The importance of menaquinones in human nutrition.

Bacterially produced menaquinones, 2-methyl-1,4-naphthoquinones with an unsaturated polyisoprenoid chain at the 3-position, are biologically active forms of vitamin K that are present in high concentrations in the human lower bowel. Menaquinones are found in human liver and circulate in human plasma at much higher concentrations than previously thought. Numerous case reports of antibiotic-induced, vitamin K-responsive hypothrombinemias have been taken as evidence that menaquinones contribute importantly to satisfying the human vitamin K requirement. However, more recent production of symptoms of vitamin K insufficiency in normal human subjects by dietary restriction of vitamin K argues against their nutritional significance. Current data support the view that menaquinones may partially satisfy the human requirement but that their contribution is much less than previously thought.