Vitamin K is a fat-soluble nutrient historically recognized for its function in blood clotting. It exists in two main forms: Vitamin K1 (phylloquinone), found primarily in leafy green vegetables, and Vitamin K2 (menaquinone). Vitamin K2 is particularly important for regulating calcium metabolism, helping to direct calcium to the bones and teeth while preventing its accumulation in soft tissues like arteries. The different forms of K2, known as menaquinones, have distinct properties that influence their effectiveness for supplementation.
The Two Primary Forms of Vitamin K2
The menaquinone family is categorized by the length of its chemical side chain. The most commonly studied forms are menaquinone-4 (MK-4) and menaquinone-7 (MK-7), which differ fundamentally in molecular structure and origin. The MK-4 molecule possesses a shorter side chain, influencing how the body processes and utilizes it.
MK-4 is found in animal-based foods, such as meat, egg yolks, and high-fat dairy products. It is also produced endogenously in the body through the conversion of other Vitamin K forms. However, the MK-4 used in most supplements is synthesized in a laboratory setting.
The MK-7 form features a longer side chain that grants it unique stability. This longer-chain menaquinone is predominantly sourced from bacterial fermentation processes. The richest natural source of MK-7 is natto, a traditional Japanese food made from fermented soybeans.
Bioavailability and Duration of Activity
The structural differences between MK-4 and MK-7 result in vastly different behaviors concerning their half-life and distribution. MK-4 is rapidly absorbed but quickly disappears, exhibiting a very short half-life of approximately two to eight hours. This rapid clearance means that MK-4 must be consumed in high, milligram-range doses multiple times daily to maintain consistent blood levels.
Despite its short half-life, MK-4 shows a strong affinity for specific organs, such as the brain, pancreas, and arterial walls, suggesting a localized action. However, at typical nutritional doses, MK-4 is often not detectable in the serum, limiting its ability to provide sustained, systemic support.
Conversely, MK-7 exhibits a significantly longer half-life, remaining in the bloodstream for up to 68 to 72 hours following ingestion. This extended duration allows MK-7 to accumulate in the blood, providing a steady and sustained supply to the body’s tissues.
This stability means MK-7 is more effective at reaching extrahepatic tissues, including bone and the vascular system. There, it activates proteins responsible for bone mineralization and the inhibition of arterial calcification. Due to its prolonged presence, MK-7 is effective at much lower, microgram-level doses taken just once daily.
Practical Guide to Supplement Selection
When selecting a Vitamin K2 supplement, the choice of form should be guided by its pharmacokinetic profile and desired health outcome. MK-7 is generally preferred for systemic support of bone and cardiovascular health due to its long half-life and effectiveness at lower doses. It is important to look for MK-7 supplements that specify an “all-trans” purity level, as this isomer represents the biologically active form.
The “all-trans” designation confirms the molecular structure is optimal for biological function, with high-quality products often listing a purity of 97% or higher. The MK-7 form requires a much smaller daily dose, typically 100 to 200 micrograms. This contrasts with the substantial 45-milligram daily dose required for MK-4 to be clinically effective for bone density.
The pairing of Vitamin K2 with Vitamin D3 is an important consideration for synergistic health benefits. Vitamin D3 increases calcium absorption and stimulates the production of Vitamin K-dependent proteins, which require K2 for activation. A general guideline is to consume approximately 45 micrograms of Vitamin K2 for every 1,000 International Units (IU) of Vitamin D3.