Vitamin K is a fat-soluble vitamin that exists in two primary natural forms: phylloquinone (K1), found predominantly in green leafy vegetables, and menaquinones (K2), synthesized by intestinal bacteria and found in fermented foods and animal products. It serves as an essential cofactor for gamma-carboxylation of specific glutamate residues in proteins critical to coagulation and bone metabolism. Serum or plasma phylloquinone (K1) is the most commonly measured clinical form, though functional assays such as PIVKA-II (protein induced by vitamin K absence) may better reflect tissue-level sufficiency. Vitamin K functions as a cofactor for the enzyme gamma-glutamyl carboxylase, enabling post-translational carboxylation of vitamin K-dependent proteins including coagulation factors II, VII, IX, and X, as well as anticoagulant proteins C and S, and osteocalcin in bone. This carboxylation is essential for calcium binding and the proper biological activity of these proteins. Vitamin K undergoes an epoxide cycle in the liver, where it is continuously recycled between its active (hydroquinone) and inactive (epoxide) forms, a process inhibited by warfarin-class anticoagulants.
Vitamin K is a fat-soluble vitamin found mainly in green leafy vegetables and some fermented foods that your body needs to help blood clot properly and to keep bones healthy. When levels are too low, your blood may not clot as well as it should, which can cause unusual bruising or bleeding. Most healthy adults who eat a balanced diet get enough vitamin K, but certain medical conditions that affect fat absorption, some medications like warfarin, or very restricted diets can lead to deficiency. If you take a blood thinner like warfarin, the amount of vitamin K in your diet can affect how well your medication works, so it is important to keep your intake consistent. Your doctor may check your vitamin K level or related blood tests if there are concerns about bleeding, bone health, or how your medications are working.
When elevated: Elevated circulating vitamin K1 levels are generally not associated with toxicity, as phylloquinone has a low intrinsic toxicity profile. However, high levels may indicate recent high dietary intake or supplementation and can antagonize the therapeutic effect of vitamin K antagonist anticoagulants such as warfarin, leading to subtherapeutic anticoagulation and increased thromboembolic risk in patients on these medications. When low: Low vitamin K levels are associated with impaired hemostasis due to reduced carboxylation of coagulation factors, manifesting clinically as easy bruising, mucosal bleeding, or prolonged clotting times. Subclinical deficiency has been associated with reduced bone mineral density, increased fracture risk, and potentially accelerated vascular calcification, though causality for these outcomes requires further confirmation in large randomized trials.
Vitamin K supports bone mineralization and may help maintain skeletal integrity under the mechanical stress of training. Adequate K1 and K2 status supports optimal coagulation function, relevant for injury recovery and cardiovascular health in endurance athletes, though deficiency is rare in well-nourished populations.
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