Understanding the Vitamin K Family
Vitamin K was first discovered for its role in blood coagulation, deriving its name from the German word "Koagulationsvitamin". Over time, scientists learned that what was initially considered a single vitamin is actually a group of compounds, with the two most prominent forms being vitamin K1 (phylloquinone) and vitamin K2 (menaquinones). These different forms have unique dietary sources, absorption rates, and functions in the body, which directly influences whether you should focus on supplementing with vitamin K or K2.
Vitamin K1 (Phylloquinone)
Vitamin K1 is the most common form of vitamin K in the human diet, primarily sourced from leafy green vegetables such as kale, spinach, and broccoli. Its main function is in the liver, where it plays a critical role in activating proteins necessary for blood clotting. For a minor injury, this function prevents excessive bleeding. The body, however, tends to absorb K1 poorly from plant sources, and it has a relatively short half-life, staying in the bloodstream for only a few hours.
Vitamin K2 (Menaquinones)
Vitamin K2 is found predominantly in fermented foods like natto and certain animal products, including cheese and egg yolks. It exists in various subtypes, known as menaquinones (MK-n), which differ in their side-chain length. The most well-researched subtypes for supplementation are MK-4 and MK-7. Unlike K1, which is primarily concentrated in the liver, K2 is distributed more widely throughout the body, including extra-hepatic tissues like bones and blood vessels. This form of the vitamin has higher bioavailability and a significantly longer half-life, with MK-7 lasting for several days in the blood.
Specialized Benefits of Vitamin K2
While both forms are essential, K2 is responsible for specific health benefits that go beyond K1's primary clotting role. These benefits are tied to K2's function in directing calcium throughout the body.
Enhanced Bone Health
For those concerned with bone strength and osteoporosis, K2 is the more potent form. It activates osteocalcin, a protein that binds calcium to the bone matrix, ensuring proper bone mineralization. Research shows that K2, particularly the MK-7 subtype, can improve bone mineral density and reduce fracture risk, especially in postmenopausal women. Vitamin D is crucial for absorbing calcium, but K2 is the nutrient that directs this calcium to where it needs to go: the bones.
Promotes Cardiovascular Health
Calcium buildup in the arteries is a significant risk factor for heart disease. Vitamin K2 plays a key role in preventing this by activating another protein, Matrix Gla Protein (MGP), which inhibits vascular calcification. Studies have linked higher dietary intake of vitamin K2 with a reduced risk of coronary heart disease. This anti-calcification effect is primarily associated with K2, not K1.
K1 vs. K2: Key Differences
To simplify the comparison, here is a breakdown of the two primary forms of vitamin K.
| Feature | Vitamin K1 (Phylloquinone) | Vitamin K2 (Menaquinones) |
|---|---|---|
| Primary Source | Leafy green vegetables (kale, spinach, broccoli) | Fermented foods (natto), animal products (egg yolks, cheese) |
| Primary Function | Blood clotting in the liver | Bone mineralization and cardiovascular protection (extra-hepatic) |
| Absorption | Poorly absorbed from plant sources | Absorbed more readily, especially MK-7 |
| Half-Life | Short (hours) | Long (days), especially MK-7 |
| Key Benefits | Essential for blood coagulation | Directs calcium to bones, prevents arterial calcification |
Which Supplement to Choose: Vitamin K1, MK-4, or MK-7?
If your goal is to address potential deficiencies in bone and cardiovascular health, a vitamin K2 supplement is the more targeted approach. When choosing a K2 supplement, the form matters:
- MK-4: Found in animal products, this subtype has a short half-life and requires larger, more frequent doses for therapeutic effects. It is also converted from K1 in the body, but this process is often inefficient.
- MK-7: This long-chain subtype is derived from fermented foods, most notably natto. Due to its superior bioavailability and long half-life, MK-7 remains in the blood longer and is more effective at lower doses, making it the preferred choice for supplementation.
How to Get Your K1 and K2
While supplements are an option, prioritizing food sources is always recommended.
Vitamin K1-rich foods:
- Kale
- Spinach
- Collard greens
- Broccoli
Vitamin K2-rich foods:
- Natto (contains very high levels of MK-7)
- Hard and soft cheeses (contain various MKs)
- Egg yolks
- Goose or chicken liver
- Fermented dairy products
It is wise to consume these fat-soluble vitamins with a source of fat to enhance absorption.
Conclusion: The Final Verdict
For blood clotting, your body primarily relies on the K1 it gets from leafy greens and its liver processing. For extra-hepatic benefits like directing calcium away from arteries and into bones, vitamin K2 is the better and more targeted choice, especially in supplement form. The differences in bioavailability and half-life make K2, particularly the MK-7 subtype, a more efficient way to support long-term bone and cardiovascular health. For most people, focusing on a diverse diet rich in both K1 and K2 sources is the best approach. However, if supplementation is considered, consulting a healthcare provider is essential, particularly for individuals on blood-thinning medications.
For more detailed information on vitamin K's functions and dietary intake, the NIH Office of Dietary Supplements provides extensive resources on the topic.
