Vitamin K is a fat-soluble nutrient recognized for its involvement in several physiological processes, particularly blood clotting. This compound exists in nature in a few distinct forms, with the two main categories being Vitamin K1 and Vitamin K2. Vitamin K1 (phylloquinone) is primarily found in plants, whereas Vitamin K2 encompasses a group of compounds called menaquinones, which are largely produced by bacteria. Menaquinone-7, or MK-7, is a specific subtype of Vitamin K2 noted for its unique chemical structure and superior behavior within the human body.
Defining MK-7 and the Vitamin K Family
Vitamin K molecules share a common chemical structure but are differentiated by the length of their side chain. The menaquinones (Vitamin K2) are designated MK-n, where ‘n’ represents the number of isoprene units in that side chain. MK-7 is a long-chain menaquinone, indicating seven repeating isoprene units attached to the main structure.
This longer side chain gives MK-7 distinct advantages over other forms, particularly Vitamin K1 and the short-chain menaquinone MK-4. Unlike K1 and MK-4, which are quickly cleared from the bloodstream, MK-7 is more hydrophobic. This allows it to circulate in the blood for days, providing a long half-life of approximately three days. This superior systemic availability means MK-7 can reach tissues outside the liver, known as extrahepatic tissues, more effectively.
Primary Biological Function
The primary function of MK-7 is to act as a cofactor for the carboxylation of specific proteins, often referred to as Vitamin K-Dependent Proteins (VKDPs). This process is a chemical modification that allows these proteins to bind to calcium ions. Without sufficient Vitamin K, these proteins remain inactive and cannot perform their intended biological roles.
MK-7’s most recognized actions involve two specific VKDPs: Osteocalcin (OC) and Matrix Gla Protein (MGP). Osteocalcin is produced by bone-forming cells and requires carboxylation to integrate calcium into the bone mineral matrix, promoting bone mineralization. By activating Osteocalcin, MK-7 helps ensure that ingested calcium is directed into the skeletal structure.
MGP is involved in maintaining healthy blood vessels and preventing soft tissue calcification. When MGP is fully carboxylated by MK-7, it actively inhibits the buildup of calcium deposits within the walls of arteries. This dual action demonstrates how MK-7 manages calcium, simultaneously promoting its deposition in bone while preventing its accumulation in the arterial system. Research has shown that MK-7 supplementation can dose-dependently decrease levels of uncarboxylated MGP, which is a marker of improved vitamin K status in the vasculature.
Dietary Sources and Bioavailability
MK-7 is primarily synthesized through bacterial fermentation, which makes it abundant in certain fermented foods. The richest known natural source of MK-7 is Natto, a traditional Japanese dish made from fermented soybeans. The high concentration of MK-7 in Natto is due to the action of the specific bacteria used in its production, Bacillus subtilis.
While other fermented products like certain cheeses, egg yolk, and some animal products contain various menaquinones, Natto offers an exceptionally high dose of MK-7. The superior bioavailability of MK-7 is a result of both its long half-life and its effective absorption in the small intestine. Unlike Vitamin K1, which is often tightly bound within the plant structure of leafy greens, MK-7 is more readily absorbed by the body. This sustained presence in the circulation provides a greater opportunity for the nutrient to activate VKDPs throughout the body.
Supplementation and Safety Considerations
Because MK-7 is a fat-soluble vitamin, its absorption is enhanced when consumed with dietary fat. Therefore, it is advisable to take MK-7 supplements with a meal that contains some fat to maximize its uptake. Typical dosages found in supplements range from 45 µg to 360 µg per day, with studies showing improvements in the carboxylation of proteins like MGP and Osteocalcin within this range.
A serious safety consideration involves individuals taking anticoagulant medications, particularly Warfarin (a Vitamin K antagonist). Warfarin works by blocking the recycling of Vitamin K, which is necessary to reduce the blood’s clotting ability. Supplementing with MK-7 can compete with Warfarin, making the medication less effective and dangerously increasing the risk of blood clotting.
Even low doses of MK-7, such as 10 µg, have been shown to influence anticoagulation sensitivity in some individuals. For this reason, individuals on Warfarin or similar Vitamin K antagonist therapy must avoid MK-7 supplements and maintain a consistent, monitored intake of Vitamin K-rich foods. Any decision to start an MK-7 supplement should be made only after consultation with a healthcare provider.