Vitamin K refers to a group of fat-soluble vitamins involved in the homeostasis of blood coagulation and calcium. Discovered and studied beginning in the 1930s, the name is derived from the German word "Koagulation," reflecting the vitamins' role as cofactors in the activation—via carboxylation—of certain blood-clotting factors.

Vitamin K2 (or menaquinone), in particular, plays a fundamental role in maintaining a proper balance of calcium deposits within the body; it promotes the accumulation of calcium in tissues where it is needed—such as bone—while simultaneously preventing its formation in tissues where it could be harmful, such as the walls of blood vessels or cartilage.

This vitamin is involved in the activation, through carboxylation, of matrix Gla proteins, which are produced by chondrocytes and vascular smooth muscle cells. Matrix Gla proteins inhibit the calcification of blood vessels—a process linked to an increased cardiovascular risk—as well as the calcification of soft tissues, such as cartilage, where they appear to prevent the formation of abnormal calcium deposits that can contribute to joint issues like osteoarthritis.

Another important substrate for carboxylation promoted by vitamin K2 is osteocalcin, a protein responsible for incorporating calcium into bones—a process that improves bone quality and reduces the risk of fractures. In addition to osteocalcin, there are other vitamin K-dependent proteins that are important for bone health, such as periostin, Gas6, and protein S. Furthermore, vitamin K contributes to bone homeostasis through other mechanisms involving osteoblasts and osteoclasts, the cells responsible for the continuous remodeling of bone tissue. A deficiency in vitamin K2 is associated with the presence of vascular calcifications and osteoporosis. Vitamin K1 is synthesized by plants and cyanobacteria and is particularly abundant in green leafy vegetables, though significant amounts can also be found in other vegetables, fruits, grains, and dairy products. Vitamin K1 is, therefore, the primary form of vitamin K found in the Western diet. Vitamin K2, on the other hand, is synthesized by certain bacteria. Its dietary sources include animal liver, but—most importantly—fermented foods, such as certain cheeses in the Western diet and natto (fermented soybeans) in the Eastern diet.