Introduction to the Vitamin K Family
While many people are familiar with vitamin K, they may not realize it exists in two primary natural forms: vitamin K1 (phylloquinone) and vitamin K2 (menaquinones). The body's need for vitamin K was discovered in the 1920s when a nutrient deficiency in animals resulted in severe bleeding disorders. The different forms of vitamin K perform distinct roles within the body and are absorbed and transported differently. Vitamin K1, sourced mainly from green leafy vegetables, is crucial for activating blood-clotting proteins in the liver. In contrast, vitamin K2, or menaquinone, has a wider range of functions beyond the liver, influencing bone density and cardiovascular health.
Menaquinone: The Real Name for Vitamin K2
Menaquinone is the collective term for the vitamin K2 family, which consists of various subtypes known as MK-n, where 'n' denotes the number of isoprene units in its side chain. The most clinically relevant and well-studied menaquinone subtypes are menaquinone-4 (MK-4) and menaquinone-7 (MK-7). Each subtype has a unique chemical structure and biological activity, influencing its bioavailability, half-life, and efficacy within the body.
The Subtypes of Menaquinone
- MK-4 (Menatetrenone): This is a short-chain menaquinone and is the most common form of vitamin K in animal tissues, including the brain, pancreas, and reproductive organs. While it can be found in some animal products, it is also converted by the body from dietary vitamin K1. It has a shorter half-life, meaning it doesn't circulate in the bloodstream for as long as MK-7.
- MK-7: This is a long-chain menaquinone produced by certain bacteria during fermentation. It is particularly abundant in the Japanese fermented soybean dish, natto, but also found in some cheeses. Because MK-7 has a longer half-life, it remains in the body for a much longer period, making it a highly bioavailable form of vitamin K2.
- Other Menaquinones (MK-8, MK-9, etc.): Other longer-chain menaquinones are also produced by bacteria and can be found in different fermented foods, particularly certain cheeses. The precise health implications of these specific subtypes are still being investigated, but they contribute to overall menaquinone intake.
Functions of Vitamin K2 (Menaquinone)
Beyond blood clotting, menaquinone plays a critical role in calcium metabolism, a function that vitamin K1 does not effectively support. Vitamin K2 is necessary to activate specific vitamin K-dependent proteins (VKDPs) that bind calcium.
- Bone Health: Vitamin K2 activates osteocalcin, a protein produced by osteoblasts (bone-building cells), which helps bind calcium to the bone matrix, promoting proper mineralization. This process is crucial for developing and maintaining strong, healthy bones and reducing the risk of osteoporosis.
- Cardiovascular Health: Menaquinone activates matrix Gla protein (MGP), a potent inhibitor of soft tissue calcification. By keeping calcium out of the arteries and blood vessels, MGP helps maintain arterial elasticity and reduces the risk of cardiovascular damage. This protective effect is more strongly linked to K2 intake than K1.
The Importance of Bioavailability
Due to its longer side chain, MK-7 is more bioavailable than MK-4 and remains in circulation for several days. This prolonged presence allows for more sustained activation of vitamin K-dependent proteins in extra-hepatic tissues, such as bones and blood vessels, leading to more consistent health benefits. While MK-4 is also effective, it requires multiple daily doses at a much higher concentration to achieve similar effects.
Comparison of MK-4 and MK-7
| Feature | Menaquinone-4 (MK-4) | Menaquinone-7 (MK-7) |
|---|---|---|
| Primary Source | Found in animal products (e.g., egg yolks, meat, dairy); also converted from K1 in animal tissues. | Produced by bacterial fermentation; highest concentration in natto, also found in some cheeses. |
| Half-Life | Short (approx. 6-8 hours), requiring more frequent or higher dosing for sustained effect. | Long (approx. 72 hours), providing a stable, circulating supply. |
| Distribution | Concentrates in specific tissues like the brain, kidneys, and pancreas. | Circulates in the bloodstream for longer, reaching extra-hepatic tissues effectively. |
| Effective Dosage | Requires very high doses (milligrams) for therapeutic bone benefits, often administered in multiple daily doses. | Effective at much lower, microgram-level doses taken once daily due to high bioavailability. |
| Application | Historically used for bone health, but modern research favors MK-7 for supplementation due to its better stability and bioavailability. | Widely used in supplements for bone and cardiovascular health, leveraging its long half-life and efficient extra-hepatic action. |
Conclusion: Understanding the Name and Nuances of Vitamin K2
The real name for vitamin K2 is menaquinone, but that's only part of the story. Recognizing the different menaquinone subtypes, especially MK-4 and MK-7, is crucial for understanding its sources, function, and bioavailability. While vitamin K1 is essential for blood clotting, it is the menaquinone family that directs calcium away from soft tissues and into the bones, thereby supporting cardiovascular health and maintaining bone density. The specific differences in half-life and absorption between MK-4 and MK-7 highlight why certain dietary sources and supplements may be more effective than others for long-term health benefits, particularly for bone mineralization and cardiovascular protection. For most people, incorporating a variety of K2-rich foods or a quality MK-7 supplement is the most practical way to ensure adequate intake. For further information on the broader vitamin K family, including K1 and K2, refer to the Linus Pauling Institute.
