Vitamin K is a fat-soluble vitamin essential for coagulation of blood and for metabolic pathways in bone and other tissue. It exists in two forms, vitamin K1 and vitamin K2. Vitamin K1 is synthesised by plants and is directly involved in photosynthesis. Vitamin K2 is the main storage form in animals. Colonic bacteria can also convert K1 to K2.
The existence of an anti-hemorrhagic dietary factor was proven in 1929 and isolated in 1936. It was given the name vitamin K, but in fact it is a group of several similar liposoluble substances that permit blood coagulation in certain conditions through complex biochemical mechanisms. Vitamin K is traditionally administered by injection following the intoxication of anticoagulants (e.g. accidental ingestion of rodenticides).
Vitamin K is a co-factor for many enzymes, which means that these enzymes cannot be active without it. As a consequence, it is essential to some blood coagulation factors. It also has a role in protein metabolism, helping to bind calcium in bone.
A vitamin K deficiency is responsible for digestive, nasal, skin and cerebral haemorrhages, as blood coagulation is no longer possible. In time, these minor, sometimes imperceptible haemorrhages lead to anaemia (lack of the red cells that transport oxygen in the blood).
Vitamin K1 is found chiefly in leafy green vegetables such as dandelion greens, spinach, silver beet, and Brassica (e.g. cabbage, kale, cauliflower, broccoli, and brussels sprouts). Some fruits, such as avocado, kiwifruit and grapes, are also high in vitamin K. Absorption is often greater when accompanied by dietary fats such as butter or oils. Colonic bacteria synthesise a significant portion of mammalian vitamin K needs and consequently dietary supplementation is unnecessary for most dogs and cats.