THE IMPORTANCE OF MENAQUINONES IN HUMAN-NUTRITION

Bacterially produced menaquinones, 2-methyl-1,4-naphthoquinones with a n 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 circu late in human plasma at much higher concentrations than previously tho ught. Numerous case reports of antibiotic-induced, vitamin K-responsiv e hypothrombinemias have been taken as evidence that menaquinones cont ribute importantly to satisfying the human vitamin K requirement. Howe ver, more recent production of symptoms of vitamin K insufficiency in normal human subjects by dietary restriction of vitamin K argues again st their nutritional significance. Current data support the view that menaquinones may partially satisfy the human requirement but that thei r contribution is much less than previously thought.