Understanding the Two Forms of Vitamin K
Before diving into the specifics of fermented foods, it's essential to distinguish between the two primary types of vitamin K found in food: K1 and K2.
- Vitamin K1 (Phylloquinone): This form is primarily found in green leafy vegetables like kale, spinach, and broccoli. It is directly involved in blood clotting and is typically the main dietary source of total vitamin K in Western diets.
- Vitamin K2 (Menaquinones): The K2 family consists of several menaquinones (MK-4 to MK-13) that are mostly synthesized by bacteria. This form is found in certain animal products and is notably abundant in specific fermented foods. K2 is known for its role in bone health and cardiovascular health by helping to regulate calcium.
The Bacterial Engine of Vitamin K2 Production
Vitamin K2 in fermented foods is a direct result of bacterial synthesis. The specific bacterial strains present during fermentation determine the quantity and type of menaquinone produced.
For example, the Japanese fermented soybean product, natto, is created using the bacterium Bacillus subtilis. This particular bacteria strain is an exceptionally efficient producer of a long-chain menaquinone called MK-7, making natto the richest known dietary source of K2. Similarly, lactic acid bacteria (LAB) used in the fermentation of dairy products and vegetables, such as Lactobacillus species, can also produce various menaquinone forms. The longer the fermentation process, the higher the potential yield of menaquinone in some products.
Fermented Foods: Your Source of Vitamin K2
While leafy greens provide K1, a variety of fermented foods can significantly boost your intake of the bacterially-produced K2.
Fermented Soybeans
Natto stands out as the ultimate vitamin K2 superfood. Its rich content of MK-7 is a result of the specific fermentation process using Bacillus subtilis. Its unique flavor and texture make it a popular part of the Japanese diet and a potent health booster.
Fermented Dairy
Many types of cheese, especially aged and hard varieties, are excellent sources of vitamin K2, predominantly in the form of MK-9. Examples include Gouda, Edam, and Brie. The K2 content can vary based on the starter cultures used and the length of the aging process. Kefir, a fermented milk drink, is also a source of vitamin K2, with a 100g serving potentially providing around 13µg of the nutrient.
Fermented Vegetables
Sauerkraut, or fermented cabbage, is another good source of vitamin K2, although its content can be more variable than natto. The bacteria present during fermentation synthesize menaquinones, adding K2 to the K1 already present in the cabbage. Kimchi, a Korean fermented vegetable dish, also contains both forms of vitamin K, with studies quantifying both K1 and K2 in various products.
Other Fermented Products
- Kombucha: This fermented tea contains various B vitamins and other nutrients, but typically has very low levels of vitamin K2. The different microbial cultures and substrates used mean its K2 contribution is minimal compared to other fermented foods.
- Yogurt: While some yogurts contain K2 from the fermentation process, the levels are generally lower and more dependent on the bacterial strains used than with hard cheeses or kefir.
Comparison of Vitamin K Content in Fermented Foods
To illustrate the variance, here is a comparison table of vitamin K content in several popular fermented foods. Note that these figures can vary significantly based on processing methods and bacterial strains.
| Fermented Food | Primary Vitamin K Form | Approx. K2 Content (μg per 100g) | Primary Bacterial Type | Notes |
|---|---|---|---|---|
| Natto | K2 (MK-7) | >1000 | Bacillus subtilis natto | Richest source, highly variable based on preparation |
| Aged Cheese | K2 (MK-9) | 30-80 | Lactic Acid Bacteria | Varies widely with cheese type and aging duration |
| Sauerkraut | K1 & K2 | ~5.5 (K2) | Lactobacillus species | Contains both K1 from cabbage and K2 from fermentation |
| Kefir | K2 (MK-9, MK-7) | ~13 (K2) | Kefir grains (yeast and bacteria) | Content varies based on starter culture and fat content |
| Kimchi | K1 & K2 | ~5-12 (K2) | Lactobacillus species | K1 content is also present from the cabbage base |
Health Benefits of K2 from Fermented Foods
Getting vitamin K2 from fermented foods offers distinct health advantages, primarily through its unique physiological functions.
- Bone Health: Vitamin K2 helps activate osteocalcin, a protein that regulates calcium in bone metabolism. Studies suggest that high K2 intake is linked to increased bone mineral density and a reduced risk of bone fractures.
- Heart Health: K2 also helps prevent arterial calcification by directing calcium to the bones and away from soft tissues like arteries. Regular consumption of K2-rich foods has been associated with a lower risk of heart disease.
- Improved Bioavailability: The menaquinone-7 (MK-7) found in foods like natto has a higher bioavailability and longer half-life than K1, meaning it stays active in the body for a longer period.
Conclusion: Making Informed Dietary Choices
In summary, fermented foods are indeed a significant source of vitamin K, specifically the crucial K2 form. The amount varies greatly, with natto being the standout performer for MK-7, while certain cheeses, kefir, and sauerkraut provide valuable amounts as well. For those looking to increase their intake of this important nutrient, incorporating a variety of fermented foods into their diet is an excellent strategy. However, it is always wise to consult a healthcare provider or a registered dietitian if you have concerns about your vitamin K intake, especially if you are on blood-thinning medication. For more detailed information on the health aspects of vitamins, you can refer to authoritative sources like the National Institutes of Health (NIH). NIH Health Professional Fact Sheet on Vitamin K
