Understanding the Role of Vitamin K2 in the Body
Vitamin K is a fat-soluble nutrient essential for activating proteins involved in several critical physiological processes. While vitamin K1 (phylloquinone) is primarily used by the liver for blood clotting, vitamin K2 (menaquinone) directs its action toward tissues outside the liver. The primary function of vitamin K2 is to activate key proteins that regulate the body's calcium, ensuring it is deposited where it's needed, like in bones and teeth, and kept out of soft tissues, such as arteries.
Two of the most important vitamin K2-dependent proteins are:
- Osteocalcin: Secreted by bone-building cells (osteoblasts), activated osteocalcin directs calcium to integrate into the bone matrix, improving bone density and strength.
- Matrix Gla Protein (MGP): Found in the walls of blood vessels, activated MGP inhibits the buildup of calcium in arteries, thereby protecting against arterial calcification and stiffening.
The Two Primary Forms of Vitamin K2: MK-4 and MK-7
The most extensively studied forms of vitamin K2 are menaquinone-4 (MK-4) and menaquinone-7 (MK-7), which differ significantly in their sources, metabolism, and effects within the body. MK-4 has a shorter chemical side chain, while MK-7 has a longer one.
- MK-4: This form is found in animal products like meat, eggs, and dairy, particularly from grass-fed animals. Interestingly, MK-4 is the form that all other forms of vitamin K are converted into within the body's tissues. However, the MK-4 found in food is poorly absorbed at a nutritional intake.
- MK-7: This form is a long-chain menaquinone produced by bacteria during fermentation. It is most famously and abundantly found in the Japanese fermented soybean dish nattō.
MK-4 vs. MK-7: A Comparison of Bioavailability and Half-Life
One of the most critical factors distinguishing MK-4 from MK-7 is their half-life in the bloodstream, which directly impacts their bioavailability and efficacy at a consistent intake. Half-life refers to how long a substance remains in the body before half of it is eliminated.
MK-4's half-life is quite short, lasting only a few hours. For MK-4 to be effective, some research suggests a higher, often pharmacological, intake (in the milligram range) may be necessary to maintain adequate levels in the bloodstream. In contrast, MK-7 boasts a much longer half-life, circulating in the blood for up to 72 hours. This extended availability allows for consistent activation of K2-dependent proteins throughout the body with a single, much lower daily intake (in the microgram range).
MK-4 vs. MK-7 for Bone and Heart Health
Clinical studies have explored the benefits of both forms, but their differing pharmacokinetics lead to different considerations for bone and heart health.
Benefits for Bone Health
- MK-4: Historically, MK-4 has been used in Japan at high intakes to address osteoporosis. Meta-analyses of these high-intake trials have shown a reduction in fractures. Some studies have also indicated that MK-4 can improve bone quality indices.
- MK-7: Research shows that consistent, nutritional intakes of MK-7 can help maintain bone mineral content and density in healthy postmenopausal women. Its longer half-life provides a steady supply for osteocalcin activation, which is crucial for bone mineralization.
Benefits for Cardiovascular Health
- MK-7: Evidence strongly supports MK-7's role in cardiovascular health by activating MGP, which helps prevent arterial calcification. A long-term study in postmenopausal women found that regular intake of MK-7 for three years significantly improved arterial elasticity and reduced arterial stiffness compared to a placebo.
- MK-4: Due to its rapid clearance, MK-4 is less effective at activating MGP in extrahepatic tissues. Higher, therapeutic intakes may be required to achieve vascular benefits, which are not typically found in nutritional supplements.
MK-4 vs. MK-7: A Comparison at a Glance
| Feature | MK-4 | MK-7 |
|---|---|---|
| Primary Source | Animal products (meat, eggs, dairy) | Fermented foods (nattō, some cheeses) |
| Natural Production | Converted in human tissues from other K vitamins | Produced by bacteria |
| Half-Life | Short (a few hours) | Long (up to 72+ hours) |
| Bioavailability (nutritional intake) | Poorly absorbed and low serum levels | Well-absorbed and high serum levels |
| Effective Intake (nutritional goals) | May require large, milligram-range intakes for effect | Effective at lower, microgram-range intakes |
| Efficacy for Bone Health | High-intake evidence for reducing fractures | Nutritional-intake evidence for maintaining bone mineral density |
| Efficacy for Heart Health | Less effective at nutritional intakes | Strong evidence for improving arterial health and elasticity |
Conclusion: Which Form of K2 is Best?
When considering which form of K2 is best for general bone and cardiovascular health via supplementation, MK-7 is often a preferred choice due to its high bioavailability and long half-life. A consistent daily intake of MK-7 can be effective at activating K2-dependent proteins throughout the body, providing steady support for bone mineralization and arterial flexibility.
MK-4, on the other hand, may require significantly higher, more frequent supplementation to achieve therapeutic effects due to its poor absorption and short half-life. While potentially effective for specific clinical applications like high-intake support for bone health, MK-4 supplements may be less practical for general health maintenance for some individuals. Ultimately, for many consumers, a high-quality all-trans MK-7 supplement, often combined synergistically with vitamin D3, offers an efficient approach to optimizing vitamin K2 status.
Integrating K2 into Your Diet
For those interested in natural dietary sources, consuming foods rich in these vitamins is an excellent approach. As mentioned, a notable food source of MK-7 is natto. Other fermented foods like some cheeses also contain menaquinones. MK-4 can be obtained from certain animal products such as liver, eggs, and meat. However, a significant intake of these foods may be necessary to achieve levels comparable to a supplement.
Important Considerations for Supplementation
When starting any new supplement, it is wise to consult a healthcare professional. This is especially true for vitamin K, as it can interfere with blood-thinning medications like warfarin. For those not taking such medications, a combination of vitamins D3 and K2 is often recommended to help ensure proper calcium utilization.
Disclaimer: This article is for informational purposes only and is not a substitute for medical advice. Please consult with a healthcare professional before making any dietary or supplemental changes. For more authoritative information on nutrition, the Office of Dietary Supplements at the National Institutes of Health is an excellent resource.
