Understanding Vitamin K2: Forms and Functions
Vitamin K is a group of fat-soluble vitamins essential for various bodily functions, primarily blood clotting and calcium metabolism. It is primarily split into two forms: Vitamin K1 (phylloquinone), found in leafy greens, and Vitamin K2 (menaquinone), found in animal products and fermented foods. Within Vitamin K2, there are further subtypes, the most notable being MK-4 and MK-7. MK-4 is a shorter chain menaquinone found in animal fats and has a shorter half-life, while MK-7 is a longer chain version produced by bacterial fermentation and has a longer half-life, making it more bioavailable.
The Role of K2 in the Body
Vitamin K2 plays a crucial role in directing calcium to the right places in the body. It activates specific proteins, like osteocalcin, which helps bind calcium to the bone matrix, improving bone mineral density and strength. At the same time, it activates Matrix Gla Protein (MGP), which prevents calcium from depositing in soft tissues and arteries, reducing the risk of cardiovascular disease.
Fish that are High in K2
While many fish contain some level of vitamin K2, the concentrations are often far lower than in other animal or fermented sources. However, some fish stand out as more significant contributors.
Eel: The Top Fish Source
According to nutritional data, eel is one of the richest fish sources of vitamin K2 (MK-4). A 100-gram serving of eel can provide a significant amount of this nutrient. This makes it a standout option for those seeking K2 from aquatic sources.
Fatty Fish: A Moderate Source
Fatty fish, such as salmon, sardines, and mackerel, contain smaller amounts of vitamin K2. For instance, a 100-gram serving of salmon or mackerel offers a comparatively small amount of K2. However, these fish offer a range of other benefits, including high levels of omega-3 fatty acids and vitamin D, both of which work synergistically with K2 for optimal bone and heart health.
Other Fish Products
- Fermented Cod Liver Oil: While regular cod liver oil primarily provides vitamins A and D, certain fermented versions, particularly those fortified with ingredients like natto, can be excellent sources of vitamin K2 (MK-7). This process boosts the K2 content significantly, making it a potent supplement.
- Canned Tuna (in oil): Some vitamin K can be found in canned tuna, but often it is derived from the oil it's packed in rather than the fish itself. The amount can vary depending on the oil used.
Comparison of K2 Food Sources
To put fish-based sources into perspective, here is a comparison of vitamin K2 content in a 100-gram serving of various foods, primarily showing the MK-4 content unless otherwise noted.
| Food Source | Primary K2 Form | Approx. K2 Content (µg/100g) | Notes |
|---|---|---|---|
| Natto (fermented soybeans) | MK-7 | 1000+ | Richest known source |
| Goose Liver Pâté | MK-4 | 369 | Very high MK-4 content |
| Eel | MK-4 | 63.1 | Highest known fish source |
| Soft Cheeses (e.g., Camembert) | MK-7, MK-9 | ~56 | Content varies by type |
| Hard Cheeses (e.g., Gouda) | MK-8, MK-9 | ~76 | Content varies by age and type |
| Chicken Liver (pan-fried) | MK-4 | 12.6 | A strong meat-based source |
| Egg Yolks (from pasture-raised) | MK-4 | ~15-30 | Content varies based on hen's diet |
| Salmon | MK-4 | ~0.5-0.6 | A moderate source |
Key Considerations for K2 Intake
When planning your diet, remember that not all vitamin K2 is created equal. The longer-chain menaquinones like MK-7, typically from fermented foods, are more bioavailable and have a longer half-life than the MK-4 form found in animal products. Furthermore, the K2 content in animal products, including fish, is highly dependent on the animal's diet. For instance, K2 levels are higher in products from grass-fed animals that consume K1-rich plants.
Conclusion
While fish like eel and some fermented fish products are respectable sources of vitamin K2, they are not the most concentrated dietary source available. Other animal products like goose liver pâté, hard and soft cheeses, and egg yolks, especially from pasture-raised animals, offer higher concentrations of the MK-4 form. For the more bioavailable MK-7 form, fermented foods like natto are unparalleled. A balanced approach incorporating a variety of these foods is the most effective way to ensure adequate vitamin K2 intake for bone and cardiovascular health. For those seeking concentrated K2 without relying heavily on these specific foods, fortified fermented cod liver oil can be a viable supplement option.
Frequently Asked Questions
What is the difference between vitamin K1 and K2?
Vitamin K1 is found in leafy green vegetables and is primarily involved in blood clotting. Vitamin K2 is found in animal products and fermented foods and is crucial for directing calcium to bones and keeping it out of arteries.
Does salmon have a lot of vitamin K2?
No, salmon is a moderate source at best. While healthy, it contains far less vitamin K2 than other animal products or fermented foods.
What is the richest food source of vitamin K2?
The richest source of vitamin K2 is natto, a traditional Japanese food made from fermented soybeans, which is especially high in the long-chain MK-7 form.
Is fermented cod liver oil a good source of K2?
Yes, specifically when it is fermented, as this process can increase K2 levels. Some brands also fortify their product with natto for a higher MK-7 concentration.
What are the main benefits of getting enough vitamin K2?
Adequate vitamin K2 helps ensure that calcium is used to build strong bones and teeth and is prevented from accumulating in soft tissues and arteries, which supports heart health.
Can I get enough K2 from my diet alone?
For most people consuming a Western diet, getting enough K2 from diet alone is difficult. It often requires incorporating specific foods like fermented products, certain cheeses, or pasture-raised egg yolks.
How does vitamin K2 work with vitamin D?
Vitamin K2 and Vitamin D work synergistically to support bone and heart health. Vitamin D aids in calcium absorption, while K2 ensures that absorbed calcium is deposited correctly in bones and not in arteries.
