Vitamin K is a group of fat-soluble compounds required for several fundamental bodily processes. The vitamin K family is broadly categorized into two natural forms: Vitamin K1 (phylloquinone) and Vitamin K2 (menaquinones). This article focuses specifically on menaquinone-7, or MK-7, a highly researched form of Vitamin K2.
Classifying Vitamin K and MK-7
The Vitamin K family shares a common chemical foundation, but the forms differ based on the structure of their side chains. Vitamin K1, or phylloquinone, is produced by plants and is the main form found in leafy green vegetables. Its primary physiological role is in the liver, where it is utilized to synthesize proteins involved in blood clotting.
Vitamin K2 comprises a series of compounds called menaquinones, abbreviated as MK-n, where the ‘n’ represents the number of repeating isoprenoid units in the molecule’s side chain. The forms of K2 range from MK-4 to MK-13, with the most studied being MK-4 and the long-chain menaquinone, MK-7. MK-7 is distinguished by its seven repeating isoprenoid residues.
MK-4 is the shortest-chain menaquinone and is primarily found in animal products, often synthesized by animal tissues from dietary K1. In contrast, MK-7 is a long-chain menaquinone typically produced by bacterial fermentation.
How MK-7 Regulates Calcium Distribution
The importance of MK-7 lies in its role as a cofactor required for activating specific proteins that manage calcium throughout the body. These are known as Vitamin K-dependent proteins (VKDPs), which require the vitamin to undergo gamma-carboxylation. This reaction converts specific glutamic acid residues on the proteins into Gla residues, enabling them to bind to calcium ions.
MK-7 activates two primary VKDPs that govern the movement of calcium: Matrix Gla Protein (MGP) and Osteocalcin. MGP is widely distributed in soft tissues, including the walls of blood vessels and the kidneys. Once activated by MK-7, MGP works as an inhibitor, binding to excess calcium to prevent its accumulation and the subsequent hardening of arteries, known as vascular calcification.
The second protein, Osteocalcin, is synthesized by bone-building cells called osteoblasts. When MK-7 catalyzes its activation, the carboxylated Osteocalcin gains the ability to attract and secure calcium. This channels calcium directly into the bone structure where it is needed for mineralization.
This dual action of activating MGP to inhibit soft tissue calcification and activating Osteocalcin to promote bone mineralization is often referred to as “calcium channeling.” Without sufficient MK-7, these proteins remain inactive, which can contribute to the displacement of calcium in the body.
Practical Sources and Bioavailability
MK-7 is not widely present in the average Western diet, as its primary source is food that has undergone bacterial fermentation. The single most concentrated source of this menaquinone is Natto, a traditional Japanese dish made from fermented soybeans. Certain hard and soft cheeses also contain menaquinones, though the specific MK-7 content varies depending on the bacterial cultures used.
MK-7 is often the preferred form of Vitamin K2 in dietary supplements due to its superior absorption and stability in the bloodstream. Unlike Vitamin K1, which is rapidly cleared by the liver, MK-7 is well-absorbed in the small intestine and transported via lipoproteins to extrahepatic tissues like the bone and blood vessels.
The long-chain structure of MK-7 allows it to have a significantly longer half-life, remaining in the circulation for up to 72 hours. This extended presence ensures a continuous supply of the vitamin for the activation of calcium-regulating proteins.