Vitamin K is a fat-soluble vitamin that was first identified in the 1930s when Danish researcher Henrik Dam discovered that chickens fed a fat-free diet developed haemorrhagic disease. He named the missing compound "Koagulationsvitamin" — vitamin K. For decades, vitamin K was understood primarily in the context of blood clotting. More recent research has identified a second, distinct role — in calcium metabolism and vascular calcification — that is mediated by a different form of the vitamin and operates largely independently of clotting.
K1 (phylloquinone)
Vitamin K1 — phylloquinone — is the form found in green leafy vegetables (kale, spinach, Brussels sprouts, broccoli, and parsley are particularly rich sources). It is the predominant dietary form and the form with the strongest evidence for coagulation function. After absorption, K1 is rapidly cleared by the liver, where it activates the clotting factors II, VII, IX, and X. Because it is so efficiently used by the liver for clotting, relatively little K1 circulates to extrahepatic tissues like bone and blood vessels.
K2 (menaquinones)
Vitamin K2 comprises a family of compounds called menaquinones, distinguished by the length of their side chain — written as MK-n, where n is the number of isoprene units. The two most relevant for human supplementation are:
- MK-4 (menaquinone-4): The shortest-chain menaquinone. Found in animal products, including egg yolk, meat, butter, and some cheeses. It has a short half-life in the body (approximately 1 hour) and is thought to be particularly active in certain tissues including the brain and testes.
- MK-7 (menaquinone-7): Produced by bacterial fermentation and found in highest concentrations in nattō (Japanese fermented soybean). Has a much longer half-life (approximately 72 hours) and produces more stable, sustained blood levels than MK-4. The majority of K2 supplement research uses MK-7.
K2 and calcium regulation: osteocalcin and MGP
K2's distinct role from K1 is in activating two vitamin K-dependent proteins outside the liver:
- Osteocalcin: A protein secreted by osteoblasts (bone-forming cells) that, when activated by K2 (carboxylated), helps bind calcium to the bone matrix. Under-carboxylated osteocalcin — a marker of K2 insufficiency — is associated with reduced bone mineral density in observational studies.
- Matrix Gla Protein (MGP): An inhibitor of vascular calcification, produced by smooth muscle cells in blood vessel walls. MGP must be activated by K2 to function. Uncarboxylated MGP (ucMGP) — a biomarker of K2 insufficiency — correlates with arterial stiffness and cardiovascular risk in large population studies.
The D3+K2 pairing
Vitamin D3 increases calcium absorption and osteocalcin synthesis. Adequate K2 is required to carboxylate the osteocalcin that D3 stimulates, directing calcium into bone rather than soft tissues. This is the biological rationale for pairing D3 and K2 in combination supplements — an article covered separately in our blog.
Recent research
- Dietary Intake and Adequacy of Vitamin K — Journal of Nutrition, 1998. Authors: Booth SL, Suttie JW. Phylloquinone (K1) is cleared rapidly from plasma with a half-life of 1–2 hours, while MK-7 persists for 72+ hours.
- Vitamin K-containing dietary supplements: comparison of synthetic vitamin K1 and natto-derived menaquinone-7 — Blood, 2007. Authors: Schurgers LJ et al. MK-7 supplementation increased carboxylated osteocalcin concentrations to a greater degree than equivalent doses of K1.
- Dietary Intake of Menaquinone Is Associated with a Reduced Risk of Coronary Heart Disease — Journal of Nutrition, 2004. Authors: Geleijnse JM et al. Higher dietary MK-7 intake was associated with reduced arterial calcification scores as measured by CT scanning.
Vitamin K in Lebanon
Supplements & More carries Horbäach Vitamin D3 + K2 MK-7 from the US, available with cash on delivery across Lebanon.