The Vitamin K Family: An Overview
Vitamin K is a fat-soluble vitamin essential for several bodily processes, including blood coagulation, calcium metabolism, and bone health. It exists as a group of structurally similar compounds, but the most well-known are the natural forms, vitamin K1 (phylloquinone) and vitamin K2 (menaquinone), and the synthetic variant, vitamin K3 (menadione). These versions share a core chemical structure but differ in their side chains, which significantly affects their absorption, transport, and function in the body.
Vitamin K1 (Phylloquinone)
Vitamin K1 is the primary form of vitamin K found in the human diet, accounting for up to 90% of our total vitamin K intake.
Origin and Primary Function
- Source: Vitamin K1 is synthesized by plants and found most abundantly in green, leafy vegetables.
- Role: Its main function is to activate proteins in the liver that are necessary for blood coagulation. This process is what prevents excessive bleeding from injuries. For individuals taking blood-thinning medication like warfarin, consistent vitamin K1 intake is crucial because it can interfere with the drug's effectiveness.
Food Sources of K1
The best sources of K1 are primarily plants. It is a fat-soluble vitamin, so consuming it with some fat can enhance absorption.
- Kale
- Spinach
- Collard greens
- Broccoli
- Brussels sprouts
- Cabbage
- Soybean oil and canola oil
Vitamin K2 (Menaquinone)
Vitamin K2 is the lesser-known but equally vital form of vitamin K, with critical functions beyond the liver in extra-hepatic tissues.
Origin and Primary Function
- Source: Vitamin K2 is produced by bacteria, both in the human gut and during the fermentation of foods. It is also found in animal products where bacteria play a role in its creation.
- Role: K2's most important functions relate to regulating calcium distribution. It activates matrix Gla-protein (MGP) to prevent calcium from building up in the arterial walls, which is a major risk factor for heart disease. It also activates osteocalcin, a protein that directs calcium to be deposited into bones and teeth, improving bone density.
Subtypes and Food Sources of K2
There are several subtypes of K2, known as menaquinones (MK-n), which differ in their side chain length. MK-4 and MK-7 are the most studied.
- MK-4: Found in animal products like egg yolks and meat. It has a shorter half-life than MK-7.
- MK-7: Found most prominently in fermented foods, particularly natto (fermented soybeans). MK-7 has a longer half-life, meaning it remains active in the body longer and is more bioavailable for extra-hepatic tissues like bones and arteries.
Vitamin K3 (Menadione)
Unlike the other two, vitamin K3 is not a natural food source of vitamin K and is considered unsafe for humans.
Origin and Safety Concerns
- Source: Vitamin K3 is a synthetic compound created in a lab.
- Safety: Human studies from the 1980s and 1990s linked vitamin K3 to liver damage and the destruction of red blood cells. As a result, it is no longer permitted in human supplements or fortified foods in the United States and other regions.
Use in Animals
Despite its toxicity in humans, vitamin K3 is commonly added to animal feed for livestock and pets, where it can be converted into active vitamin K forms.
Comparison of Vitamin K1, K2, and K3
| Feature | Vitamin K1 (Phylloquinone) | Vitamin K2 (Menaquinone) | Vitamin K3 (Menadione) |
|---|---|---|---|
| Origin | Natural (plants) | Natural (bacteria/animals) | Synthetic (man-made) |
| Food Sources | Green, leafy vegetables (kale, spinach, broccoli), some vegetable oils | Fermented foods (natto, sauerkraut), animal products (egg yolks, liver, some cheeses) | None (banned from human consumption) |
| Primary Function | Activates liver proteins for blood clotting factors | Activates calcium-regulating proteins in bones and soft tissues | Converts to K2 in animals, toxic to humans |
| Absorption/Half-Life | Poorly absorbed from plants, shorter half-life | Better absorbed, longer half-life (especially MK-7) | Not applicable for human use |
| Human Supplement | Available as a supplement | Available as a supplement | Not legally sold for human supplements |
Key Takeaways for Your Diet
For a healthy and well-rounded diet, it is important to include sources of both vitamin K1 and K2. While K1 is more readily available and crucial for blood clotting, K2 offers unique benefits for cardiovascular and bone health that K1 cannot provide as effectively. Since K2 is less common in a Western diet, strategic food choices or supplementation may be necessary.
Here are some tips for incorporating both forms into your diet:
- Prioritize green leafy vegetables: Since K1 is abundant in greens, ensure you eat them regularly. Cooking them and adding a fat source like olive oil or avocado can improve absorption.
- Include fermented foods: Consider adding fermented items like sauerkraut or, if you are adventurous, natto to your meals for a potent dose of K2.
- Enjoy some animal products: Foods like eggs and some cheeses, particularly hard cheeses, are good sources of vitamin K2. Grass-fed dairy and meat may offer higher levels.
- Consider supplements with K2: For those unable to consistently obtain enough K2 from food, particularly the longer-lasting MK-7 form, a supplement might be beneficial, especially when combined with vitamin D for synergistic effects on bone and heart health.
Conclusion
The family of vitamin K includes the plant-derived K1 (phylloquinone), the bacterially-derived K2 (menaquinone), and the synthetic K3 (menadione). The primary difference lies in their functions and food sources: K1 primarily supports blood clotting in the liver, while K2 targets calcium regulation in the arteries and bones. The key difference in safety is that K3 is toxic and should not be consumed by humans. By consuming a diverse range of foods, including leafy greens, fermented products, and some animal sources, you can ensure adequate intake of both natural and beneficial forms of this vital nutrient.
For more information on the critical roles of Vitamin K and other micronutrients in your health, refer to authoritative health resources.
