Do Fermented Foods Contain Vitamin K? An Expert Guide to K2 Sources

December 22, 2025 •

4 min read

Some of the world's most nutrient-dense foods are created through fermentation, a process known for producing beneficial probiotics. A lesser-known but equally significant byproduct is vitamin K, specifically the menaquinone (K2) form, which certain bacteria synthesize during fermentation.

Quick Summary

Yes, fermented foods contain vitamin K, mainly K2, which is synthesized by bacteria. Content varies widely; natto is an exceptionally rich source, while some cheeses, kefir, and fermented vegetables also provide it.

Key Points

Bacteria Produce Vitamin K2: The fermentation process synthesizes vitamin K2 (menaquinone), not vitamin K1, which is found in plants.
Natto is the Richest Source: The fermented soybean dish natto is by far the highest dietary source of vitamin K2, specifically MK-7.
Cheese and Kefir Vary: Hard and aged cheeses contain significant K2, as does kefir, but concentrations vary greatly depending on bacterial cultures and processing.
Sauerkraut Contains Both K1 and K2: Fermented cabbage provides vitamin K1 from the cabbage itself and some K2 from the bacteria, particularly if unpasteurized.
K2 Is Important for Bone and Heart Health: Unlike K1, which is primarily for blood clotting, K2 helps regulate calcium metabolism for strong bones and prevents arterial calcification.
Fermentation Time Matters: Longer fermentation, up to a point, can lead to higher vitamin K2 production in some foods.

The Surprising Source of Vitamin K2

While many people associate vitamin K with leafy green vegetables, which contain vitamin K1 (phylloquinone), fermented foods are a unique and often superior dietary source of vitamin K2 (menaquinone). Unlike plants, the vitamin K in fermented products is not inherent to the raw ingredients but is actively produced by the bacteria involved in the fermentation process. The specific strains of bacteria and the fermentation duration heavily influence the final concentration and type of menaquinones (MK-n) found in the food.

The Role of Bacteria in Vitamin K Production

The synthesis of vitamin K2 is a key function for many bacteria, including those used in food fermentation. One of the most potent examples is Bacillus subtilis, the bacterium responsible for fermenting soybeans to create natto. This specific strain is a prolific producer of menaquinone-7 (MK-7), a form of K2 with a high bioavailability and a long half-life in the human body. Other lactic acid bacteria (LAB) found in dairy fermentation, such as Lactococcus and Propionibacterium, also produce various menaquinones (MK-4 to MK-10), contributing to the vitamin K content of cheeses and other cultured dairy products. Even the bacteria in the human colon produce menaquinones, although their absorption and contribution to overall vitamin status remain uncertain.

Fermented Foods Rich in Vitamin K2

Natto: This traditional Japanese dish of fermented soybeans is the most concentrated dietary source of vitamin K2, specifically MK-7. The powerful bacteria Bacillus subtilis is responsible for this exceptional nutrient profile. Though it has a strong flavor and sticky texture that takes some getting used to, a small serving of natto can provide several times the daily recommended intake of vitamin K.
Fermented Dairy and Cheese: Certain cheeses, especially aged and hard varieties, are good sources of K2. Bacteria like Propionibacterium in Swiss-type cheeses and other microbes in Gouda and blue cheese produce menaquinones. Kefir, a fermented milk drink, also contains varying levels of K2, with some studies showing higher levels in versions made with natural kefir grains. The K2 content in dairy is often higher in full-fat varieties and is influenced by the starter cultures used.
Fermented Vegetables (Sauerkraut and Kimchi): Fermented cabbage products like sauerkraut and kimchi contain both vitamin K1 from the original vegetable and a smaller, but still beneficial, amount of vitamin K2 synthesized by lactic acid bacteria. The K2 levels in these foods are typically lower than in natto but can still contribute to your overall intake, especially if they are unpasteurized.

Vitamin K1 vs. K2 from Fermented Foods

Understanding the difference between vitamin K1 and K2 is crucial for appreciating the benefits of fermented foods. Vitamin K1, found predominantly in leafy greens, is primarily used by the liver for blood clotting. In contrast, the menaquinone (K2) produced during fermentation is more readily absorbed by the body and plays a critical role in regulating calcium metabolism for bone health and preventing arterial calcification, supporting a healthy heart. The subtype MK-7, abundant in natto, is particularly effective due to its longer half-life compared to other forms.

Factors Affecting Vitamin K Content

The vitamin K2 content of fermented foods is not standardized and can vary significantly. Key factors include:

Bacterial Strains

Different bacteria produce different forms and amounts of menaquinones. For instance, the Bacillus subtilis strain used for natto is exceptionally efficient at producing MK-7, while the bacteria in cheese and yogurt may produce a mix of menaquinones (e.g., MK-8, MK-9).

Fermentation Time and Conditions

Longer fermentation periods can often lead to a higher synthesis of vitamin K2, up to a certain point. Fermentation conditions, including temperature and starter cultures, are also critical. For example, kefir made with traditional grains can have higher MK-7 content than versions fermented with commercial starters.

Raw Ingredients

The starting material's composition, such as the fat content in milk for kefir and cheese, can also affect the final vitamin K2 concentration.

Comparative Look: Vitamin K in Fermented Foods

To illustrate the difference in vitamin K content, here is a comparison of typical amounts found in several fermented foods, noting that values can vary widely.


Food Type	Form of Vitamin K	Typical Content (per 100g)	Primary Bacterial Synthesizer
Natto	K2 (MK-7)	Extremely High (up to 1,080 mcg)	Bacillus subtilis natto
Aged Gouda Cheese	K2 (MK-9)	High (around 32 mcg)	Propionibacterium, Lactic Acid Bacteria
Kefir	K2 (MK-7, MK-8, MK-9)	Varies (2-13 mcg)	Lactic Acid Bacteria and Yeasts
Sauerkraut (Unpasteurized)	K1 and K2 (MK-n)	Lower K2 (approx. 3-6 mcg)	Lactobacillus species

Conclusion: Making Informed Dietary Choices

Yes, fermented foods contain vitamin K, with certain types being exceptional sources of vitamin K2. While leafy greens provide K1, the fermentation process creates the highly bioavailable K2, which is essential for bone mineralization and cardiovascular health. For the highest concentration, natto is unparalleled, though aged cheeses, kefir, and fermented vegetables can also contribute meaningfully to your intake. For specific vitamin K recommendations, consult a healthcare professional, especially if you are on blood-thinning medication. Incorporating a variety of fermented foods can enhance your overall nutritional intake and support a healthy gut microbiome.

How to get the most K2 from your fermented foods

To maximize your intake, focus on unpasteurized and longer-fermented products. For example, choose aged cheeses over fresh ones and seek out traditional or natural kefir grains for fermentation. The bacteria in these products are the true vitamin factories. For a potent, direct source of MK-7, try adding natto to your diet.

Outbound Link for Further Reading

For more detailed information on vitamin K, including its different forms and functions, consult the National Institutes of Health Fact Sheet on Vitamin K.

Frequently Asked Questions

Vitamin K1 (phylloquinone) is found in plants like leafy greens and is essential for blood clotting. Vitamin K2 (menaquinone) is produced by bacteria during fermentation and supports bone and cardiovascular health by directing calcium to the right places.

Natto, a traditional Japanese fermented soybean dish, is recognized as the richest dietary source of vitamin K2, containing exceptionally high levels of the MK-7 subtype.

No, not all fermented foods contain significant amounts of K2. Production depends on the type of bacteria involved. Foods fermented with specific bacteria like Bacillus subtilis (natto) or certain dairy cultures (cheese, kefir) are known for their K2 content.

Yes, sauerkraut contains some vitamin K2, produced by lactic acid bacteria during fermentation. It also provides vitamin K1 from the cabbage itself. The K2 content is typically lower than in natto and is higher in unpasteurized versions.

Kefir can be a good source of vitamin K2, produced by the bacteria and yeasts in the kefir grains. The content varies depending on the starter culture and fat content, but some studies have shown beneficial levels of menaquinones.

Aging and specific bacterial cultures significantly affect K2 content in cheese. Hard and aged cheeses often have higher K2 concentrations. Different bacteria strains used in cheese making produce various forms of menaquinones, influencing the final nutritional profile.

Vitamin K2, particularly the MK-7 form found in natto, has better bioavailability and a longer half-life in the body compared to plant-derived K1. This means more of it can be absorbed and utilized by the body for functions like bone and cardiovascular health.