As with man, overt vitamin K deficiency is rare in animals and several examples will be identified. The general understanding of vitamin K in the physiology and pathophysiology of animal health and well‐being is less well developed than it is for man. It is also worth noting that, in the rat, vitamin K1 can be converted to menaquinone‐4 without the need for bacterial mediators. In contrast to bacteria‐mediated generation of biologically functional vitamin K2, it is also possible for the specific form of vitamer, menaquinone‐4, to be synthesized in animal tissues from vitamin K3. Menaquinone‐4 is found less frequently in bacteria, those species where it has been reported as a major menaquinone are usually extremophiles and microaerophiles, and it may be the evolutionary hinge between vitamin K1 using phytoplankton and photosynthetic cyanobacteria. All the long side‐chain menaquinones are derived from bacterial sources.
There are several forms of vitamin K: a single vitamer from plants, vitamin K1, or phylloquinone, and a family of K2 vitamins, the menaquinones, which are distinguished by the number of isoprene units in their side chain at position 3 of the 2‐methyl‐1,4‐naphthoquinone moiety. The post‐translational gamma‐carboxylation of proteins by vitamin K is common across the animal kingdom, yet the organisms most studied for the relationship between this vitamin in health and disease are predominantly man and the rat.
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