Chemistry, nutritional sources, tissue distribution and metabolism of vitamin K with special reference to bone health.
Vitamin K occurs in nature as a series of compounds with a common 2-methyl- 1,4 naphthoquinone nucleus and differing isoprenoid side chains at the 3 position. They comprise a single major plant form, phylloquinone with a phytyl side chain and a family of bacterially synthesized menaquinones (MKs) with multiprenyl side chains. The major dietary source to humans is phylloquinone for which the chief food contributors are green, leafy vegetables followed by certain vegetable oils (soybean, rapeseed and olive oils). Recent analyses by high pressure liquid chromatography are now providing a wide-ranging database of phylloquinone in foods. Menaquinones are found in moderate concentrations in only a few foods such as cheeses (MK-8 and MK-9). A wider spectrum of MKs is synthesized by the gut microflora, and their intestinal absorption probably accounts for most of the hepatic stores, particularly those with very long side chains (MKs-10-13) synthesized by members of the genus Bacteroides. The site of absorption of floral MKs is not known, but reasonable concentrations are found in the terminal ileum where bile salt-mediated absorption is possible. Both phylloquinone and menaquinones are bioactive in hepatic gamma-carboxylation but long-chain MKs are less well absorbed. Liver stores of vitamin K are relatively small and predominantly MKs-7-13. The hepatic reserves of phylloquinone (approximately 10% of the total) are labile and turn over at a faster rate than menaquinones. Trabecular and cortical bone appear to contain substantial concentrations of both phylloquinone and menaquinones. A majority (approximately 60-70%) of the daily dietary intake of phylloquinone is lost to the body by excretion, which emphasizes the need for a continuous dietary supply to maintain tissue reserves.
At the present time the human requirements for vitamin K are based solely on its classical function in coagulation being listed as a Recommended Dietary Allowance (RDA) in the United States (Suttie 1992) and a Safe and Adequate Intake in the United Kingdom (Department of Health Report 1991). In both cases these requirements were set at a value of 1 mcg/kg/d. If, as argued by Vermeer et al. and Kohlmeier et al. in this volume, vitamin K is important to bone health and its requirements for this bone function are greater than for its hepatic function, a great challenge to researchers and future committees alike is to determine whether these putative extra demands can be quantified more precisely. Finally, it should be noted that the concept of reexamining the optimal intake of a vitamin with respect to the extra health benefits, which may be conferred by giving amounts over and above those required to protect against the originally discovered deficiency disease, is not new. There is already a recognition of the newer and often unexpected roles played by several other vitamins including in some cases the beneficial effects of extra intakes (Sauberlich and Machlin 1992).