Vitamin K is essential for normal blood clotting in the human body.
The liver uses vitamin K to help activate several proteins involved in coagulation, including prothrombin and factors VII, IX, and X. Vitamin K-dependent proteins also include anticoagulant proteins and proteins involved in bone metabolism, so its biological role extends beyond stopping bleeding.
The vitamin K cycle allows the body to reuse vitamin K during the activation of clotting proteins. The anticoagulant drug warfarin works partly by interfering with this cycle, thereby reducing the blood's ability to clot. This is why changes in vitamin K intake can matter for people taking warfarin.
Vitamin K1 is found especially in green leafy vegetables, while vitamin K2 forms are associated with bacterial production and certain foods. Newborns have low vitamin K stores and commonly receive vitamin K soon after birth to reduce the risk of vitamin-K-deficiency bleeding.