Vitamin K

What Is Vitamin K?

Vitamin K is a fat-soluble vitamin named after the German word Koagulation (coagulation), reflecting its discovery in 1929 as a factor essential for blood clotting. It exists in two main dietary forms: vitamin K1 (phylloquinone), synthesized by plants and the predominant dietary form, and vitamin K2 (menaquinones, MK-4 through MK-13), produced by bacteria and found in fermented foods and certain animal products. Gut microbiota also produce menaquinones, contributing to K2 status.

Rather than an RDA, an Adequate Intake (AI) is used: 120 mcg/day for adult men and 90 mcg/day for adult women. No established UL exists for natural forms, but high-dose vitamin K2 supplements can interact with warfarin anticoagulant therapy.

Key Functions

• Blood coagulation: Vitamin K is an essential cofactor for the enzyme gamma-glutamyl carboxylase, which activates coagulation factors II (prothrombin), VII, IX, and X, as well as anticoagulant proteins C and S. Carboxylation adds calcium-binding capacity, enabling these proteins to function in clotting cascades.
• Bone metabolism: Osteocalcin (bone Gla protein) and matrix Gla protein (MGP) require vitamin K-dependent carboxylation. Undercarboxylated osteocalcin is associated with reduced bone mineral density and increased fracture risk.
• Vascular calcification prevention: MGP, the most potent inhibitor of arterial calcification, requires carboxylation by vitamin K2 for activity, potentially explaining observed associations between vitamin K2 intake and cardiovascular health.

Food Sources

Vitamin K1 is abundant in dark green leafy vegetables: kale (817 mcg/100 g cooked), collard greens, spinach, broccoli, and Brussels sprouts. Vitamin K2 is found in fermented soy (natto is the richest source at approximately 900 mcg/100 g as MK-7), hard cheeses, butter, egg yolks, and chicken. Absorption of both forms is enhanced by dietary fat consumed in the same meal.