Do Probiotics Contain K2? The Surprising Connection Between Gut Health and Vitamin K2

October 7, 2025 •

3 min read

While most people know that certain probiotic bacteria are essential for gut health, a lesser-known fact is that specific strains of these microorganisms are capable of producing vitamin K2. This endogenous production means the vibrant ecosystem within your gut can be a direct contributor to your body's supply of this crucial, fat-soluble vitamin.

Quick Summary

This article explores the link between probiotics and vitamin K2, revealing that specific gut bacteria can synthesize this essential vitamin. It details which fermented foods are the best sources of K2 and explains the difference between various menaquinone forms. The content also covers the benefits of a symbiotic relationship between your gut and K2 and cautions against relying solely on probiotics for your daily intake.

Key Points

Bacteria Produce K2: Specific probiotic bacteria strains in fermented foods and the gut microbiome can naturally synthesize vitamin K2.
Not All Probiotics Have K2: While many fermented foods are rich in K2 due to bacterial activity, not all probiotic supplements contain it unless specifically formulated with K2-producing strains.
Fermented Foods are Key: Dietary intake from fermented foods like natto, aged cheeses, and kefir is a reliable way to get high-quality vitamin K2.
MK-7 is Highly Bioavailable: The MK-7 variant of K2, primarily from fermentation, has a longer half-life and superior bioavailability for supporting bone and cardiovascular health.
Gut Production is Insufficient: You cannot rely solely on your gut's endogenous K2 production to meet your nutritional needs, as the quantity and absorption are inconsistent.

The Gut's Role in Vitamin K2 Production

For years, vitamin K was primarily associated with its role in blood coagulation. However, more recent research has highlighted the unique importance of vitamin K2 (menaquinones, or MKs) for extrahepatic functions, such as bone health and cardiovascular protection. Unlike vitamin K1, which primarily comes from leafy green vegetables, K2 is largely of bacterial origin. This is where your gut microbiome enters the picture.

Your gut is home to trillions of bacteria, and certain species, particularly in the large intestine, can synthesize various menaquinone forms. Bacteria use menaquinones in their electron transfer chain to produce energy, making K2 available for absorption by the body. Research shows that bacterially synthesized K2 can be absorbed, contributing to your overall vitamin K status.

The quantity and type of K2 produced depend on the specific bacterial species. For example, Escherichia coli produces MK-8, while Bacteroides species can produce longer-chain versions like MK-10 and MK-11. The precise amount your gut produces and absorbs varies due to diet, age, and individual microbiome composition.

Fermented Foods: A Direct Source of K2-Producing Probiotics

While your intestinal flora plays a part, one of the most reliable ways to get bacterially-produced vitamin K2 is through fermented foods. Many fermented products use specific bacteria as starter cultures known to produce menaquinones. Here are some of the best dietary sources:

Natto: This Japanese dish of fermented soybeans is an exceptionally rich source of MK-7.
Cheese: Aged varieties like Gouda, Brie, and Cheddar contain notable levels of K2, which vary based on the fermentation process.
Kefir: This fermented milk drink is a good source of K2, particularly MK-9, and provides various probiotics.
Sauerkraut: Fermented cabbage contains beneficial bacteria that contribute to its K2 content.
Other Fermented Dairy: Yogurt and buttermilk can also contain K2, depending on the bacterial strains used.

MK-4 vs. MK-7: Why the Difference Matters

Vitamin K2 exists in different forms, with MK-4 and MK-7 being the most common, differing in their sources and how the body uses them.

| Feature | MK-4 | MK-7 | Source of Production | Primarily from animal products and the conversion of K1 in tissues | Predominantly from bacterial synthesis, especially during fermentation | Dietary Sources | Meat, liver, eggs, and dairy fat | Fermented foods like natto, cheese, and kefir | Bioavailability | Lower bioavailability with a short half-life (6-8 hours), requiring more frequent doses | Superior bioavailability with a long half-life (several days), allowing for once-daily dosing | Health Implications | Effective but requires consistent intake due to rapid clearance | Better suited for maintaining consistent blood levels for bone and cardiovascular health |

MK-7's longer half-life means it stays in your system longer, supporting bone mineralization and helping prevent arterial calcification.

Can You Rely Solely on Probiotics for K2?

While your gut microbiome produces K2, the amount is highly variable and often insufficient for optimal needs. Fermented foods rich in specific K2-producing bacteria offer a more direct and reliable way to get enough. Probiotic supplements typically focus on other gut health benefits and don't necessarily contain K2 unless specifically formulated with K2-producing strains. Therefore, a diet including K2-rich fermented foods and a balanced approach is best.

Conclusion

While probiotics themselves don't automatically contain K2, the beneficial bacteria in a healthy gut microbiome and those used in fermented foods do produce this essential vitamin. This highlights the link between gut health and nutrition. Including K2-rich fermented foods in your diet can support bone and cardiovascular health. However, due to variable production in the gut, dietary sources or specific supplements are the most reliable ways to ensure sufficient K2 intake.

Key Takeaways

Bacteria Produce K2: Certain bacteria in the gut and fermented foods naturally produce vitamin K2.
Fermented foods are a key source: Natto, aged cheeses, kefir, and sauerkraut are excellent dietary sources of K2.
MK-7 is a superior form: MK-7, common in fermented foods, has better bioavailability and a longer half-life than MK-4.
Gut production is variable: The amount of K2 your gut produces varies and shouldn't be your only source.
Diet is the best approach: A diet rich in K2-producing fermented foods is the most effective way to boost K2 intake.
Probiotic supplements vary: Standard probiotic supplements may not contain K2 unless explicitly stated.

Frequently Asked Questions

Vitamin K1 (phylloquinone) is found mainly in leafy green vegetables and is crucial for blood clotting. Vitamin K2 (menaquinone) is produced by bacteria in the gut and during fermentation, and it is more involved in extrahepatic functions like directing calcium to bones and preventing its accumulation in arteries.

Several bacterial strains, including certain species of Lactococcus lactis, Lactobacillus plantarum, and Bacillus subtilis, are known to produce vitamin K2. The specific type and amount of menaquinone produced can vary significantly by strain.

No, vitamin K2 is a collection of compounds called menaquinones (MKs) with varying chain lengths. The most well-studied are MK-4 and MK-7, which differ in their sources, bioavailability, and half-life in the body.

It is not recommended to rely solely on probiotic supplements for vitamin K2. Most supplements are not designed for K2 production, and the amount produced by your gut flora is often insufficient and unreliable. Dietary intake from specific fermented foods is a more dependable approach.

For healthy individuals, consuming K2 from fermented foods is safe. However, relying on endogenous gut production is inconsistent. In rare cases, individuals with compromised immune systems have experienced side effects from certain probiotics, but this is not specific to K2 production.

The K2 produced by your gut bacteria can be absorbed and utilized by your body, helping with bone mineralization and cardiovascular health by activating proteins that regulate calcium. A healthy gut microbiome, therefore, plays a supportive role in maintaining vitamin K2 status.

The most effective way is through a balanced diet rich in K2-producing fermented foods like natto, aged cheeses, and kefir. For some, a supplement containing a highly bioavailable form like MK-7 may also be beneficial, especially if dietary intake is low.