Vitamin K is a fat-soluble nutrient known for its function in blood clotting. However, it is a family of compounds with diverse roles. Menaquinone-7 (MK-7) is a specific form of Vitamin K2 that is gaining attention for its significant impact on long-term health beyond coagulation. Its unique chemical structure allows it to remain in circulation for a prolonged period, making it highly effective at reaching tissues throughout the body.
The Identity of Menaquinone-7
Vitamin K is broadly classified into two main natural forms: Vitamin K1 (phylloquinone) and Vitamin K2 (menaquinone). Phylloquinone is primarily found in green leafy vegetables, while menaquinones (MKs) are synthesized by bacteria and found in fermented foods and animal products. The K2 family is further divided into subtypes designated by a number, such as MK-4 and MK-7, which refers to the length of the isoprenyl side chain attached to the naphthoquinone ring.
MK-7 possesses a longer side chain than the short-chain MK-4, which is the reason for its superior pharmacokinetic profile. This longer structure provides MK-7 with a significantly extended half-life, allowing it to remain active in the bloodstream for up to 72 hours. In comparison, Vitamin K1 is rapidly cleared from the circulation, primarily stored in the liver. MK-7’s prolonged presence enables it to efficiently reach and activate proteins in tissues outside the liver, known as extrahepatic tissues.
Key Biological Function: Directing Calcium
The primary function of MK-7 is to serve as a necessary cofactor for the enzyme \(\gamma\)-glutamyl carboxylase. This enzyme is responsible for activating a group of proteins known as Vitamin K-dependent proteins (VKDPs) through a process called \(\gamma\)-carboxylation. Without sufficient MK-7, these proteins remain in an inactive state, unable to perform their designated functions.
MK-7’s most recognized role involves the precise management and distribution of calcium within the body. This is achieved by activating two key VKDPs: Matrix Gla Protein (MGP) and Osteocalcin.
MGP is found in the walls of blood vessels and soft tissues, acting as a potent inhibitor of calcification. When MGP is activated by MK-7, it binds to calcium ions, effectively preventing them from depositing in the arteries and other soft tissues, which can lead to vascular stiffness and compromise cardiovascular health.
Conversely, Osteocalcin is a protein synthesized by osteoblasts, the cells responsible for building bone tissue. Once MK-7 fully activates Osteocalcin, the protein gains the ability to bind calcium. This activated form then directs the circulating calcium toward the bone matrix, ensuring its proper incorporation into the skeleton.
Dietary and Supplemental Sources
Menaquinone-7 is mainly sourced from foods that have undergone bacterial fermentation. The single most concentrated source of MK-7 is the traditional Japanese food Natto, which is made from soybeans fermented with Bacillus subtilis natto. Natto contains exceptionally high levels of MK-7, providing a substantial amount in a single serving.
While Natto remains a niche food in many Western diets, other fermented products contain varying levels of menaquinones. Certain hard and soft cheeses, such as Gouda and Brie, contain MK-7, as well as other long-chain menaquinones like MK-8 and MK-9. These fermented dairy products contribute to dietary K2 intake, though typically at lower concentrations than Natto. Because the Western diet generally lacks these high-MK-7 foods, many individuals turn to dietary supplements to ensure a consistent intake.
Determining Appropriate Intake
Currently, there is no formal Recommended Dietary Allowance (RDA) specifically established for Vitamin K2 (menaquinones), unlike the general Vitamin K recommendation which is primarily based on K1’s role in coagulation. However, clinical research has established common supplemental ranges for MK-7 that have shown efficacy in improving markers of Vitamin K status. These ranges often fall between 90 and 375 micrograms (\(\mu\)g) per day for promoting the carboxylation of extrahepatic proteins like MGP and Osteocalcin.
Individuals considering MK-7 supplementation should be aware that, as a form of Vitamin K, it directly affects the body’s coagulation pathways. Due to this function, MK-7 can interact with certain anticoagulant medications, particularly Vitamin K antagonists such as Warfarin. Patients taking these medications must consult with a healthcare professional before beginning any MK-7 supplementation to manage the potential for drug interactions. MK-7 is generally considered safe, but medical guidance is necessary when managing existing health conditions.