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vitamin K2, the main storage form in animals, has several subtypes, which differ in isoprenoid chain length. These vitamin K2 homologues are called menaquinones, and are characterized by the number of isoprenoid residues in their side chains. Menaquinones are abbreviated MK-n, where M stands for menaquinone, the K stands for vitamin K, and the n represents the number of isoprenoid side chain residues.
Vitamin K2 has only begun to be studied, and the few studies on humans suffer from either a small number of test subjects or that the study has yet to be reproduced by an independent team. The possible health benefits which those studies have suggested may be worth further investigation are mostly related to bone strength and arterial health (reducing calcification or even decalcifying, with a possible reduction in blood pressure).
The European Food Safety Authority (EU) and the US Institute of Medicine, on reviewing existing evidence, have decided there is insufficient evidence to publish a dietary reference value for vitamin K or for K2. They have, however, published an adequate intake (AI) for vitamin K, but no value specifically for K2. Evidence suggests K2 is converted from dietary K1, and thus no dietary intake of K2 may be needed.
Parts of the scientific literature, dating back to 1998, suggest that the AI values are based only on the hepatic requirements (i.e. related to the liver). This hypothesis is supported by the fact that the majority of the Western population exhibits a substantial fraction of undercarboxylated extra-hepatic proteins. Thus, complete activation of coagulation factors is satisfied, but there does not seem to be enough vitamin K2 for the carboxylation of osteocalcin in bone and MGP in the vascular system.
There is no known toxicity associated with high doses of menaquinones (vitamin K2). Unlike the other fat-soluble vitamins, vitamin K is not stored in any significant quantity in the liver; therefore the toxic level is not a described problem. All data available as of 2017 demonstrate that vitamin K has no adverse effects in healthy subjects. The recommendations for the daily intake of vitamin K, as issued recently by the US Institute of Medicine, also acknowledge the wide safety margin of vitamin K: "A search of the literature revealed no evidence of toxicity associated with the intake of either K1 or K2". Animal models involving rats, if generalisable to humans, show that MK-7 is well-tolerated.