Figure 1 Chemical structures of phylloquinone, menaquinones, menadione, warfarin, and dicumarol.
quantitative source; however, its availability for absorption from these foods is thought to be relatively poor. Certain plant-derived oils, notably soya and canola oils, are also rich in the vitamin, which is probably much more readily available from such sources than it is from leaves. Menaquinones are typically obtained from foods, such as cheeses or Japanese 'natto' (fermented bean curd), in which bacterial fermentation has occurred. Smaller amounts of both phylloquinones and menaquinones are obtained from liver and other animal-derived foods.
Phylloquinone is highly lipophilic; however, at low concentrations it is transported by a saturable,
Table 1 Mean estimate food contents of phylloquinone and selected menaquinones
Food item ^g/100g wet weight or ^g/100mla
Phylloquinone MK-4 MK-7 MK-8 MK-9 (vitamin K1)
Olive oil 53.7
Corn oil 2.9
aA dashed line means not detectable. Values obtained for MK-5 and MK-6 are omitted from this summary. The data demonstrate clearly (i) the huge difference in vitamin K contents between different foods and (ii) the preponderence of phylloquinone in some foods and of menaquinones (of several different chain lengths) in others.
bA Japanese food made from fermented soya bean curd. Data from Schurgers LJ and Vermeer C (2000) Determination of phylloquinone and menaqunones in food. Haemostasis 30: 298-307, Table 2.
energy-dependent transport system across the gut wall, mainly in the upper small intestine. Phylloqui-none in foods consisting of plant tissues is much less readily bioavailable for absorption than the pure vitamin since it is tightly bound to the thylakoid membranes of the chloroplasts, and the absorption of vitamin K from plant foods is considerably improved by including additional fat in the meal. Its absorption also depends on the stimulation of bile salt and pancreatic lipase secretions. The long-chain menaquinones, which are even more lipophilic, are only passively absorbed and are much less bioavail-able for absorption than phylloquinone. However, if given by injection (e.g., intracardially), they can be even more functionally active than phylloquinone. The relative bioavailability and bioactivity of the different forms and food sources of vitamin K need more research. Preliminary studies with
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