What is Vitamin K2 and How is it Made?
Vitamin K is a group of fat-soluble vitamins, with the two most prominent forms being K1 (phylloquinone) and K2 (menaquinone). While vitamin K1 is primarily found in leafy green vegetables, vitamin K2 is a collection of menaquinones (MKs) that are largely synthesized by bacteria. This bacterial production during fermentation is what makes many fermented foods potential sources of K2. Different types of bacteria produce different lengths of the menaquinone side chain, which results in various subtypes of K2, such as MK-7 and MK-9. The specific microbial cultures used, along with fermentation conditions like time and temperature, are major factors that influence the final K2 content and its subtypes in fermented products.
The K2 Profile of Various Fermented Foods
While many people associate fermented foods with probiotics, their vitamin K2 content is a lesser-known but significant benefit. The amount of K2, however, varies dramatically. It's not as simple as assuming all fermented products are rich in this vitamin. The source material and the specific bacteria doing the work are crucial. For example, Bacillus subtilis bacteria are responsible for the exceptionally high MK-7 levels in natto, while different lactic acid bacteria contribute to the menaquinones found in cheeses.
Fermented Soybeans
- Natto: Far and away the richest food source of vitamin K2, particularly the highly bioavailable MK-7 form. One tablespoon can provide more than the recommended daily amount for most people. The unique fermentation process is key to this high concentration.
- Other Soy Products: Other fermented soy products like miso and tempeh also contain some vitamin K2, but in significantly smaller amounts compared to natto.
Fermented Dairy
- Cheeses: Certain cheeses, especially aged, hard, and blue varieties, contain appreciable amounts of vitamin K2. Gouda and blue cheese are notable sources, rich in longer-chain menaquinones like MK-8 and MK-9. The K2 content increases with the cheese's age and depends on the specific starter cultures used.
- Kefir and Fermented Milk: These fermented milk products also contain K2, with kefir grains producing higher levels of MK-7 and MK-9 compared to some yogurts. However, the content can vary based on the specific bacteria used and the fermentation process.
Fermented Vegetables
- Sauerkraut: This fermented cabbage is a source of vitamin K2, but its concentration is relatively low compared to natto or some cheeses. The K2 is synthesized during the lacto-fermentation process. For maximum benefit, it should be raw and unpasteurized.
Comparison of K2 Sources
To put the varying levels into perspective, here is a comparison table showcasing the approximate vitamin K2 content in different food sources. It’s important to remember that these values can fluctuate based on the specific product, processing methods, and origin.
| Food Source | Approximate K2 Content (μg/100g) | Main Menaquinone Type(s) | Notes |
|---|---|---|---|
| Natto (fermented soybeans) | ~1,000+ | MK-7 | Highest known food source, vegan friendly. |
| Blue Cheese | Up to 110 | MK-8, MK-9 | Moderate source, dependent on cheese type and age. |
| Gouda Cheese | ~30-40 | MK-8, MK-9 | Content varies; best with aged versions. |
| Chicken Thigh (with skin) | ~53 | MK-4 | Non-fermented animal source; contains the MK-4 subtype. |
| Egg Yolk (variable) | ~15-32 | MK-4 | Content depends on the animal's diet. |
| Sauerkraut | ~4.8 | MK-4 to MK-9 (various) | Lower concentration, but a plant-based source. |
| Butter | ~15 | MK-4 | Non-fermented dairy source, content tied to fat. |
Health Benefits of Vitamin K2
The significance of vitamin K2 extends far beyond its bacterial origins. Its primary health benefits are rooted in its ability to activate specific proteins in the body. These functions set it apart from its plant-based counterpart, vitamin K1.
- Bone Health: Vitamin K2 activates osteocalcin, a protein that helps bind calcium to the bone matrix. This process is crucial for maintaining bone density and reducing the risk of conditions like osteoporosis, especially in postmenopausal women.
- Cardiovascular Health: One of K2's most important roles is activating Matrix Gla Protein (MGP), which helps prevent the accumulation of calcium in soft tissues, including the arteries. This helps reduce arterial calcification, a major risk factor for heart disease.
- Synergy with Vitamin D: K2 and vitamin D work together synergistically. While vitamin D aids in calcium absorption, vitamin K2 directs that calcium to where it's needed (bones and teeth) and away from where it can cause problems (arteries).
Conclusion
So, are fermented foods high in vitamin K2? The answer is that it depends heavily on the type of food and the fermentation process. While some, like natto, are exceptionally rich sources of the powerful MK-7 subtype, others like sauerkraut and some cheeses offer more moderate levels. The bacterial synthesis of menaquinones during fermentation makes these foods unique contributors to K2 intake, providing benefits for bone and heart health. For those seeking to increase their dietary K2, it is valuable to diversify your intake by exploring different types of fermented foods and considering sources from both bacterial and animal origins to obtain a full spectrum of menaquinone subtypes.
For more in-depth information on the function and sources of vitamin K, the National Institutes of Health provides comprehensive fact sheets.
