Menaquinones, often referred to as vitamin K2, represent a group of compounds that play a critical role in human health, primarily in bone metabolism and blood coagulation. Unlike vitamin K1 (phylloquinone), which is predominantly found in green leafy vegetables, menaquinones are found in fermented foods and certain animal products.
Structure and Types:
Menaquinones are characterized by their chemical structure, which includes a quinone ring and a side chain of isoprenoid units. The length of the isoprenoid side chain varies, leading to different forms of vitamin K2, designated as MK-n, where "n" represents the number of isoprenoid units. MK-4 through MK-10 are the most common forms in the human diet, with MK-4 (with 4 isoprenoid units) and MK-7 (with 7 isoprenoid units) being the most extensively studied.
Sources:
- MK-4: Found in animal products like meat, eggs, and dairy, MK-4 is synthesized by certain animals from vitamin K1.
- MK-7 and other long-chain menaquinones: These are predominantly found in fermented foods, such as natto (fermented soybeans), which is particularly high in MK-7. Cheese and other fermented dairy products also contain various menaquinones.
Biological Roles:
- Bone Health: Menaquinones are crucial for the carboxylation of osteocalcin, a protein that helps bind calcium to the bone matrix, thereby improving bone strength and reducing the risk of fractures.
- Cardiovascular Health: Vitamin K2 is believed to inhibit vascular calcification by carboxylating matrix Gla protein, which prevents the deposition of calcium in blood vessel walls.
- Blood Coagulation: While vitamin K1 is more directly involved in clotting factor synthesis, menaquinones also play a role in this process, albeit to a lesser extent.
Absorption and Metabolism:
Menaquinones are absorbed in the small intestine and transported to the liver and other tissues. MK-4, being shorter in chain length, is rapidly cleared from the blood, while longer-chain menaquinones like MK-7 have a longer half-life, leading to more sustained levels in the bloodstream.
Measurement and Analysis:
The quantification of menaquinones in plasma or serum can provide insights into an individual's vitamin K status. High-performance liquid chromatography (HPLC) is commonly used for this purpose, often coupled with mass spectrometry (MS) for enhanced specificity and sensitivity.
Health Implications:
Ensuring adequate intake of vitamin K2 is important for maintaining bone health and minimizing the risk of cardiovascular diseases. While deficiency symptoms specific to vitamin K2 are not well-defined, a deficiency in vitamin K overall can lead to impaired blood clotting and increased bleeding risk.
In summary, menaquinones or vitamin K2 (MK-4 to MK-10) are essential nutrients with key roles in bone health, cardiovascular health, and coagulation. Understanding their dietary sources, biological roles, and methods for their quantification can aid in assessing and improving vitamin K status for overall health maintenance.
References:
https://www.vitas.no/about/publications/vitamin-k2-in-different-bovine-muscles-and-breeds#