Vitamin K is a fat-soluble nutrient essential for various physiological processes in the body. For decades, its most well-known function was its critical role in blood coagulation, but modern research has illuminated its broader impact, particularly on bone and cardiovascular health. The term “vitamin K” is an umbrella for several compounds, but the two main forms found in the diet are vitamin K1 (phylloquinone) and vitamin K2 (menaquinones). The question of which form of vitamin K is better is not straightforward, as each has distinct properties and functions within the body.
Understanding the Different Forms of Vitamin K
Vitamin K1 (Phylloquinone)
Vitamin K1 is the most abundant dietary source of vitamin K, comprising roughly 75–90% of a person's total intake. Its name, phylloquinone, reflects its origin in plant sources, specifically the chloroplast membranes of leafy green vegetables. While K1 is crucial for producing the clotting factors in the liver, its bioavailability from plant foods is notoriously low—some studies suggest less than 10% is absorbed. To improve K1 absorption, consuming it with a source of dietary fat is beneficial.
Key sources of vitamin K1 include:
- Leafy Greens: Kale, spinach, collard greens, turnip greens, and Swiss chard.
- Other Vegetables: Broccoli, Brussels sprouts, and cabbage.
Vitamin K2 (Menaquinones)
Vitamin K2 represents a family of compounds called menaquinones (MKs). Unlike K1, which comes from plants, K2 is primarily synthesized by bacteria. It is found in animal products and certain fermented foods, and some is also produced by the bacteria in our gut. Vitamin K2 is further categorized into subtypes, from MK-4 to MK-13, based on the length of its side chain. The two most-researched subtypes are MK-4 and MK-7, which have different properties.
- MK-4: A short-chain menaquinone found in animal products like meat, eggs, and high-fat dairy from grass-fed animals. The body can also convert K1 into MK-4 in tissues like the brain and kidneys.
- MK-7: A long-chain menaquinone produced by certain bacteria. The richest source is natto, a traditional Japanese dish of fermented soybeans. MK-7 is particularly known for its high bioavailability and long half-life, meaning it stays in the bloodstream longer than K1 and MK-4.
K1 vs. K2: A Comparison of Key Features
| Feature | Vitamin K1 (Phylloquinone) | Vitamin K2 (Menaquinones, e.g., MK-7) |
|---|---|---|
| Primary Function | Activating clotting factors in the liver. | Activating proteins related to bone and cardiovascular health in extra-hepatic tissues. |
| Food Sources | Leafy green vegetables and vegetable oils. | Fermented foods (natto, some cheeses) and animal products (eggs, meat). |
| Bioavailability | Relatively low absorption from plant sources. | High bioavailability, especially MK-7. |
| Half-Life | Short, lasting only a few hours in the bloodstream. | Long, with MK-7 remaining in circulation for days. |
| Tissue Distribution | Primarily concentrated and utilized by the liver. | Widely distributed to extra-hepatic tissues like bones and blood vessels. |
Bioavailability and Tissue Distribution
The difference in bioavailability is one of the most critical distinctions between K1 and K2. When you consume vitamin K1 from vegetables, it is tightly bound to chloroplasts, which hinders its absorption. Once absorbed, the liver rapidly uptakes and uses K1, primarily for activating blood-clotting proteins. This rapid hepatic utilization means less K1 is available for other tissues.
In contrast, vitamin K2, especially the long-chain MK-7, is more readily absorbed and has a significantly longer half-life, circulating in the blood for several days. This prolonged presence allows it to be effectively distributed to extra-hepatic tissues, such as bones and the arterial walls, where it performs functions beyond blood clotting. In fact, some studies show MK-7 is a better supplier of MK-4 to extra-hepatic tissues than nutritional doses of MK-4 itself.
Health Benefits: K1 vs. K2
Blood Clotting
Both K1 and K2 are cofactors for the enzyme gamma-glutamylcarboxylase, which activates the proteins needed for blood clotting. Vitamin K1, given its primary residence in the liver, is the most well-known form for this purpose. However, high doses of K2, particularly MK-7, can also influence clotting factors, a consideration for individuals on anticoagulant medications like warfarin.
Bone Health
Vitamin K plays an important role in bone metabolism by activating a protein called osteocalcin, which helps bind calcium to the bone matrix. While both K1 and K2 can contribute, research suggests that K2, especially MK-7, may be more effective for bone health due to its better distribution to bone tissue. Studies on postmenopausal women, for example, have shown that MK-7 supplementation can improve bone mineral density and reduce bone loss.
Cardiovascular Health
Perhaps the most significant difference lies in their impact on cardiovascular health. Vitamin K activates matrix Gla protein (MGP), which helps prevent calcium from depositing in the walls of arteries and other soft tissues. This process is crucial for preventing plaque buildup and arterial stiffening. Multiple studies, including the Rotterdam Study, have found that higher intake of K2 (but not K1) is associated with a reduced risk of coronary heart disease and other cardiovascular events. This is because K2 is more effectively distributed to vascular tissues.
Which form is better?
Deciding which form of vitamin K is better ultimately depends on the specific health outcome you're targeting. For general coagulation functions, the adequate intake of vitamin K, mainly derived from K1 in the diet, is usually sufficient for most healthy adults. However, for supporting long-term bone density and inhibiting arterial calcification, vitamin K2 (particularly MK-7) holds significant advantages due to its superior bioavailability and distribution to extra-hepatic tissues. The scientific community is still actively researching the nuances, but the evidence for K2's benefits in bone and heart health is substantial.
Sourcing Vitamin K from Your Diet
To ensure a comprehensive intake of both forms, a varied diet is key. Here are some examples:
- Vitamin K1 Sources: Kale, spinach, collard greens, broccoli, Swiss chard, and parsley.
- Vitamin K2 Sources: Natto (fermented soybeans, especially high in MK-7), hard and soft cheeses (e.g., Gouda, Brie, Camembert), grass-fed animal products (egg yolks, chicken), and sauerkraut.
Consuming vitamin K with fats, like adding olive oil to a salad or cooking vegetables with butter, can help improve absorption.
Supplements: K1 vs. K2 vs. Combination
For those with specific health concerns or dietary restrictions, supplements are an option. Supplements are available in K1, K2 (often as MK-7 or MK-4), or as a combination. Because MK-7 has such high bioavailability and a long half-life, it is a very popular choice for supplementation targeting bone and heart health. Some supplements combine vitamin K2 with vitamin D3, as these nutrients work synergistically for optimal bone and cardiovascular health. However, individuals on blood-thinning medication like warfarin must consult a healthcare provider before taking any vitamin K supplement, as changes in intake can interfere with its effectiveness.
Conclusion
In the debate over which form of vitamin K is better, there is no single winner. Both vitamin K1 and vitamin K2 are essential, but they play different, complementary roles in the body. Vitamin K1 is crucial for hepatic functions like blood clotting, an area where K2 plays a lesser role. On the other hand, vitamin K2 shines in its capacity to benefit extra-hepatic tissues, particularly supporting bone and cardiovascular health. Its superior bioavailability, especially the MK-7 subtype, allows for more effective action in these vital areas. For the most comprehensive support, incorporating both K1-rich leafy greens and K2-rich fermented and animal products into your diet is the optimal strategy. If considering a supplement, a K2 (MK-7) formula may offer distinct advantages for targeting bone density and arterial calcification, but a discussion with a healthcare professional is paramount, especially when on medication.
