Menaquinone: The Common Name for Vitamin K2
Vitamin K refers to a group of fat-soluble compounds rather than a single nutrient. The primary forms are vitamin K1 (phylloquinone) and vitamin K2 (menaquinone). Menaquinone, the common name for vitamin K2, is primarily sourced from animal products and fermented foods. Phylloquinone is found in leafy green vegetables. Despite their similar structures, the side chains of menaquinones affect their function and absorption. K2 plays a more significant role in non-liver tissues like bones and arteries.
The Diverse Forms of Vitamin K2 (Menaquinones)
The menaquinones, or vitamin K2, are categorized based on their side chain length, represented as MK-n, where 'n' denotes the number of isoprene units. Menaquinone-4 (MK-4) and menaquinone-7 (MK-7) are the most studied. Each possesses unique characteristics influencing its function.
- Menaquinone-4 (MK-4): This is the main form of vitamin K2 in mammalian tissues, such as the brain and reproductive organs. Unlike other menaquinones, the body synthesizes MK-4 from vitamin K1 (phylloquinone). It has a shorter half-life, circulating in the blood for a few hours. Dietary sources include meat, egg yolks, and high-fat dairy from grass-fed animals.
- Menaquinone-7 (MK-7): This menaquinone is known for its bioavailability and longer half-life, remaining in circulation for several days. Bacteria produce it during fermentation. Natto (fermented soybeans) contains high concentrations of MK-7.
Why Vitamin K2 Matters: Beyond Clotting
Both K1 and K2 activate blood-clotting proteins, but vitamin K2 has a more pronounced effect on calcium metabolism outside the liver. This is crucial for bone and cardiovascular health.
- Bone Health: Vitamin K2 activates proteins like osteocalcin, which binds calcium to bone, promoting mineralization. A deficiency may weaken bones, increasing fracture risk. K2, especially MK-7, may increase bone mineral density in postmenopausal women.
- Heart Health: Menaquinones help prevent vascular calcification, where calcium builds up in arteries, leading to stiffness and heart disease. Vitamin K2 activates matrix Gla protein (MGP), preventing calcium deposits in soft tissues and redirecting them to the bones. Higher K2 intake may reduce heart disease risk.
- Potential Anti-inflammatory and Antioxidant Effects: Vitamin K2 may have anti-inflammatory and antioxidant properties. This suggests a protective role in overall health, though more research is required.
Comparison of MK-4 and MK-7 Subtypes
| Feature | Menaquinone-4 (MK-4) | Menaquinone-7 (MK-7) |
|---|---|---|
| Primary Source | Animal products (egg yolks, dairy), synthesized from K1 | Fermented foods (natto, cheese), bacterial synthesis |
| Half-Life | Short (clears within 6-8 hours) | Long (circulates for several days) |
| Bioavailability | Lower, requires multiple doses for sustained levels | Higher, accumulates in the body for constant supply |
| Distribution | Concentrated in specific tissues (brain, pancreas, testes) | Better distributed to extrahepatic tissues (bones, arteries) |
| Research Use | Historically used in high doses for osteoporosis in Japan | Increasingly used in supplements for bone and heart health |
Natural Dietary Sources of Menaquinone
Dietary intake of menaquinones is important. Excellent sources of vitamin K2 include:
- Natto: This fermented soybean product is one of the richest sources of MK-7.
- Cheeses: Aged and hard cheeses, such as Gouda and some fermented cheeses, contain menaquinones.
- Egg Yolks: Egg yolks from pasture-raised hens contain MK-4.
- Organ Meats: Beef and chicken liver contain vitamin K2, specifically MK-4.
- Fermented Dairy: Some fermented dairy products may contain menaquinones.
Conclusion
The common name for vitamin K2 is menaquinone, which helps regulate calcium for bone and cardiovascular health. Its two main forms, MK-4 and MK-7, differ in dietary sources, half-life, and bioavailability. Including menaquinone-rich foods in your diet can support your body's calcium utilization and promote long-term health. For more details, consult the National Institutes of Health (NIH) Office of Dietary Supplements.
