Overview

Vitamin K refers to a group of fat‑soluble compounds required for normal blood clotting and for regulating where calcium is deposited in the body. It is best known for its role in the blood coagulation cascade and for activating proteins that bind calcium. The vitamin was first identified and characterized in the 20th century; the discovery of its role in coagulation earned the Danish chemist Henrik Dam a Nobel Prize. Historically the vitamin has been studied in relation to bleeding disorders and later for bone and cardiovascular health.

Chemical forms and absorption

Vitamin K exists in several forms. The major dietary form in plants is phylloquinone (often called K1), while menaquinones (collectively called K2) are produced by bacteria and occur in fermented foods and some animal products. Synthetic analogues also exist. Because vitamin K is a fat‑soluble nutrient, its absorption from the intestine depends on dietary fat and normal bile function; the gut microbiota contributes to K2 supply in many people. Absorbed vitamin K is transported in the bloodstream with lipoproteins and delivered to tissues where it participates in enzyme reactions that modify specific proteins.

Biological functions

The most familiar role of vitamin K is supporting the synthesis of clotting factors in the liver. Without adequate vitamin K, the blood’s ability to form a stable clot is impaired and a person who bleeds may lose more blood than normal. Other important functions include the activation of proteins involved in bone mineralization and in preventing inappropriate calcification of blood vessels and soft tissues. These functions operate through vitamin K–dependent carboxylation of glutamate residues on target proteins.

  • Coagulation: activation of clotting proteins synthesized by the liver (prothrombin and others).
  • Bone health: activation of osteocalcin, a protein that helps bind calcium in bone.
  • Vascular health: activation of matrix Gla protein, which helps inhibit calcium deposition in arteries.

Dietary sources and supplements

Common sources of vitamin K include leafy green vegetables and certain fermented foods. Examples include green vegetables such as spinach, lettuce, broccoli and cabbage. Natto and other fermented soy products are rich in K2, while some animal products contain menaquinones in varying amounts. Because it is fat‑soluble, vitamin K is better absorbed when eaten with a source of dietary fat. Supplements and pharmaceutical preparations are available and are used in specific clinical situations.

Deficiency, clinical relevance and interactions

Clinically significant vitamin K deficiency is uncommon in healthy adults but can occur in people with poor fat absorption, very low intake, or when intestinal bacteria are suppressed. Newborn infants are particularly vulnerable to bleeding from vitamin K deficiency and many health systems give a prophylactic injection at birth to reduce the risk. Vitamin K antagonists (for example, the anticoagulant drug warfarin) interfere with vitamin K’s action and are used therapeutically to reduce clotting; monitoring and adjustment are necessary because dietary vitamin K can modify anticoagulant effect. Severe untreated deficiency can lead to dangerous bleeding that may result in loss of consciousness or, in extreme cases, death.

Notable facts and practical points

Vitamin K is measured indirectly in clinical practice by tests of clotting function when deficiency or excess anticoagulation is suspected. Its role outside coagulation—particularly in bone and cardiovascular systems—continues to be an area of active research. The nutrient’s discovery and the early work tying it to clotting are historical milestones attributed to a Danish researcher, and modern recommendations emphasize obtaining vitamin K from a balanced diet of vegetables, fermented foods, and appropriate medical guidance when supplements or anticoagulant medications are involved. For basic information on clotting and related terminology see coagulation and for general information about blood see blood. Background on the discoverer and historical context is available via links about the Danish research tradition and the original chemist who reported the vitamin.

Further reading and resources: calcium regulation, clinical guidance on anticoagulant reversal, and public health recommendations vary by region; consult local guidelines and healthcare professionals for personalized advice. Additional authoritative summaries and reviews can be found through medical databases and professional societies (fat‑soluble vitamin overview, dietary sources).

Note: This article provides a concise overview and does not replace clinical guidance. If you have concerns about bleeding risk, medication interactions, or newborn care, seek advice from a healthcare provider.