods and high-quality animal sources is the ideal approach. While the extraordinary potency of natto makes it the richest single source of MK-7, incorporating aged cheeses, grass-fed dairy and eggs, and occasional organ meats provides a broader spectrum of menaquinones. Recognizing the difference in bioavailability and focusing on consistent intake of these fat-soluble nutrients is crucial for supporting both bone mineralization and cardiovascular health. For those looking for simple changes, adding a bit of aged cheese, some fermented vegetables, or cooking with grass-fed butter can make a big difference in ensuring adequate K2 intake.
To learn more about the scientific basis for Vitamin K's health benefits, a detailed review is available from the National Institutes of Health.
